[PATCH] D148631: ValueTracking: fdiv sign handling in computeKnownFPClass

[Jay Foad via Phabricator via llvm-commits]

foad added inline comments.


================
Comment at: llvm/include/llvm/Analysis/ValueTracking.h:357-358
+  void signBitMustBeZero() {
     KnownFPClasses = (KnownFPClasses & fcPositive) |
                      (KnownFPClasses & fcNan);
     SignBit = false;
----------------



================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:4564
 
-    if (KnowSomethingUseful && WantInfNan) {
+    if (KnowSomethingUseful && (WantInfNan || WantNegative)) {
       computeKnownFPClass(Op->getOperand(0), DemandedElts,
----------------
`WantInfNan || WantNegative` is true, else we would have bailed out


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:4582-4584
+    // X / -0.0 is -Inf (or NaN).
+    if (KnownLHS.cannotBeOrderedLessThanZero() && KnownRHS.signBitIsZeroOrNaN())
+      Known.knownNot(KnownFPClass::OrderedLessThanZeroMask);
----------------
This may be true but it seems weirdly specific, and the comment doesn't really say what it does.

I think it generalises to:
```
  if (KnownLHS.isKnownNot(fcNegative) && KnownRHS.isKnownNot(fcNegative))
    Known.knownNot(fcNegative);
```
... plus three other cases for the other combinations of LHS/RHS known not positive/negative.

This is basically saying that ignoring nans, the sign of the result is the xor of the signs of the arguments, which is also true for FMul.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D148631/new/

https://reviews.llvm.org/D148631