[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).


> 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.
>         * 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.
>         * tree.c (make_node_stat) <tcc_declaration>: Set function alignment to
>         (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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