[InstSimplify] fold select (fcmp X, Y), X, Y

This is NFCI for InstCombine because it calls InstSimplify, 
so I left the tests for this transform there. As noted in
the code comment, we can allow this fold more often by using
FMF and/or value tracking.

llvm-svn: 346169

1 files changed, 31 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index db929aa7059..fd6f4ba476e 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -3874,6 +3874,34 @@ static Value *simplifySelectWithICmpCond(Value *CondVal, Value *TrueVal,
   return nullptr;
 }
 
+/// Try to simplify a select instruction when its condition operand is a
+/// floating-point comparison.
+static Value *simplifySelectWithFCmp(Value *Cond, Value *T, Value *F) {
+  FCmpInst::Predicate Pred;
+  if (!match(Cond, m_FCmp(Pred, m_Specific(T), m_Specific(F))) &&
+      !match(Cond, m_FCmp(Pred, m_Specific(F), m_Specific(T))))
+    return nullptr;
+
+  // TODO: The transform may not be valid with -0.0. An incomplete way of
+  // testing for that possibility is to check if at least one operand is a
+  // non-zero constant.
+  const APFloat *C;
+  if ((match(T, m_APFloat(C)) && C->isNonZero()) ||
+      (match(F, m_APFloat(C)) && C->isNonZero())) {
+    // (T == F) ? T : F --> F
+    // (F == T) ? T : F --> F
+    if (Pred == FCmpInst::FCMP_OEQ)
+      return F;
+
+    // (T != F) ? T : F --> T
+    // (F != T) ? T : F --> T
+    if (Pred == FCmpInst::FCMP_UNE)
+      return T;
+  }
+
+  return nullptr;
+}
+
 /// Given operands for a SelectInst, see if we can fold the result.
 /// If not, this returns null.
 static Value *SimplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
@@ -3910,6 +3938,9 @@ static Value *SimplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
           simplifySelectWithICmpCond(Cond, TrueVal, FalseVal, Q, MaxRecurse))
     return V;
 
+  if (Value *V = simplifySelectWithFCmp(Cond, TrueVal, FalseVal))
+    return V;
+
   if (Value *V = foldSelectWithBinaryOp(Cond, TrueVal, FalseVal))
     return V;