[patch] Get rid of stack trampolines for nested functions
Richard Biener
richard.guenther@gmail.com
Thu Jun 30 08:26:00 GMT 2016
On Thu, Jun 30, 2016 at 12:08 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
> Hi,
>
> this patch implements generic support for the elimination of stack trampolines
> and, consequently, of the need to make the stack executable when pointers to
> nested functions are used. That's done on a per-language and per-target basis
> (i.e. there is 1 language hook and 1 target hook to parameterize it) and there
> are no changes whatsoever in code generation if both are not turned on (and
> the patch implements a -ftrampolines option to let the user override them).
>
> The idea is based on the fact that, for targets using function descriptors as
> per their ABI like IA-64, AIX or VMS platforms, stack trampolines "degenerate"
> into descriptors built at run time on the stack and thus made up of data only,
> which in turn means that the stack doesn't need to be made executable.
As you are not able to handle all trampolines that way I always wondered if we
can allocate space for the trampolines statically for those cases we can compute
do not happen in a recursive path (or avoid them if we do not need a
static chain
anyway, not sure if we optimize for this case currently).
Richard.
> This descriptor-based scheme is implemented generically for nested functions,
> i.e. the nested function lowering pass builds generic descriptors instead of
> trampolines on the stack when encountering pointers to nested functions, which
> means that there are 2 kinds of pointers to functions and therefore a run-time
> identification mechanism is needed for indirect calls to distinguish them.
>
> Because of that, enabling the support breaks binary compatibility (for code
> manipulating pointers to nested functions). That's OK for Ada and nested
> functions are first-class citizens in the language anyway so we really need
> this, but not for C so for example Ada doesn't use it at the interface with C
> (when objects have "convention C" in Ada parlance).
>
> This was bootstrapped/regtested on x86_64-suse-linux but AdaCore has been
> using it on native platforms (Linux, Windows, Solaris, etc) for years.
>
> OK for the mainline?
>
>
> 2016-06-29 Eric Botcazou <ebotcazou@adacore.com>
>
> PR ada/37139
> PR ada/67205
> * common.opt (-ftrampolines): New option.
> * doc/invoke.texi (Code Gen Options): Document it.
> * doc/tm.texi.in (Trampolines): Add TARGET_CUSTOM_FUNCTION_DESCRIPTORS
> * doc/tm.texi: Regenerate.
> * builtins.def: Add init_descriptor and adjust_descriptor.
> * builtins.c (expand_builtin_init_trampoline): Do not issue a warning
> on platforms with descriptors.
> (expand_builtin_init_descriptor): New function.
> (expand_builtin_adjust_descriptor): Likewise.
> (expand_builtin) <BUILT_IN_INIT_DESCRIPTOR>: New case.
> <BUILT_IN_ADJUST_DESCRIPTOR>: Likewise.
> * calls.c (prepare_call_address): Remove SIBCALLP parameter and add
> FLAGS parameter. Deal with indirect calls by descriptor and adjust.
> Set STATIC_CHAIN_REG_P on the static chain register, if any.
> (call_expr_flags): Set ECF_BY_DESCRIPTOR for calls by descriptor.
> (expand_call): Likewise. Move around call to prepare_call_address
> and pass all flags to it.
> * cfgexpand.c (expand_call_stmt): Reinstate CALL_EXPR_BY_DESCRIPTOR.
> * gimple.h (enum gf_mask): New GF_CALL_BY_DESCRIPTOR value.
> (gimple_call_set_by_descriptor): New setter.
> (gimple_call_by_descriptor_p): New getter.
> * gimple.c (gimple_build_call_from_tree): Set CALL_EXPR_BY_DESCRIPTOR.
> (gimple_call_flags): Deal with GF_CALL_BY_DESCRIPTOR.
> * langhooks.h (struct lang_hooks): Add custom_function_descriptors.
> * langhooks-def.h (LANG_HOOKS_CUSTOM_FUNCTION_DESCRIPTORS): Define.
> (LANG_HOOKS_INITIALIZER): Add LANG_HOOKS_CUSTOM_FUNCTION_DESCRIPTORS.
> * rtl.h (STATIC_CHAIN_REG_P): New macro.
> * rtlanal.c (find_first_parameter_load): Skip static chain registers.
> * target.def (custom_function_descriptors): New POD hook.
> * tree.h (FUNC_ADDR_BY_DESCRIPTOR): New flag on ADDR_EXPR.
> (CALL_EXPR_BY_DESCRIPTOR): New flag on CALL_EXPR.
> * tree-core.h (ECF_BY_DESCRIPTOR): New mask.
> Document FUNC_ADDR_BY_DESCRIPTOR and CALL_EXPR_BY_DESCRIPTOR.
> * tree.c (make_node_stat) <tcc_declaration>: Set function alignment to
> DEFAULT_FUNCTION_ALIGNMENT instead of FUNCTION_BOUNDARY.
> (build_common_builtin_nodes): Initialize init_descriptor and
> adjust_descriptor.
> * tree-nested.c: Include target.h.
> (struct nesting_info): Add 'any_descr_created' field.
> (get_descriptor_type): New function.
> (lookup_element_for_decl): New function extracted from...
> (create_field_for_decl): Likewise.
> (lookup_tramp_for_decl): ...here. Adjust.
> (lookup_descr_for_decl): New function.
> (convert_tramp_reference_op): Deal with descriptors.
> (build_init_call_stmt): New function extracted from...
> (finalize_nesting_tree_1): ...here. Adjust and deal with descriptors.
> * defaults.h (DEFAULT_FUNCTION_ALIGNMENT): Define.
> (TRAMPOLINE_ALIGNMENT): Set to above instead of FUNCTION_BOUNDARY.
> * config/aarch64/aarch64.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS):Define
> * config/alpha/alpha.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): Likewise.
> * config/arm/arm.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): Likewise.
> * config/arm/arm.c (arm_function_ok_for_sibcall): Return false for an
> indirect call by descriptor if all the argument registers are used.
> * config/i386/i386.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): Define.
> * config/ia64/ia64.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): Likewise.
> * config/mips/mips.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): Likewise.
> * config/pa/pa.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): Likewise.
> * config/rs6000/rs6000.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS):Likewise
> * config/sparc/sparc.h (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): Likewise.
> ada/
> * gcc-interface/misc.c (LANG_HOOKS_CUSTOM_FUNCTION_DESCRIPTORS):Define
> * gcc-interface/trans.c (Attribute_to_gnu) <Attr_Access>: Deal with
> a zero TARGET_CUSTOM_FUNCTION_DESCRIPTORSspecially for 'Code_Address.
> Otherwise, if TARGET_CUSTOM_FUNCTION_DESCRIPTORS is positive, set
> FUNC_ADDR_BY_DESCRIPTOR for 'Access/'Unrestricted_Access of nested
> subprograms if the type can use an internal representation.
> (call_to_gnu): Likewise, but set CALL_EXPR_BY_DESCRIPTOR on indirect
> calls if the type can use an internal representation.
>
>
> 2016-06-29 Eric Botcazou <ebotcazou@adacore.com>
>
> * gnat.dg/trampoline3.adb: New test.
> * gnat.dg/trampoline4.adb: Likewise.
>
> --
> Eric Botcazou
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