[ValueTracking] determine sign of 0.0 from select when matching min/max FP

In PR39475:
https://bugs.llvm.org/show_bug.cgi?id=39475
..we may fail to recognize/simplify fabs() in some cases because we do not 
canonicalize fcmp with a -0.0 operand.

Adding that canonicalization can cause regressions on min/max FP tests, so 
that's this patch: for the purpose of determining whether something is min/max, 
let the value returned by the select determine how we treat a 0.0 operand in the fcmp.

This patch doesn't actually change the -0.0 to +0.0. It just changes the analysis, so 
we don't fail to recognize equivalent min/max patterns that only differ in the 
signbit of 0.0.

Differential Revision: https://reviews.llvm.org/D54001

llvm-svn: 346097

1 files changed, 21 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 6e08272c64e..ed17441d1e4 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4760,6 +4760,27 @@ static SelectPatternResult matchSelectPattern(CmpInst::Predicate Pred,
                                               Value *TrueVal, Value *FalseVal,
                                               Value *&LHS, Value *&RHS,
                                               unsigned Depth) {
+  if (CmpInst::isFPPredicate(Pred)) {
+    // IEEE-754 ignores the sign of 0.0 in comparisons. So if the select has one
+    // 0.0 operand, set the compare's 0.0 operands to that same value for the
+    // purpose of identifying min/max. Disregard vector constants with undefined
+    // elements because those can not be back-propagated for analysis.
+    Value *OutputZeroVal = nullptr;
+    if (match(TrueVal, m_AnyZeroFP()) && !match(FalseVal, m_AnyZeroFP()) &&
+        !cast<Constant>(TrueVal)->containsUndefElement())
+      OutputZeroVal = TrueVal;
+    else if (match(FalseVal, m_AnyZeroFP()) && !match(TrueVal, m_AnyZeroFP()) &&
+             !cast<Constant>(FalseVal)->containsUndefElement())
+      OutputZeroVal = FalseVal;
+
+    if (OutputZeroVal) {
+      if (match(CmpLHS, m_AnyZeroFP()))
+        CmpLHS = OutputZeroVal;
+      if (match(CmpRHS, m_AnyZeroFP()))
+        CmpRHS = OutputZeroVal;
+    }
+  }
+
   LHS = CmpLHS;
   RHS = CmpRHS;