[InstCombine] narrow select to match condition operands' size

This is the planned enhancement to D47163 / rL333611.
We want to match cmp/select sizes because that will be recognized
as min/max more easily and lead to better codegen (especially for
vector types).

As mentioned in D47163, this improves some of the tests that would
also be folded by D46380, so we may want to adjust that patch to
match the new patterns where the extend op occurs after the select.

llvm-svn: 333689

1 files changed, 11 insertions, 8 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 9fe3c59e5e0..e70078a73c5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1164,6 +1164,11 @@ static Instruction *foldAddSubSelect(SelectInst &SI,
 }
 
 Instruction *InstCombiner::foldSelectExtConst(SelectInst &Sel) {
+  Constant *C;
+  if (!match(Sel.getTrueValue(), m_Constant(C)) &&
+      !match(Sel.getFalseValue(), m_Constant(C)))
+    return nullptr;
+
   Instruction *ExtInst;
   if (!match(Sel.getTrueValue(), m_Instruction(ExtInst)) &&
       !match(Sel.getFalseValue(), m_Instruction(ExtInst)))
@@ -1173,20 +1178,18 @@ Instruction *InstCombiner::foldSelectExtConst(SelectInst &Sel) {
   if (ExtOpcode != Instruction::ZExt && ExtOpcode != Instruction::SExt)
     return nullptr;
 
-  // TODO: Handle larger types? That requires adjusting FoldOpIntoSelect too.
+  // If we are extending from a boolean type or if we can create a select that
+  // has the same size operands as its condition, try to narrow the select.
   Value *X = ExtInst->getOperand(0);
   Type *SmallType = X->getType();
-  if (!SmallType->isIntOrIntVectorTy(1))
-    return nullptr;
-
-  Constant *C;
-  if (!match(Sel.getTrueValue(), m_Constant(C)) &&
-      !match(Sel.getFalseValue(), m_Constant(C)))
+  Value *Cond = Sel.getCondition();
+  auto *Cmp = dyn_cast<CmpInst>(Cond);
+  if (!SmallType->isIntOrIntVectorTy(1) &&
+      (!Cmp || Cmp->getOperand(0)->getType() != SmallType))
     return nullptr;
 
   // If the constant is the same after truncation to the smaller type and
   // extension to the original type, we can narrow the select.
-  Value *Cond = Sel.getCondition();
   Type *SelType = Sel.getType();
   Constant *TruncC = ConstantExpr::getTrunc(C, SmallType);
   Constant *ExtC = ConstantExpr::getCast(ExtOpcode, TruncC, SelType);