[PATCH 3/5] x86: Add -mfunction-return=

[Jeff Law]

On 01/07/2018 03:59 PM, H.J. Lu 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:
> 	lfence
> 	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=lfence 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.
> 
> gcc/testsuite/
> 
> 	* gcc.target/i386/indirect-thunk-1.c (dg-options): Add
> 	-mfunction-return=keep.
> 	* gcc.target/i386/indirect-thunk-2.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-3.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-4.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-5.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-6.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-7.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-attr-1.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-attr-2.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-attr-3.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-attr-4.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-attr-5.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-attr-6.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-attr-7.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-attr-8.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-bnd-1.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-bnd-2.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-bnd-3.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-bnd-4.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-extern-1.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-extern-2.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-extern-3.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-extern-4.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-extern-5.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-extern-6.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-extern-7.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-inline-1.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-inline-2.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-inline-3.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-inline-4.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-inline-5.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-inline-6.c: Likewise.
> 	* gcc.target/i386/indirect-thunk-inline-7.c: Likewise.
> 	* gcc.target/i386/ret-thunk-1.c: New test.
> 	* gcc.target/i386/ret-thunk-10.c: Likewise.
> 	* gcc.target/i386/ret-thunk-11.c: Likewise.
> 	* gcc.target/i386/ret-thunk-12.c: Likewise.
> 	* gcc.target/i386/ret-thunk-13.c: Likewise.
> 	* gcc.target/i386/ret-thunk-14.c: Likewise.
> 	* gcc.target/i386/ret-thunk-15.c: Likewise.
> 	* gcc.target/i386/ret-thunk-16.c: Likewise.
> 	* gcc.target/i386/ret-thunk-2.c: Likewise.
> 	* gcc.target/i386/ret-thunk-3.c: Likewise.
> 	* gcc.target/i386/ret-thunk-4.c: Likewise.
> 	* gcc.target/i386/ret-thunk-5.c: Likewise.
> 	* gcc.target/i386/ret-thunk-6.c: Likewise.
> 	* gcc.target/i386/ret-thunk-7.c: Likewise.
> 	* gcc.target/i386/ret-thunk-8.c: Likewise.
> 	* gcc.target/i386/ret-thunk-9.c: Likewise.
Same high level comments apply here.  ie, expecting Uros to chime in and
thus final approval rests with Uros.  Is there significant value in
having return thunks in the object file?

Is this something that is currently used or requested by the kernel teams?

> @@ -28539,6 +28610,43 @@ ix86_output_indirect_jmp (rtx call_op, bool ret_p)
>      return "%!jmp\t%A0";
>  }
>  
> +const char *
> +ix86_output_function_return (bool long_p)
Needs function comment.


> diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
> index f3e43da0aed..2b62363973d 100644
> --- a/gcc/config/i386/i386.opt
> +++ b/gcc/config/i386/i386.opt
> @@ -1026,9 +1026,13 @@ mindirect-branch=
>  Target Report RejectNegative Joined Enum(indirect_branch) Var(ix86_indirect_branch) Init(indirect_branch_keep)
>  Convert indirect call and jump.
>  
> +mfunction-return=
> +Target Report RejectNegative Joined Enum(indirect_branch) Var(ix86_function_return) Init(indirect_branch_keep)
> +Convert function return.
Again, reads poorly.