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17.30 Miscellaneous Parameters

Here are several miscellaneous parameters.

— Macro: HAS_LONG_COND_BRANCH

Define this boolean macro to indicate whether or not your architecture has conditional branches that can span all of memory. It is used in conjunction with an optimization that partitions hot and cold basic blocks into separate sections of the executable. If this macro is set to false, gcc will convert any conditional branches that attempt to cross between sections into unconditional branches or indirect jumps.

— Macro: HAS_LONG_UNCOND_BRANCH

Define this boolean macro to indicate whether or not your architecture has unconditional branches that can span all of memory. It is used in conjunction with an optimization that partitions hot and cold basic blocks into separate sections of the executable. If this macro is set to false, gcc will convert any unconditional branches that attempt to cross between sections into indirect jumps.

— Macro: CASE_VECTOR_MODE

An alias for a machine mode name. This is the machine mode that elements of a jump-table should have.

— Macro: CASE_VECTOR_SHORTEN_MODE (min_offset, max_offset, body)

Optional: return the preferred mode for an addr_diff_vec when the minimum and maximum offset are known. If you define this, it enables extra code in branch shortening to deal with addr_diff_vec. To make this work, you also have to define INSN_ALIGN and make the alignment for addr_diff_vec explicit. The body argument is provided so that the offset_unsigned and scale flags can be updated.

— Macro: CASE_VECTOR_PC_RELATIVE

Define this macro to be a C expression to indicate when jump-tables should contain relative addresses. You need not define this macro if jump-tables never contain relative addresses, or jump-tables should contain relative addresses only when -fPIC or -fPIC is in effect.

— Target Hook: unsigned int TARGET_CASE_VALUES_THRESHOLD (void)

This function return the smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. The default is four for machines with a casesi instruction and five otherwise. This is best for most machines.

— Macro: WORD_REGISTER_OPERATIONS

Define this macro if operations between registers with integral mode smaller than a word are always performed on the entire register. Most RISC machines have this property and most CISC machines do not.

— Macro: LOAD_EXTEND_OP (mem_mode)

Define this macro to be a C expression indicating when insns that read memory in mem_mode, an integral mode narrower than a word, set the bits outside of mem_mode to be either the sign-extension or the zero-extension of the data read. Return SIGN_EXTEND for values of mem_mode for which the insn sign-extends, ZERO_EXTEND for which it zero-extends, and UNKNOWN for other modes.

This macro is not called with mem_mode non-integral or with a width greater than or equal to BITS_PER_WORD, so you may return any value in this case. Do not define this macro if it would always return UNKNOWN. On machines where this macro is defined, you will normally define it as the constant SIGN_EXTEND or ZERO_EXTEND.

You may return a non-UNKNOWN value even if for some hard registers the sign extension is not performed, if for the REGNO_REG_CLASS of these hard registers CANNOT_CHANGE_MODE_CLASS returns nonzero when the from mode is mem_mode and the to mode is any integral mode larger than this but not larger than word_mode.

You must return UNKNOWN if for some hard registers that allow this mode, CANNOT_CHANGE_MODE_CLASS says that they cannot change to word_mode, but that they can change to another integral mode that is larger then mem_mode but still smaller than word_mode.

— Macro: SHORT_IMMEDIATES_SIGN_EXTEND

Define this macro if loading short immediate values into registers sign extends.

— Target Hook: unsigned int TARGET_MIN_DIVISIONS_FOR_RECIP_MUL (enum machine_mode mode)

When -ffast-math is in effect, GCC tries to optimize divisions by the same divisor, by turning them into multiplications by the reciprocal. This target hook specifies the minimum number of divisions that should be there for GCC to perform the optimization for a variable of mode mode. The default implementation returns 3 if the machine has an instruction for the division, and 2 if it does not.

— Macro: MOVE_MAX

The maximum number of bytes that a single instruction can move quickly between memory and registers or between two memory locations.

— Macro: MAX_MOVE_MAX

The maximum number of bytes that a single instruction can move quickly between memory and registers or between two memory locations. If this is undefined, the default is MOVE_MAX. Otherwise, it is the constant value that is the largest value that MOVE_MAX can have at run-time.

— Macro: SHIFT_COUNT_TRUNCATED

A C expression that is nonzero if on this machine the number of bits actually used for the count of a shift operation is equal to the number of bits needed to represent the size of the object being shifted. When this macro is nonzero, the compiler will assume that it is safe to omit a sign-extend, zero-extend, and certain bitwise `and' instructions that truncates the count of a shift operation. On machines that have instructions that act on bit-fields at variable positions, which may include `bit test' instructions, a nonzero SHIFT_COUNT_TRUNCATED also enables deletion of truncations of the values that serve as arguments to bit-field instructions.

If both types of instructions truncate the count (for shifts) and position (for bit-field operations), or if no variable-position bit-field instructions exist, you should define this macro.

However, on some machines, such as the 80386 and the 680x0, truncation only applies to shift operations and not the (real or pretended) bit-field operations. Define SHIFT_COUNT_TRUNCATED to be zero on such machines. Instead, add patterns to the md file that include the implied truncation of the shift instructions.

You need not define this macro if it would always have the value of zero.

— Target Hook: unsigned HOST_WIDE_INT TARGET_SHIFT_TRUNCATION_MASK (enum machine_mode mode)

This function describes how the standard shift patterns for mode deal with shifts by negative amounts or by more than the width of the mode. See shift patterns.

