x86: Add -mfunction-return=

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.

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