sivachandra added inline comments.
================
Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:22193
+ DAG.getConstant(0x45, DL, MVT::i8));
+
+ return DAG.getSetCC(DL, ResultVT, Extract, DAG.getConstant(1, DL, MVT::i8),
----------------
While I do not understand the code mechanics of this patch, I am mostly in
agreement with the general direction of this patch. However, it has lead to a
change in behavior wrt 80-bit x86 floating point numbers. Unlike the 32-bit and
64-bit floating point numbers, 80-bit numbers have an additional class of
"Unsupported Numbers". Those numbers were previously treated as NaNs. Since
this change uses the `fxam` instruction to classify the input number, that is
not the case any more as the `fxam` instruction distinguishes between
unsupported numbers and NaNs. So, to restore the previous behavior, can we
extend this patch to treat unsupported numbers as NaNs? At a high level, what I
am effectively saying is that we should implement `isnan` this way:
```
bool isnan(long double x) {
uint16_t status;
__asm__ __volatile__("fldt %0" : : "m"(x));
__asm__ __volatile__("fxam");
__asm__ __volatile__("fnstsw %0": "=m"(status):);
uint16_t c0c2c3 = (status >> 8) & 0x45;
return c0c2c3 <= 1; // This patch seems to be only doing c0c2c3 == 1 check.
}
```
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D104854/new/
https://reviews.llvm.org/D104854
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