On many machines, the shift patterns will apply a mask m to the shift count, meaning that a fixed-width shift of x by y is equivalent to an arbitrary-width shift of x by y & m. If this is true for mode mode, the function should return m, otherwise it should return 0. A return value of 0 indicates that no particular behavior is guaranteed.

Note that, unlike SHIFT_COUNT_TRUNCATED, this function does not apply to general shift rtxes; it applies only to instructions that are generated by the named shift patterns.

The default implementation of this function returns GET_MODE_BITSIZE (mode) - 1 if SHIFT_COUNT_TRUNCATED and 0 otherwise. This definition is always safe, but if SHIFT_COUNT_TRUNCATED is false, and some shift patterns nevertheless truncate the shift count, you may get better code by overriding it.

— Macro: TRULY_NOOP_TRUNCATION (outprec, inprec)

A C expression which is nonzero if on this machine it is safe to “convert” an integer of inprec bits to one of outprec bits (where outprec is smaller than inprec) by merely operating on it as if it had only outprec bits.

On many machines, this expression can be 1.

When TRULY_NOOP_TRUNCATION returns 1 for a pair of sizes for modes for which MODES_TIEABLE_P is 0, suboptimal code can result. If this is the case, making TRULY_NOOP_TRUNCATION return 0 in such cases may improve things.

— Target Hook: int TARGET_MODE_REP_EXTENDED (enum machine_mode mode, enum machine_mode rep_mode)

The representation of an integral mode can be such that the values are always extended to a wider integral mode. Return SIGN_EXTEND if values of mode are represented in sign-extended form to rep_mode. Return UNKNOWN otherwise. (Currently, none of the targets use zero-extended representation this way so unlike LOAD_EXTEND_OP, TARGET_MODE_REP_EXTENDED is expected to return either SIGN_EXTEND or UNKNOWN. Also no target extends mode to rep_mode so that rep_mode is not the next widest integral mode and currently we take advantage of this fact.)

Similarly to LOAD_EXTEND_OP you may return a non-UNKNOWN value even if the extension is not performed on certain hard registers as long as for the REGNO_REG_CLASS of these hard registers CANNOT_CHANGE_MODE_CLASS returns nonzero.

Note that TARGET_MODE_REP_EXTENDED and LOAD_EXTEND_OP describe two related properties. If you define TARGET_MODE_REP_EXTENDED (mode, word_mode) you probably also want to define LOAD_EXTEND_OP (mode) to return the same type of extension.

In order to enforce the representation of mode, TRULY_NOOP_TRUNCATION should return false when truncating to mode.

— Macro: STORE_FLAG_VALUE

A C expression describing the value returned by a comparison operator with an integral mode and stored by a store-flag instruction (‘cstoremode4’) when the condition is true. This description must apply to all the ‘cstoremode4’ patterns and all the comparison operators whose results have a MODE_INT mode.

A value of 1 or −1 means that the instruction implementing the comparison operator returns exactly 1 or −1 when the comparison is true and 0 when the comparison is false. Otherwise, the value indicates which bits of the result are guaranteed to be 1 when the comparison is true. This value is interpreted in the mode of the comparison operation, which is given by the mode of the first operand in the ‘cstoremode4’ pattern. Either the low bit or the sign bit of STORE_FLAG_VALUE be on. Presently, only those bits are used by the compiler.

If STORE_FLAG_VALUE is neither 1 or −1, the compiler will generate code that depends only on the specified bits. It can also replace comparison operators with equivalent operations if they cause the required bits to be set, even if the remaining bits are undefined. For example, on a machine whose comparison operators return an SImode value and where STORE_FLAG_VALUE is defined as ‘0x80000000’, saying that just the sign bit is relevant, the expression

          (ne:SI (and:SI x (const_int power-of-2)) (const_int 0))

can be converted to

          (ashift:SI x (const_int n))

where n is the appropriate shift count to move the bit being tested into the sign bit.

There is no way to describe a machine that always sets the low-order bit for a true value, but does not guarantee the value of any other bits, but we do not know of any machine that has such an instruction. If you are trying to port GCC to such a machine, include an instruction to perform a logical-and of the result with 1 in the pattern for the comparison operators and let us know at gcc@gcc.gnu.org.

Often, a machine will have multiple instructions that obtain a value from a comparison (or the condition codes). Here are rules to guide the choice of value for STORE_FLAG_VALUE, and hence the instructions to be used:

Many machines can produce both the value chosen for STORE_FLAG_VALUE and its negation in the same number of instructions. On those machines, you should also define a pattern for those cases, e.g., one matching

          (set A (neg:m (ne:m B C)))

Some machines can also perform and or plus operations on condition code values with less instructions than the corresponding ‘cstoremode4’ insn followed by and or plus. On those machines, define the appropriate patterns. Use the names incscc and decscc, respectively, for the patterns which perform plus or minus operations on condition code values. See rs6000.md for some examples. The GNU Superoptimizer can be used to find such instruction sequences on other machines.

If this macro is not defined, the default value, 1, is used. You need not define STORE_FLAG_VALUE if the machine has no store-flag instructions, or if the value generated by these instructions is 1.

— Macro: FLOAT_STORE_FLAG_VALUE (mode)

A C expression that gives a nonzero REAL_VALUE_TYPE value that is returned when comparison operators with floating-point results are true. Define this macro on machines that have comparison operations that return floating-point values. If there are no such operations, do not define this macro.

