[PATCH] D142178: [X86] Change precision control to FP80 during u64->fp32 conversion on Windows.

Sat Feb 4 04:04:56 PST 2023

icedrocket added inline comments.

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Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:22045
+  // Windows needs the precision control changed to 80bits around this add.
+  if (Subtarget.isOSWindows() && DstVT == MVT::f32)
+    Opc = X86ISD::FP80_ADD;
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icedrocket wrote:
> Should we only apply this to the conversion to f32? Conversion to f64 might also have precision issues though I can't prove it.
LowerUINT_TO_FP_i64 also uses FP64 addition. If u64 to f64 conversion is problematic because FP80 addition is not used, then implementations using SSE2 will also be problematic. So unless we find a broken case, I think it's better to leave it alone.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D142178/new/

https://reviews.llvm.org/D142178