[SROA] Also insert a bit piece expression if only one piece is needed

Summary: If SROA creates only one piece (e.g. because the other is not needed),
it still needs to create a bit_piece expression if that bit piece is smaller
than the original size of the alloca.

Reviewers: aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16187

llvm-svn: 257795

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/SROA.cpp b/llvm/lib/Transforms/Scalar/SROA.cpp
index a7361b5fe08..4fa7a9fcfba 100644
--- a/llvm/lib/Transforms/Scalar/SROA.cpp
+++ b/llvm/lib/Transforms/Scalar/SROA.cpp
@@ -4024,12 +4024,12 @@ bool SROA::splitAlloca(AllocaInst &AI, AllocaSlices &AS) {
     auto *Var = DbgDecl->getVariable();
     auto *Expr = DbgDecl->getExpression();
     DIBuilder DIB(*AI.getModule(), /*AllowUnresolved*/ false);
-    bool IsSplit = Pieces.size() > 1;
+    uint64_t AllocaSize = DL.getTypeSizeInBits(AI.getAllocatedType());
     for (auto Piece : Pieces) {
       // Create a piece expression describing the new partition or reuse AI's
       // expression if there is only one partition.
       auto *PieceExpr = Expr;
-      if (IsSplit || Expr->isBitPiece()) {
+      if (Piece.Size < AllocaSize || Expr->isBitPiece()) {
         // If this alloca is already a scalar replacement of a larger aggregate,
         // Piece.Offset describes the offset inside the scalar.
         uint64_t Offset = Expr->isBitPiece() ? Expr->getBitPieceOffset() : 0;
@@ -4043,6 +4043,9 @@ bool SROA::splitAlloca(AllocaInst &AI, AllocaSlices &AS) {
           Size = std::min(Size, AbsEnd - Start);
         }
         PieceExpr = DIB.createBitPieceExpression(Start, Size);
+      } else {
+        assert(Pieces.size() == 1 &&
+               "partition is as large as original alloca");
       }
 
       // Remove any existing dbg.declare intrinsic describing the same alloca.