— Macro: VECTOR_STORE_FLAG_VALUE (mode)

A C expression that gives a rtx representing the nonzero true element for vector comparisons. The returned rtx should be valid for the inner mode of mode which is guaranteed to be a vector mode. Define this macro on machines that have vector comparison operations that return a vector result. If there are no such operations, do not define this macro. Typically, this macro is defined as const1_rtx or constm1_rtx. This macro may return NULL_RTX to prevent the compiler optimizing such vector comparison operations for the given mode.

— Macro: CLZ_DEFINED_VALUE_AT_ZERO (mode, value)
— Macro: CTZ_DEFINED_VALUE_AT_ZERO (mode, value)

A C expression that indicates whether the architecture defines a value for clz or ctz with a zero operand. A result of 0 indicates the value is undefined. If the value is defined for only the RTL expression, the macro should evaluate to 1; if the value applies also to the corresponding optab entry (which is normally the case if it expands directly into the corresponding RTL), then the macro should evaluate to 2. In the cases where the value is defined, value should be set to this value.

If this macro is not defined, the value of clz or ctz at zero is assumed to be undefined.

This macro must be defined if the target's expansion for ffs relies on a particular value to get correct results. Otherwise it is not necessary, though it may be used to optimize some corner cases, and to provide a default expansion for the ffs optab.

Note that regardless of this macro the “definedness” of clz and ctz at zero do not extend to the builtin functions visible to the user. Thus one may be free to adjust the value at will to match the target expansion of these operations without fear of breaking the API.

— Macro: Pmode

An alias for the machine mode for pointers. On most machines, define this to be the integer mode corresponding to the width of a hardware pointer; SImode on 32-bit machine or DImode on 64-bit machines. On some machines you must define this to be one of the partial integer modes, such as PSImode.

The width of Pmode must be at least as large as the value of POINTER_SIZE. If it is not equal, you must define the macro POINTERS_EXTEND_UNSIGNED to specify how pointers are extended to Pmode.

— Macro: FUNCTION_MODE

An alias for the machine mode used for memory references to functions being called, in call RTL expressions. On most CISC machines, where an instruction can begin at any byte address, this should be QImode. On most RISC machines, where all instructions have fixed size and alignment, this should be a mode with the same size and alignment as the machine instruction words - typically SImode or HImode.

— Macro: STDC_0_IN_SYSTEM_HEADERS

In normal operation, the preprocessor expands __STDC__ to the constant 1, to signify that GCC conforms to ISO Standard C. On some hosts, like Solaris, the system compiler uses a different convention, where __STDC__ is normally 0, but is 1 if the user specifies strict conformance to the C Standard.

Defining STDC_0_IN_SYSTEM_HEADERS makes GNU CPP follows the host convention when processing system header files, but when processing user files __STDC__ will always expand to 1.

— C Target Hook: const char * TARGET_C_PREINCLUDE (void)

Define this hook to return the name of a header file to be included at the start of all compilations, as if it had been included with #include <file>. If this hook returns NULL, or is not defined, or the header is not found, or if the user specifies -ffreestanding or -nostdinc, no header is included.

This hook can be used together with a header provided by the system C library to implement ISO C requirements for certain macros to be predefined that describe properties of the whole implementation rather than just the compiler.

— C Target Hook: bool TARGET_CXX_IMPLICIT_EXTERN_C (const char*)

Define this hook to add target-specific C++ implicit extern C functions. If this function returns true for the name of a file-scope function, that function implicitly gets extern "C" linkage rather than whatever language linkage the declaration would normally have. An example of such function is WinMain on Win32 targets.

— Macro: NO_IMPLICIT_EXTERN_C

Define this macro if the system header files support C++ as well as C. This macro inhibits the usual method of using system header files in C++, which is to pretend that the file's contents are enclosed in ‘extern "C" {...}’.

— Macro: REGISTER_TARGET_PRAGMAS ()

Define this macro if you want to implement any target-specific pragmas. If defined, it is a C expression which makes a series of calls to c_register_pragma or c_register_pragma_with_expansion for each pragma. The macro may also do any setup required for the pragmas.

The primary reason to define this macro is to provide compatibility with other compilers for the same target. In general, we discourage definition of target-specific pragmas for GCC.

If the pragma can be implemented by attributes then you should consider defining the target hook ‘TARGET_INSERT_ATTRIBUTES’ as well.

Preprocessor macros that appear on pragma lines are not expanded. All ‘#pragma’ directives that do not match any registered pragma are silently ignored, unless the user specifies -Wunknown-pragmas.

— Function: void c_register_pragma (const char *space, const char *name, void (*callback) (struct cpp_reader *))
— Function: void c_register_pragma_with_expansion (const char *space, const char *name, void (*callback) (struct cpp_reader *))

Each call to c_register_pragma or c_register_pragma_with_expansion establishes one pragma. The callback routine will be called when the preprocessor encounters a pragma of the form

          #pragma [space] name ...

space is the case-sensitive namespace of the pragma, or NULL to put the pragma in the global namespace. The callback routine receives pfile as its first argument, which can be passed on to cpplib's functions if necessary. You can lex tokens after the name by calling pragma_lex. Tokens that are not read by the callback will be silently ignored. The end of the line is indicated by a token of type CPP_EOF. Macro expansion occurs on the arguments of pragmas registered with c_register_pragma_with_expansion but not on the arguments of pragmas registered with c_register_pragma.

Note that the use of pragma_lex is specific to the C and C++ compilers. It will not work in the Java or Fortran compilers, or any other language compilers for that matter. Thus if pragma_lex is going to be called from target-specific code, it must only be done so when building the C and C++ compilers. This can be done by defining the variables c_target_objs and cxx_target_objs in the target entry in the config.gcc file. These variables should name the target-specific, language-specific object file which contains the code that uses pragma_lex. Note it will also be necessary to add a rule to the makefile fragment pointed to by tmake_file that shows how to build this object file.

