[InstCombine] remove flop from lerp patterns

(Y * (1.0 - Z)) + (X * Z) -->
Y - (Y * Z) + (X * Z) -->
Y + Z * (X - Y)

This is part of solving:
https://bugs.llvm.org/show_bug.cgi?id=42716

Factoring eliminates an instruction, so that should be a good canonicalization.
The potential conversion to FMA would be handled by the backend based on target
capabilities.

Differential Revision: https://reviews.llvm.org/D65305

llvm-svn: 367101

1 files changed, 20 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index ba15b023f2a..891370e2c76 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1308,6 +1308,22 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
   return Changed ? &I : nullptr;
 }
 
+/// Eliminate an op from a linear interpolation (lerp) pattern.
+static Instruction *factorizeLerp(BinaryOperator &I,
+                                  InstCombiner::BuilderTy &Builder) {
+  Value *X, *Y, *Z;
+  if (!match(&I, m_c_FAdd(m_OneUse(m_c_FMul(m_Value(Y),
+                                            m_OneUse(m_FSub(m_FPOne(),
+                                                            m_Value(Z))))),
+                          m_OneUse(m_c_FMul(m_Value(X), m_Deferred(Z))))))
+    return nullptr;
+
+  // (Y * (1.0 - Z)) + (X * Z) --> Y + Z * (X - Y) [8 commuted variants]
+  Value *XY = Builder.CreateFSubFMF(X, Y, &I);
+  Value *MulZ = Builder.CreateFMulFMF(Z, XY, &I);
+  return BinaryOperator::CreateFAddFMF(Y, MulZ, &I);
+}
+
 /// Factor a common operand out of fadd/fsub of fmul/fdiv.
 static Instruction *factorizeFAddFSub(BinaryOperator &I,
                                       InstCombiner::BuilderTy &Builder) {
@@ -1315,6 +1331,10 @@ static Instruction *factorizeFAddFSub(BinaryOperator &I,
           I.getOpcode() == Instruction::FSub) && "Expecting fadd/fsub");
   assert(I.hasAllowReassoc() && I.hasNoSignedZeros() &&
          "FP factorization requires FMF");
+
+  if (Instruction *Lerp = factorizeLerp(I, Builder))
+    return Lerp;
+
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   Value *X, *Y, *Z;
   bool IsFMul;