[ConstantFolding, ValueTracking] Fold constants involving bitcasts of ConstantVector

We assumed that ConstantVectors would be rather uninteresting from the
perspective of analysis.  However, this is not the case due to a quirk
of how LLVM handles vectors of i1.  Vectors of i1 are not
ConstantDataVectors like vectors of i8, i16, i32 or i64 because i1's
SizeInBits differs from it's StoreSizeInBytes.  This leads to it being
categorized as a ConstantVector instead of a ConstantDataVector.

Instead, treat ConstantVector more uniformly.

This fixes PR27591.

llvm-svn: 268479

1 files changed, 10 insertions, 7 deletions

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index ebe8a27999f..677c3d92d02 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -78,20 +78,23 @@ Constant *FoldBitCast(Constant *C, Type *DestTy, const DataLayout &DL) {
       C = ConstantExpr::getBitCast(C, SrcIVTy);
     }
 
-    ConstantDataVector *CDV = dyn_cast<ConstantDataVector>(C);
-    if (!CDV)
-      return ConstantExpr::getBitCast(C, DestTy);
-
     // Now that we know that the input value is a vector of integers, just shift
     // and insert them into our result.
     unsigned BitShift = DL.getTypeAllocSizeInBits(SrcEltTy);
     APInt Result(IT->getBitWidth(), 0);
     for (unsigned i = 0; i != NumSrcElts; ++i) {
-      Result <<= BitShift;
+      Constant *Element;
       if (DL.isLittleEndian())
-        Result |= CDV->getElementAsInteger(NumSrcElts-i-1);
+        Element = C->getAggregateElement(NumSrcElts-i-1);
       else
-        Result |= CDV->getElementAsInteger(i);
+        Element = C->getAggregateElement(i);
+
+      auto *ElementCI = dyn_cast_or_null<ConstantInt>(Element);
+      if (!ElementCI)
+        return ConstantExpr::getBitCast(C, DestTy);
+
+      Result <<= BitShift;
+      Result |= ElementCI->getValue().zextOrSelf(IT->getBitWidth());
     }
 
     return ConstantInt::get(IT, Result);