— Macro: HANDLE_PRAGMA_PACK_WITH_EXPANSION

Define this macro if macros should be expanded in the arguments of ‘#pragma pack’.

— Macro: TARGET_DEFAULT_PACK_STRUCT

If your target requires a structure packing default other than 0 (meaning the machine default), define this macro to the necessary value (in bytes). This must be a value that would also be valid to use with ‘#pragma pack()’ (that is, a small power of two).

— Macro: DOLLARS_IN_IDENTIFIERS

Define this macro to control use of the character ‘$’ in identifier names for the C family of languages. 0 means ‘$’ is not allowed by default; 1 means it is allowed. 1 is the default; there is no need to define this macro in that case.

— Macro: INSN_SETS_ARE_DELAYED (insn)

Define this macro as a C expression that is nonzero if it is safe for the delay slot scheduler to place instructions in the delay slot of insn, even if they appear to use a resource set or clobbered in insn. insn is always a jump_insn or an insn; GCC knows that every call_insn has this behavior. On machines where some insn or jump_insn is really a function call and hence has this behavior, you should define this macro.

You need not define this macro if it would always return zero.

— Macro: INSN_REFERENCES_ARE_DELAYED (insn)

Define this macro as a C expression that is nonzero if it is safe for the delay slot scheduler to place instructions in the delay slot of insn, even if they appear to set or clobber a resource referenced in insn. insn is always a jump_insn or an insn. On machines where some insn or jump_insn is really a function call and its operands are registers whose use is actually in the subroutine it calls, you should define this macro. Doing so allows the delay slot scheduler to move instructions which copy arguments into the argument registers into the delay slot of insn.

You need not define this macro if it would always return zero.

— Macro: MULTIPLE_SYMBOL_SPACES

Define this macro as a C expression that is nonzero if, in some cases, global symbols from one translation unit may not be bound to undefined symbols in another translation unit without user intervention. For instance, under Microsoft Windows symbols must be explicitly imported from shared libraries (DLLs).

You need not define this macro if it would always evaluate to zero.

— Target Hook: tree TARGET_MD_ASM_CLOBBERS (tree outputs, tree inputs, tree clobbers)

This target hook should add to clobbers STRING_CST trees for any hard regs the port wishes to automatically clobber for an asm. It should return the result of the last tree_cons used to add a clobber. The outputs, inputs and clobber lists are the corresponding parameters to the asm and may be inspected to avoid clobbering a register that is an input or output of the asm. You can use tree_overlaps_hard_reg_set, declared in tree.h, to test for overlap with regards to asm-declared registers.

— Macro: MATH_LIBRARY

Define this macro as a C string constant for the linker argument to link in the system math library, minus the initial ‘"-l"’, or ‘""’ if the target does not have a separate math library.

You need only define this macro if the default of ‘"m"’ is wrong.

— Macro: LIBRARY_PATH_ENV

Define this macro as a C string constant for the environment variable that specifies where the linker should look for libraries.

You need only define this macro if the default of ‘"LIBRARY_PATH"’ is wrong.

— Macro: TARGET_POSIX_IO

Define this macro if the target supports the following POSIX file functions, access, mkdir and file locking with fcntl / F_SETLKW. Defining TARGET_POSIX_IO will enable the test coverage code to use file locking when exiting a program, which avoids race conditions if the program has forked. It will also create directories at run-time for cross-profiling.

— Macro: MAX_CONDITIONAL_EXECUTE

A C expression for the maximum number of instructions to execute via conditional execution instructions instead of a branch. A value of BRANCH_COST+1 is the default if the machine does not use cc0, and 1 if it does use cc0.

— Macro: IFCVT_MODIFY_TESTS (ce_info, true_expr, false_expr)

Used if the target needs to perform machine-dependent modifications on the conditionals used for turning basic blocks into conditionally executed code. ce_info points to a data structure, struct ce_if_block, which contains information about the currently processed blocks. true_expr and false_expr are the tests that are used for converting the then-block and the else-block, respectively. Set either true_expr or false_expr to a null pointer if the tests cannot be converted.

— Macro: IFCVT_MODIFY_MULTIPLE_TESTS (ce_info, bb, true_expr, false_expr)

Like IFCVT_MODIFY_TESTS, but used when converting more complicated if-statements into conditions combined by and and or operations. bb contains the basic block that contains the test that is currently being processed and about to be turned into a condition.

— Macro: IFCVT_MODIFY_INSN (ce_info, pattern, insn)

A C expression to modify the PATTERN of an INSN that is to be converted to conditional execution format. ce_info points to a data structure, struct ce_if_block, which contains information about the currently processed blocks.

— Macro: IFCVT_MODIFY_FINAL (ce_info)

A C expression to perform any final machine dependent modifications in converting code to conditional execution. The involved basic blocks can be found in the struct ce_if_block structure that is pointed to by ce_info.

— Macro: IFCVT_MODIFY_CANCEL (ce_info)

A C expression to cancel any machine dependent modifications in converting code to conditional execution. The involved basic blocks can be found in the struct ce_if_block structure that is pointed to by ce_info.

— Macro: IFCVT_MACHDEP_INIT (ce_info)

A C expression to initialize any machine specific data for if-conversion of the if-block in the struct ce_if_block structure that is pointed to by ce_info.

