[ConstantFolding] Fold bitcasts of vectors w/ undef elements

An undef vector element can be treated as if it had any value.  Folding
such a vector element to 0 in a bitcast can open up further folding
opportunities.

llvm-svn: 277104

2 files changed, 17 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 010f5d62f06..60b94a903e3 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -90,6 +90,11 @@ Constant *FoldBitCast(Constant *C, Type *DestTy, const DataLayout &DL) {
       else
         Element = C->getAggregateElement(i);
 
+      if (Element && isa<UndefValue>(Element)) {
+        Result <<= BitShift;
+        continue;
+      }
+
       auto *ElementCI = dyn_cast_or_null<ConstantInt>(Element);
       if (!ElementCI)
         return ConstantExpr::getBitCast(C, DestTy);
@@ -180,7 +185,11 @@ Constant *FoldBitCast(Constant *C, Type *DestTy, const DataLayout &DL) {
       Constant *Elt = Zero;
       unsigned ShiftAmt = isLittleEndian ? 0 : SrcBitSize*(Ratio-1);
       for (unsigned j = 0; j != Ratio; ++j) {
-        Constant *Src = dyn_cast<ConstantInt>(C->getAggregateElement(SrcElt++));
+        Constant *Src = C->getAggregateElement(SrcElt++);
+        if (Src && isa<UndefValue>(Src))
+          Src = Constant::getNullValue(SrcEltTy);
+        else
+          Src = dyn_cast_or_null<ConstantInt>(Src);
         if (!Src)  // Reject constantexpr elements.
           return ConstantExpr::getBitCast(C, DestTy);
 
@@ -206,7 +215,7 @@ Constant *FoldBitCast(Constant *C, Type *DestTy, const DataLayout &DL) {
 
   // Loop over each source value, expanding into multiple results.
   for (unsigned i = 0; i != NumSrcElt; ++i) {
-    auto *Src = dyn_cast<ConstantInt>(C->getAggregateElement(i));
+    auto *Src = dyn_cast_or_null<ConstantInt>(C->getAggregateElement(i));
     if (!Src)  // Reject constantexpr elements.
       return ConstantExpr::getBitCast(C, DestTy);
 
diff --git a/llvm/test/Transforms/InstCombine/cast.ll b/llvm/test/Transforms/InstCombine/cast.ll
index 2acb68738ff..46ae14c85cc 100644
--- a/llvm/test/Transforms/InstCombine/cast.ll
+++ b/llvm/test/Transforms/InstCombine/cast.ll
@@ -1380,3 +1380,9 @@ define i64 @PR28745() {
   %b = zext i32 extractvalue ({ i32 } select (i1 icmp eq (i16 extractelement (<2 x i16> bitcast (<1 x i32> <i32 1> to <2 x i16>), i32 0), i16 0), { i32 } { i32 1 }, { i32 } zeroinitializer), 0) to i64
   ret i64 %b
 }
+
+define i32 @test89() {
+; CHECK-LABEL: @test89(
+; CHECK-NEXT:    ret i32 393216
+  ret i32 bitcast (<2 x i16> <i16 6, i16 undef> to i32)
+}