[InstCombine] narrow select with FP casts

Select doesn't change values, so truncate of extended operand cancels out.

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 53bcaeb98b4..8d8b2f9f871 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1636,6 +1636,24 @@ Instruction *InstCombiner::visitFPTrunc(FPTruncInst &FPT) {
         return BinaryOperator::CreateFNegFMF(InnerTrunc, Op);
       return UnaryOperator::CreateFNegFMF(InnerTrunc, Op);
     }
+
+    // If we are truncating a select that has an extended operand, we can
+    // narrow the other operand and do the select as a narrow op.
+    Value *Cond, *X, *Y;
+    if (match(Op, m_Select(m_Value(Cond), m_FPExt(m_Value(X)), m_Value(Y))) &&
+        X->getType() == Ty) {
+      // fptrunc (select Cond, (fpext X), Y --> select Cond, X, (fptrunc Y)
+      Value *NarrowY = Builder.CreateFPTrunc(Y, Ty);
+      Value *Sel = Builder.CreateSelect(Cond, X, NarrowY, "narrow.sel", Op);
+      return replaceInstUsesWith(FPT, Sel);
+    }
+    if (match(Op, m_Select(m_Value(Cond), m_Value(Y), m_FPExt(m_Value(X)))) &&
+        X->getType() == Ty) {
+      // fptrunc (select Cond, Y, (fpext X) --> select Cond, (fptrunc Y), X
+      Value *NarrowY = Builder.CreateFPTrunc(Y, Ty);
+      Value *Sel = Builder.CreateSelect(Cond, NarrowY, X, "narrow.sel", Op);
+      return replaceInstUsesWith(FPT, Sel);
+    }
   }
 
   if (auto *II = dyn_cast<IntrinsicInst>(FPT.getOperand(0))) {