— Target Hook: void TARGET_MACHINE_DEPENDENT_REORG (void)

If non-null, this hook performs a target-specific pass over the instruction stream. The compiler will run it at all optimization levels, just before the point at which it normally does delayed-branch scheduling.

The exact purpose of the hook varies from target to target. Some use it to do transformations that are necessary for correctness, such as laying out in-function constant pools or avoiding hardware hazards. Others use it as an opportunity to do some machine-dependent optimizations.

You need not implement the hook if it has nothing to do. The default definition is null.

— Target Hook: void TARGET_INIT_BUILTINS (void)

Define this hook if you have any machine-specific built-in functions that need to be defined. It should be a function that performs the necessary setup.

Machine specific built-in functions can be useful to expand special machine instructions that would otherwise not normally be generated because they have no equivalent in the source language (for example, SIMD vector instructions or prefetch instructions).

To create a built-in function, call the function lang_hooks.builtin_function which is defined by the language front end. You can use any type nodes set up by build_common_tree_nodes; only language front ends that use those two functions will call ‘TARGET_INIT_BUILTINS’.

— Target Hook: tree TARGET_BUILTIN_DECL (unsigned code, bool initialize_p)

Define this hook if you have any machine-specific built-in functions that need to be defined. It should be a function that returns the builtin function declaration for the builtin function code code. If there is no such builtin and it cannot be initialized at this time if initialize_p is true the function should return NULL_TREE. If code is out of range the function should return error_mark_node.

— Target Hook: rtx TARGET_EXPAND_BUILTIN (tree exp, rtx target, rtx subtarget, enum machine_mode mode, int ignore)

Expand a call to a machine specific built-in function that was set up by ‘TARGET_INIT_BUILTINS’. exp is the expression for the function call; the result should go to target if that is convenient, and have mode mode if that is convenient. subtarget may be used as the target for computing one of exp's operands. ignore is nonzero if the value is to be ignored. This function should return the result of the call to the built-in function.

— Target Hook: tree TARGET_RESOLVE_OVERLOADED_BUILTIN (unsigned int loc, tree fndecl, void *arglist)

Select a replacement for a machine specific built-in function that was set up by ‘TARGET_INIT_BUILTINS’. This is done before regular type checking, and so allows the target to implement a crude form of function overloading. fndecl is the declaration of the built-in function. arglist is the list of arguments passed to the built-in function. The result is a complete expression that implements the operation, usually another CALL_EXPR. arglist really has type ‘VEC(tree,gc)*

— Target Hook: tree TARGET_FOLD_BUILTIN (tree fndecl, int n_args, tree *argp, bool ignore)

Fold a call to a machine specific built-in function that was set up by ‘TARGET_INIT_BUILTINS’. fndecl is the declaration of the built-in function. n_args is the number of arguments passed to the function; the arguments themselves are pointed to by argp. The result is another tree, valid for both GIMPLE and GENERIC, containing a simplified expression for the call's result. If ignore is true the value will be ignored.

— Target Hook: bool TARGET_GIMPLE_FOLD_BUILTIN (gimple_stmt_iterator *gsi)

Fold a call to a machine specific built-in function that was set up by ‘TARGET_INIT_BUILTINS’. gsi points to the gimple statement holding the function call. Returns true if any change was made to the GIMPLE stream.

— Target Hook: int TARGET_COMPARE_VERSION_PRIORITY (tree decl1, tree decl2)

This hook is used to compare the target attributes in two functions to determine which function's features get higher priority. This is used during function multi-versioning to figure out the order in which two versions must be dispatched. A function version with a higher priority is checked for dispatching earlier. decl1 and decl2 are the two function decls that will be compared.

— Target Hook: tree TARGET_GET_FUNCTION_VERSIONS_DISPATCHER (void *decl)

This hook is used to get the dispatcher function for a set of function versions. The dispatcher function is called to invoke the right function version at run-time. decl is one version from a set of semantically identical versions.

— Target Hook: tree TARGET_GENERATE_VERSION_DISPATCHER_BODY (void *arg)

This hook is used to generate the dispatcher logic to invoke the right function version at run-time for a given set of function versions. arg points to the callgraph node of the dispatcher function whose body must be generated.

— Target Hook: bool TARGET_CAN_USE_DOLOOP_P (const widest_int &iterations, const widest_int &iterations_max, unsigned int loop_depth, bool entered_at_top)

Return true if it is possible to use low-overhead loops (doloop_end and doloop_begin) for a particular loop. iterations gives the exact number of iterations, or 0 if not known. iterations_max gives the maximum number of iterations, or 0 if not known. loop_depth is the nesting depth of the loop, with 1 for innermost loops, 2 for loops that contain innermost loops, and so on. entered_at_top is true if the loop is only entered from the top.

This hook is only used if doloop_end is available. The default implementation returns true. You can use can_use_doloop_if_innermost if the loop must be the innermost, and if there are no other restrictions.

— Target Hook: const char * TARGET_INVALID_WITHIN_DOLOOP (const rtx_insn *insn)

Take an instruction in insn and return NULL if it is valid within a low-overhead loop, otherwise return a string explaining why doloop could not be applied.

Many targets use special registers for low-overhead looping. For any instruction that clobbers these this function should return a string indicating the reason why the doloop could not be applied. By default, the RTL loop optimizer does not use a present doloop pattern for loops containing function calls or branch on table instructions.

— Target Hook: bool TARGET_LEGITIMATE_COMBINED_INSN (rtx_insn *insn)

Take an instruction in insn and return false if the instruction is not appropriate as a combination of two or more instructions. The default is to accept all instructions.

