On Sun, Jan 14, 2018 at 4:54 AM, H.J. Lu <hjl.to...@gmail.com> wrote: > On Sun, Jan 14, 2018 at 3:31 AM, H.J. Lu <hjl.to...@gmail.com> wrote: >> On Sun, Jan 14, 2018 at 2:48 AM, Jan Hubicka <hubi...@ucw.cz> wrote: >>>> Add -mfunction-return= option to convert function return to call and >>>> return thunks. The default is 'keep', which keeps function return >>>> unmodified. 'thunk' converts function return to call and return thunk. >>>> 'thunk-inline' converts function return to inlined call and return thunk. >>>> 'thunk-extern' converts function return to external call and return >>>> thunk provided in a separate object file. You can control this behavior >>>> for a specific function by using the function attribute function_return. >>>> >>>> Function return thunk is the same as memory thunk for -mindirect-branch= >>>> where the return address is at the top of the stack: >>>> >>>> __x86_return_thunk: >>>> call L2 >>>> L1: >>>> pause >>>> jmp L1 >>>> L2: >>>> lea 8(%rsp), %rsp|lea 4(%esp), %esp >>>> ret >>>> >>>> and function return becomes >>>> >>>> jmp __x86_return_thunk >>>> >>>> -mindirect-branch= tests are updated with -mfunction-return=keep to >>>> avoid false test failures when -mfunction-return=thunk is added to >>>> RUNTESTFLAGS for "make check". >>>> >>>> gcc/ >>>> >>>> * config/i386/i386-protos.h (ix86_output_function_return): New. >>>> * config/i386/i386.c (ix86_set_indirect_branch_type): Also >>>> set function_return_type. >>>> (indirect_thunk_name): Add ret_p to indicate thunk for function >>>> return. >>>> (output_indirect_thunk_function): Pass false to >>>> indirect_thunk_name. >>>> (ix86_output_indirect_branch): Likewise. >>>> (output_indirect_thunk_function): Create alias for function >>>> return thunk if regno < 0. >>>> (ix86_output_function_return): New function. >>>> (ix86_handle_fndecl_attribute): Handle function_return. >>>> (ix86_attribute_table): Add function_return. >>>> * config/i386/i386.h (machine_function): Add >>>> function_return_type. >>>> * config/i386/i386.md (simple_return_internal): Use >>>> ix86_output_function_return. >>>> (simple_return_internal_long): Likewise. >>>> * config/i386/i386.opt (mfunction-return=): New option. >>>> (indirect_branch): Mention -mfunction-return=. >>>> * doc/extend.texi: Document function_return function attribute. >>>> * doc/invoke.texi: Document -mfunction-return= option. >>> >>> The implementation is reasonable, but I still do not quite understand >>> if we really need this change. Celarly killing every return is going >>> to have significant impact and it is apparently not used by Linux kernel. >>> What is the main motivation for it? >>> >>> Honza >> >> See: >> >> https://gcc.gnu.org/ml/gcc-patches/2018-01/msg00540.html >> >> I will check it in. >> > > Here is the patch I am checking in. > >
Here is the backport for GCC 7. OK for gcc-7-branch? -- H.J.
