[DebugInfo] DW_OP_deref_size in PrologEpilogInserter.

The PrologEpilogInserter need to insert a DW_OP_deref_size before
prepending a memory location expression to an already implicit
expression to avoid having the existing expression act on the memory
address instead of the value behind it.

The reason for using DW_OP_deref_size and not plain DW_OP_deref is that
big-endian targets need to read the right size as simply truncating a
larger read would yield the wrong result (LSB bytes are not at the lower
address).

Differential Revision: https://reviews.llvm.org/D59687

llvm-svn: 358268

1 files changed, 13 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/PrologEpilogInserter.cpp b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
index 67834c8c352..31f962eba42 100644
--- a/llvm/lib/CodeGen/PrologEpilogInserter.cpp
+++ b/llvm/lib/CodeGen/PrologEpilogInserter.cpp
@@ -1172,12 +1172,22 @@ void PEI::replaceFrameIndices(MachineBasicBlock *BB, MachineFunction &MF,
         assert(i == 0 && "Frame indices can only appear as the first "
                          "operand of a DBG_VALUE machine instruction");
         unsigned Reg;
+        unsigned FrameIdx = MI.getOperand(0).getIndex();
+        unsigned Size = MF.getFrameInfo().getObjectSize(FrameIdx);
+
         int64_t Offset =
-            TFI->getFrameIndexReference(MF, MI.getOperand(0).getIndex(), Reg);
+            TFI->getFrameIndexReference(MF, FrameIdx, Reg);
         MI.getOperand(0).ChangeToRegister(Reg, false /*isDef*/);
         MI.getOperand(0).setIsDebug();
-        auto *DIExpr = DIExpression::prepend(MI.getDebugExpression(),
-                                             DIExpression::NoDeref, Offset);
+
+        const DIExpression *DIExpr = MI.getDebugExpression();
+        // If we already have a Implicit location expression we need to insert
+        // a deref before prepending a Memory location expression.
+        if (DIExpr->isImplicit()) {
+          SmallVector<uint64_t, 2> Ops = {dwarf::DW_OP_deref_size, Size};
+          DIExpr = DIExpression::prependOpcodes(DIExpr, Ops, DIExpression::WithStackValue);
+        }
+        DIExpr = DIExpression::prepend(DIExpr, DIExpression::NoDeref, Offset);
         MI.getOperand(3).setMetadata(DIExpr);
         continue;
       }