— Macro: MD_CAN_REDIRECT_BRANCH (branch1, branch2)

Take a branch insn in branch1 and another in branch2. Return true if redirecting branch1 to the destination of branch2 is possible.

On some targets, branches may have a limited range. Optimizing the filling of delay slots can result in branches being redirected, and this may in turn cause a branch offset to overflow.

— Target Hook: bool TARGET_CAN_FOLLOW_JUMP (const_rtx follower, const_rtx followee)

FOLLOWER and FOLLOWEE are JUMP_INSN instructions; return true if FOLLOWER may be modified to follow FOLLOWEE; false, if it can't. For example, on some targets, certain kinds of branches can't be made to follow through a hot/cold partitioning.

— Target Hook: bool TARGET_COMMUTATIVE_P (const_rtx x, int outer_code)

This target hook returns true if x is considered to be commutative. Usually, this is just COMMUTATIVE_P (x), but the HP PA doesn't consider PLUS to be commutative inside a MEM. outer_code is the rtx code of the enclosing rtl, if known, otherwise it is UNKNOWN.

— Target Hook: rtx TARGET_ALLOCATE_INITIAL_VALUE (rtx hard_reg)

When the initial value of a hard register has been copied in a pseudo register, it is often not necessary to actually allocate another register to this pseudo register, because the original hard register or a stack slot it has been saved into can be used. TARGET_ALLOCATE_INITIAL_VALUE is called at the start of register allocation once for each hard register that had its initial value copied by using get_func_hard_reg_initial_val or get_hard_reg_initial_val. Possible values are NULL_RTX, if you don't want to do any special allocation, a REG rtx—that would typically be the hard register itself, if it is known not to be clobbered—or a MEM. If you are returning a MEM, this is only a hint for the allocator; it might decide to use another register anyways. You may use current_function_is_leaf or REG_N_SETS in the hook to determine if the hard register in question will not be clobbered. The default value of this hook is NULL, which disables any special allocation.

— Target Hook: int TARGET_UNSPEC_MAY_TRAP_P (const_rtx x, unsigned flags)

This target hook returns nonzero if x, an unspec or unspec_volatile operation, might cause a trap. Targets can use this hook to enhance precision of analysis for unspec and unspec_volatile operations. You may call may_trap_p_1 to analyze inner elements of x in which case flags should be passed along.

— Target Hook: void TARGET_SET_CURRENT_FUNCTION (tree decl)

The compiler invokes this hook whenever it changes its current function context (cfun). You can define this function if the back end needs to perform any initialization or reset actions on a per-function basis. For example, it may be used to implement function attributes that affect register usage or code generation patterns. The argument decl is the declaration for the new function context, and may be null to indicate that the compiler has left a function context and is returning to processing at the top level. The default hook function does nothing.

GCC sets cfun to a dummy function context during initialization of some parts of the back end. The hook function is not invoked in this situation; you need not worry about the hook being invoked recursively, or when the back end is in a partially-initialized state. cfun might be NULL to indicate processing at top level, outside of any function scope.

— Macro: TARGET_OBJECT_SUFFIX

Define this macro to be a C string representing the suffix for object files on your target machine. If you do not define this macro, GCC will use ‘.o’ as the suffix for object files.

— Macro: TARGET_EXECUTABLE_SUFFIX

Define this macro to be a C string representing the suffix to be automatically added to executable files on your target machine. If you do not define this macro, GCC will use the null string as the suffix for executable files.

— Macro: COLLECT_EXPORT_LIST

If defined, collect2 will scan the individual object files specified on its command line and create an export list for the linker. Define this macro for systems like AIX, where the linker discards object files that are not referenced from main and uses export lists.

— Macro: MODIFY_JNI_METHOD_CALL (mdecl)

Define this macro to a C expression representing a variant of the method call mdecl, if Java Native Interface (JNI) methods must be invoked differently from other methods on your target. For example, on 32-bit Microsoft Windows, JNI methods must be invoked using the stdcall calling convention and this macro is then defined as this expression:

          build_type_attribute_variant (mdecl,
                                        build_tree_list
                                        (get_identifier ("stdcall"),
                                         NULL))
— Target Hook: bool TARGET_CANNOT_MODIFY_JUMPS_P (void)

This target hook returns true past the point in which new jump instructions could be created. On machines that require a register for every jump such as the SHmedia ISA of SH5, this point would typically be reload, so this target hook should be defined to a function such as:

          static bool
          cannot_modify_jumps_past_reload_p ()
          {
            return (reload_completed || reload_in_progress);
          }
— Target Hook: reg_class_t TARGET_BRANCH_TARGET_REGISTER_CLASS (void)

This target hook returns a register class for which branch target register optimizations should be applied. All registers in this class should be usable interchangeably. After reload, registers in this class will be re-allocated and loads will be hoisted out of loops and be subjected to inter-block scheduling.

— Target Hook: bool TARGET_BRANCH_TARGET_REGISTER_CALLEE_SAVED (bool after_prologue_epilogue_gen)

Branch target register optimization will by default exclude callee-saved registers that are not already live during the current function; if this target hook returns true, they will be included. The target code must than make sure that all target registers in the class returned by ‘TARGET_BRANCH_TARGET_REGISTER_CLASS’ that might need saving are saved. after_prologue_epilogue_gen indicates if prologues and epilogues have already been generated. Note, even if you only return true when after_prologue_epilogue_gen is false, you still are likely to have to make special provisions in INITIAL_ELIMINATION_OFFSET to reserve space for caller-saved target registers.

