[ConstantFold] Improve the bitcast folding logic for constant vectors.
The constant folder didn't know how to always fold bitcasts of constant integer
vectors. In particular, it was unable to handle the case where a constant vector
had some undef elements, and the resulting (i.e. bitcasted) vector type had more
elements than the original vector type.
Example:
%cast = bitcast <2 x i64><i64 undef, i64 2> to <4 x i32>
On a little endian target, %cast could have been folded to:
<4 x i32><i32 undef, i32 undef, i32 2, i32 0>
This patch improves the folding logic by teaching how to correctly propagate
undef elements in the folded vector.
Differential Revision: https://reviews.llvm.org/D24301
llvm-svn: 281343
diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 73cf336..1df724a 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -224,8 +224,19 @@
// Loop over each source value, expanding into multiple results.
for (unsigned i = 0; i != NumSrcElt; ++i) {
- auto *Src = dyn_cast_or_null<ConstantInt>(C->getAggregateElement(i));
- if (!Src) // Reject constantexpr elements.
+ auto *Element = C->getAggregateElement(i);
+
+ if (!Element) // Reject constantexpr elements.
+ return ConstantExpr::getBitCast(C, DestTy);
+
+ if (isa<UndefValue>(Element)) {
+ // Correctly Propagate undef values.
+ Result.append(Ratio, UndefValue::get(DstEltTy));
+ continue;
+ }
+
+ auto *Src = dyn_cast<ConstantInt>(Element);
+ if (!Src)
return ConstantExpr::getBitCast(C, DestTy);
unsigned ShiftAmt = isLittleEndian ? 0 : DstBitSize*(Ratio-1);