[PATCH] D98895: [X86][Draft] Disable long double type for -mno-x87 option

[Pengfei Wang via Phabricator via cfe-commits]

pengfei added inline comments.


================
Comment at: clang/test/Sema/x86-no-x87.c:2
+// RUN: %clang_cc1 -fsyntax-only -verify %s -triple x86_64-linux-gnu -target-feature -x87
+// RUN: %clang_cc1 -fsyntax-only -verify %s -triple i686-linux-gnu -target-feature -x87
+// RUN: %clang_cc1 -fsyntax-only -verify %s -triple x86_64-windows-msvc -target-feature -x87 -DNOERROR
----------------
asavonic wrote:
> pengfei wrote:
> > Should i686 expect no error like GCC?
> GCC seems to fallback to soft-float for i686 if -mno-80387 is used:
> 
>   long double orig(long double x, long double y)
>   {
>     long double z = x + y;
>     if (z < 0.0)
>       return z;
>     else
>       return 0.0;
>   }
> 
> i686-linux-gnu-gcc-8 -c -S -mno-80387 -O3:
> 	  call	__addxf3 at PLT
> 	  [...]
> 	  call	__ltxf2 at PLT
> 	  addl	$32, %esp
> 	  testl	%eax, %eax
> 	  js	.L3
> 	  xorl	%esi, %esi
> 	  xorl	%edi, %edi
> 	  xorl	%ebp, %ebp
>   .L3:
> 	  addl	$12, %esp
> 	  movl	%esi, %eax
> 	  movl	%edi, %edx
> 	  movl	%ebp, %ecx
> 	  popl	%ebx
> 	  popl	%esi
> 	  popl	%edi
> 	  popl	%ebp
> 	  ret
> 
> This looks like a different ABI.
> X87 instructions are not used, so no error is reported.
> 
I found it's a bit complex for 32 bits.
1. i686 ABI specifies the return of floating point type must be put in %st0, so any FP type returning should be error out w/o x87.
2. GCC doesn't respect above ABI.
3. FP types are passed from stack, so a function like `void orig(long double x, long double y, long double *z)` should not be error out w/o x87. 
x86_64 only uses ST registers when returning FP80.
Considering it is rare for case 3, I think we can ignore it this time, but I suggest we should add check for float and double on 32 bits.


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  https://reviews.llvm.org/D98895/new/

https://reviews.llvm.org/D98895