— Target Hook: bool TARGET_HAVE_CONDITIONAL_EXECUTION (void)

This target hook returns true if the target supports conditional execution. This target hook is required only when the target has several different modes and they have different conditional execution capability, such as ARM.

— Target Hook: unsigned TARGET_LOOP_UNROLL_ADJUST (unsigned nunroll, struct loop *loop)

This target hook returns a new value for the number of times loop should be unrolled. The parameter nunroll is the number of times the loop is to be unrolled. The parameter loop is a pointer to the loop, which is going to be checked for unrolling. This target hook is required only when the target has special constraints like maximum number of memory accesses.

— Macro: POWI_MAX_MULTS

If defined, this macro is interpreted as a signed integer C expression that specifies the maximum number of floating point multiplications that should be emitted when expanding exponentiation by an integer constant inline. When this value is defined, exponentiation requiring more than this number of multiplications is implemented by calling the system library's pow, powf or powl routines. The default value places no upper bound on the multiplication count.

— Macro: void TARGET_EXTRA_INCLUDES (const char *sysroot, const char *iprefix, int stdinc)

This target hook should register any extra include files for the target. The parameter stdinc indicates if normal include files are present. The parameter sysroot is the system root directory. The parameter iprefix is the prefix for the gcc directory.

— Macro: void TARGET_EXTRA_PRE_INCLUDES (const char *sysroot, const char *iprefix, int stdinc)

This target hook should register any extra include files for the target before any standard headers. The parameter stdinc indicates if normal include files are present. The parameter sysroot is the system root directory. The parameter iprefix is the prefix for the gcc directory.

— Macro: void TARGET_OPTF (char *path)

This target hook should register special include paths for the target. The parameter path is the include to register. On Darwin systems, this is used for Framework includes, which have semantics that are different from -I.

— Macro: bool TARGET_USE_LOCAL_THUNK_ALIAS_P (tree fndecl)

This target macro returns true if it is safe to use a local alias for a virtual function fndecl when constructing thunks, false otherwise. By default, the macro returns true for all functions, if a target supports aliases (i.e. defines ASM_OUTPUT_DEF), false otherwise,

— Macro: TARGET_FORMAT_TYPES

If defined, this macro is the name of a global variable containing target-specific format checking information for the -Wformat option. The default is to have no target-specific format checks.

— Macro: TARGET_N_FORMAT_TYPES

If defined, this macro is the number of entries in TARGET_FORMAT_TYPES.

— Macro: TARGET_OVERRIDES_FORMAT_ATTRIBUTES

If defined, this macro is the name of a global variable containing target-specific format overrides for the -Wformat option. The default is to have no target-specific format overrides. If defined, TARGET_FORMAT_TYPES must be defined, too.

— Macro: TARGET_OVERRIDES_FORMAT_ATTRIBUTES_COUNT

If defined, this macro specifies the number of entries in TARGET_OVERRIDES_FORMAT_ATTRIBUTES.

— Macro: TARGET_OVERRIDES_FORMAT_INIT

If defined, this macro specifies the optional initialization routine for target specific customizations of the system printf and scanf formatter settings.

— Target Hook: bool TARGET_RELAXED_ORDERING

If set to true, means that the target's memory model does not guarantee that loads which do not depend on one another will access main memory in the order of the instruction stream; if ordering is important, an explicit memory barrier must be used. This is true of many recent processors which implement a policy of “relaxed,” “weak,” or “release” memory consistency, such as Alpha, PowerPC, and ia64. The default is false.

— Target Hook: const char * TARGET_INVALID_ARG_FOR_UNPROTOTYPED_FN (const_tree typelist, const_tree funcdecl, const_tree val)

If defined, this macro returns the diagnostic message when it is illegal to pass argument val to function funcdecl with prototype typelist.

— Target Hook: const char * TARGET_INVALID_CONVERSION (const_tree fromtype, const_tree totype)

If defined, this macro returns the diagnostic message when it is invalid to convert from fromtype to totype, or NULL if validity should be determined by the front end.

— Target Hook: const char * TARGET_INVALID_UNARY_OP (int op, const_tree type)

If defined, this macro returns the diagnostic message when it is invalid to apply operation op (where unary plus is denoted by CONVERT_EXPR) to an operand of type type, or NULL if validity should be determined by the front end.

— Target Hook: const char * TARGET_INVALID_BINARY_OP (int op, const_tree type1, const_tree type2)

If defined, this macro returns the diagnostic message when it is invalid to apply operation op to operands of types type1 and type2, or NULL if validity should be determined by the front end.

— Target Hook: const char * TARGET_INVALID_PARAMETER_TYPE (const_tree type)

If defined, this macro returns the diagnostic message when it is invalid for functions to include parameters of type type, or NULL if validity should be determined by the front end. This is currently used only by the C and C++ front ends.

— Target Hook: const char * TARGET_INVALID_RETURN_TYPE (const_tree type)

If defined, this macro returns the diagnostic message when it is invalid for functions to have return type type, or NULL if validity should be determined by the front end. This is currently used only by the C and C++ front ends.

— Target Hook: tree TARGET_PROMOTED_TYPE (const_tree type)

If defined, this target hook returns the type to which values of type should be promoted when they appear in expressions, analogous to the integer promotions, or NULL_TREE to use the front end's normal promotion rules. This hook is useful when there are target-specific types with special promotion rules. This is currently used only by the C and C++ front ends.

