Re: [patch v2] Get rid of stack trampolines for nested functions (1/4)

[Ian Lance Taylor <iant@golang.org>]

On Mon, Dec 5, 2016 at 1:29 PM, Lynn A. Boger
<laboger@linux.vnet.ibm.com> wrote:
> I think you mean https://github.com/golang/go/issues/18200.

Yes, thanks, I meant to write https://golang.org/issue/18200.

Ian

> On 12/05/2016 02:52 PM, Ian Lance Taylor wrote:
>>
>> On Sun, Sep 4, 2016 at 1:10 PM, Eric Botcazou <ebotcazou@adacore.com>
>> wrote:
>>>
>>> 2016-07-04  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):
>>> AddTARGET_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):
>>> SetCALL_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>: Use
>>> FUNCTION_ALIGNMENT.
>>>          (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
>>> withdescriptors.
>>>          * defaults.h (FUNCTION_ALIGNMENT): Define.
>>>          (TRAMPOLINE_ALIGNMENT): Set to above instead of
>>> FUNCTION_BOUNDARY.
>>
>> According to https://golang.org/cl/18200, this change broke Go on PPC64le.
>>
>> I haven't investigated myself and I don't know why.  Go does not use
>> stack trampolines for function closures.  It does use function
>> closures, but they are built on the heap.  It also uses closures
>> mediated by libffi.  The Go frontend does not enable custom function
>> descriptors.
>>
>> It should be possible to recreate the problem by configuring with
>> --enable-languages=go and running `make
>> RUNTESTFLAGS="go-test.exp=recover.go" check-gcc-go`.
>>
>> Ian
>>
>>
>