pengfei added inline comments.
================
Comment at: clang/lib/Basic/Targets/X86.cpp:385
- if (!HasX87) {
- if (LongDoubleFormat == &llvm::APFloat::x87DoubleExtended())
- HasLongDouble = false;
- if (getTriple().getArch() == llvm::Triple::x86)
- HasFPReturn = false;
- }
+ if (!HasX87 && getTriple().getArch() == llvm::Triple::x86_64 &&
+ LongDoubleFormat == &llvm::APFloat::x87DoubleExtended())
----------------
asavonic wrote:
> I see that D112143 changed the ABI so that FP return values do not use x87
> registers on i386. Therefore HasFPReturn flag can be removed.
>
> However, operations with long double (x87 80-bit) should still be unsupported
> on both targets, because IIRC there is no SSE equivalent for them. GCC
> compiles them as soft-fp when -mno-x87 is set, but I haven't found 80-bit
> soft-fp implementation in LLVM.
> ```
> long double baz(long double a, long double b) {
> return a + b;
> }
> ```
>
> ```
> baz:
> [...]
> call __addxf3
> ```
> For some reason GCC only does this for for i386 target, for x86_64 it just
> emits the diagnostic about disabled x87.
Thanks for looking at this patch.
I don't think we need to exclude f80 particularly. IIUC, backend tries all
possible ways to lower a given operation. Lowering to library is always the
last choice. So the behavior is not confined to soft-fp.
It's true LLVM has problems with f80 lowering without x87. I commented it in
D112143 and hope D100091 will fix them. We don't need to bother to change it
again in future.
> For some reason GCC only does this for for i386 target, for x86_64 it just
> emits the diagnostic about disabled x87.
I think the root reason is the difference in ABI. 32-bits ABI allows passing
and returning f80 without x87 registers while 64-bits doesn't. So we have to
and only need to disable it for x86_64.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D114162/new/
https://reviews.llvm.org/D114162
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