— Target Hook: tree TARGET_CONVERT_TO_TYPE (tree type, tree expr)

If defined, this hook returns the result of converting expr to type. It should return the converted expression, or NULL_TREE to apply the front end's normal conversion rules. This hook is useful when there are target-specific types with special conversion rules. This is currently used only by the C and C++ front ends.

— Macro: TARGET_USE_JCR_SECTION

This macro determines whether to use the JCR section to register Java classes. By default, TARGET_USE_JCR_SECTION is defined to 1 if both SUPPORTS_WEAK and TARGET_HAVE_NAMED_SECTIONS are true, else 0.

— Macro: OBJC_JBLEN

This macro determines the size of the objective C jump buffer for the NeXT runtime. By default, OBJC_JBLEN is defined to an innocuous value.

— Macro: LIBGCC2_UNWIND_ATTRIBUTE

Define this macro if any target-specific attributes need to be attached to the functions in libgcc that provide low-level support for call stack unwinding. It is used in declarations in unwind-generic.h and the associated definitions of those functions.

— Target Hook: void TARGET_UPDATE_STACK_BOUNDARY (void)

Define this macro to update the current function stack boundary if necessary.

— Target Hook: rtx TARGET_GET_DRAP_RTX (void)

This hook should return an rtx for Dynamic Realign Argument Pointer (DRAP) if a different argument pointer register is needed to access the function's argument list due to stack realignment. Return NULL if no DRAP is needed.

— Target Hook: bool TARGET_ALLOCATE_STACK_SLOTS_FOR_ARGS (void)

When optimization is disabled, this hook indicates whether or not arguments should be allocated to stack slots. Normally, GCC allocates stacks slots for arguments when not optimizing in order to make debugging easier. However, when a function is declared with __attribute__((naked)), there is no stack frame, and the compiler cannot safely move arguments from the registers in which they are passed to the stack. Therefore, this hook should return true in general, but false for naked functions. The default implementation always returns true.

— Target Hook: unsigned HOST_WIDE_INT TARGET_CONST_ANCHOR

On some architectures it can take multiple instructions to synthesize a constant. If there is another constant already in a register that is close enough in value then it is preferable that the new constant is computed from this register using immediate addition or subtraction. We accomplish this through CSE. Besides the value of the constant we also add a lower and an upper constant anchor to the available expressions. These are then queried when encountering new constants. The anchors are computed by rounding the constant up and down to a multiple of the value of TARGET_CONST_ANCHOR. TARGET_CONST_ANCHOR should be the maximum positive value accepted by immediate-add plus one. We currently assume that the value of TARGET_CONST_ANCHOR is a power of 2. For example, on MIPS, where add-immediate takes a 16-bit signed value, TARGET_CONST_ANCHOR is set to ‘0x8000’. The default value is zero, which disables this optimization.

— Target Hook: unsigned HOST_WIDE_INT TARGET_ASAN_SHADOW_OFFSET (void)

Return the offset bitwise ored into shifted address to get corresponding Address Sanitizer shadow memory address. NULL if Address Sanitizer is not supported by the target.

— Target Hook: unsigned HOST_WIDE_INT TARGET_MEMMODEL_CHECK (unsigned HOST_WIDE_INT val)

Validate target specific memory model mask bits. When NULL no target specific memory model bits are allowed.

— Target Hook: unsigned char TARGET_ATOMIC_TEST_AND_SET_TRUEVAL

This value should be set if the result written by atomic_test_and_set is not exactly 1, i.e. the bool true.

— Target Hook: bool TARGET_HAS_IFUNC_P (void)

It returns true if the target supports GNU indirect functions. The support includes the assembler, linker and dynamic linker. The default value of this hook is based on target's libc.

— Target Hook: unsigned int TARGET_ATOMIC_ALIGN_FOR_MODE (enum machine_mode mode)

If defined, this function returns an appropriate alignment in bits for an atomic object of machine_mode mode. If 0 is returned then the default alignment for the specified mode is used.

— Target Hook: void TARGET_ATOMIC_ASSIGN_EXPAND_FENV (tree *hold, tree *clear, tree *update)

ISO C11 requires atomic compound assignments that may raise floating-point exceptions to raise exceptions corresponding to the arithmetic operation whose result was successfully stored in a compare-and-exchange sequence. This requires code equivalent to calls to feholdexcept, feclearexcept and feupdateenv to be generated at appropriate points in the compare-and-exchange sequence. This hook should set *hold to an expression equivalent to the call to feholdexcept, *clear to an expression equivalent to the call to feclearexcept and *update to an expression equivalent to the call to feupdateenv. The three expressions are NULL_TREE on entry to the hook and may be left as NULL_TREE if no code is required in a particular place. The default implementation leaves all three expressions as NULL_TREE. The __atomic_feraiseexcept function from libatomic may be of use as part of the code generated in *update.

— Macro: TARGET_SUPPORTS_WIDE_INT

On older ports, large integers are stored in CONST_DOUBLE rtl objects. Newer ports define TARGET_SUPPORTS_WIDE_INT to be nonzero to indicate that large integers are stored in CONST_WIDE_INT rtl objects. The CONST_WIDE_INT allows very large integer constants to be represented. CONST_DOUBLE is limited to twice the size of the host's HOST_WIDE_INT representation.

Converting a port mostly requires looking for the places where CONST_DOUBLEs are used with VOIDmode and replacing that code with code that accesses CONST_WIDE_INTs. ‘"grep -i const_double"’ at the port level gets you to 95% of the changes that need to be made. There are a few places that require a deeper look.

All and all it does not take long to convert ports that the maintainer is familiar with.