> 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
