[PATCH] D72466: [FPEnv][RFC] Invert sense of MIFlag::FPExcept flag

[Pengfei Wang via Phabricator via llvm-commits]

pengfei added inline comments.


================
Comment at: llvm/test/CodeGen/X86/fp-intrinsics-flags.ll:9
 ; CHECK: MOVSDmr %stack.0, 1, $noreg, 0, $noreg, killed [[CVTSI2SDrr]] :: (store 8 into %stack.0, align 4)
-; CHECK: [[LD_Fp64m80_:%[0-9]+]]:rfp80 = LD_Fp64m80 %stack.0, 1, $noreg, 0, $noreg, implicit-def dead $fpsw, implicit $fpcw :: (load 8 from %stack.0, align 4)
+; CHECK: [[LD_Fp64m80_:%[0-9]+]]:rfp80 = nofpexcept LD_Fp64m80 %stack.0, 1, $noreg, 0, $noreg, implicit-def dead $fpsw, implicit $fpcw :: (load 8 from %stack.0, align 4)
 ; CHECK: RET 0, killed [[LD_Fp64m80_]]
----------------
craig.topper wrote:
> pengfei wrote:
> > pengfei wrote:
> > > craig.topper wrote:
> > > > I don't know how to fix this. This is a pattern match from a any extending load instruction. There's no flag to copy from. Unfortunately, X87 generates an exception if you load a NAN from a float or double memory location. But I don't know how to represent that in SelectionDAG or IR.
> > > SSE instructions don't generate exceptions when only loading from memory to register. Maybe other targets too. And current strict FP semantics don't define a strict load. Can we add masking the `#I` before FLD and FPCW recovery operations into the pattern?
> > Oh, we also add those instructions in non-strict scenarios. So it's not practicable.
> What happens to the SNAN or denormal value that was loaded if the exception is masked. Does the SNAN get quieted or does it stay an SNAN in 80-bit format? If masking quiets it then the SNAN would never signal. For SSE it would be signalled when the SNAN is operated on by an arithmetic instruction.
sNaN will be kept sNaN in 80-bit while qNaN to be qNaN, tested by below code:
```
void foo(unsigned a) {
  fedisableexcept(FE_INVALID);
  asm("fld1\nflds %0\nfwait" :: "m"(a));
  feenableexcept(FE_INVALID);
  asm("fucom\nfwait");
}
```

But it becomes more complicated for denormal. Because any denormal in float and double will become normal value in 80-bit format. So we should keep `#D` unchanged, which means we may still raise exception when loading memory in X87, and we can't keep the same behavior with SSE.


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  https://reviews.llvm.org/D72466/new/

https://reviews.llvm.org/D72466