[DIExpression] Introduce a dedicated DW_OP_LLVM_fragment operation

so we can stop using DW_OP_bit_piece with the wrong semantics.

The entire back story can be found here:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20161114/405934.html

The gist is that in LLVM we've been misinterpreting DW_OP_bit_piece's
offset field to mean the offset into the source variable rather than
the offset into the location at the top the DWARF expression stack. In
order to be able to fix this in a subsequent patch, this patch
introduces a dedicated DW_OP_LLVM_fragment operation with the
semantics that we used to apply to DW_OP_bit_piece, which is what we
actually need while inside of LLVM. This patch is complete with a
bitcode upgrade for expressions using the old format. It does not yet
fix the DWARF backend to use DW_OP_bit_piece correctly.

Implementation note: We discussed several options for implementing
this, including reserving a dedicated field in DIExpression for the
fragment size and offset, but using an custom operator at the end of
the expression works just fine and is more efficient because we then
only pay for it when we need it.

Differential Revision: https://reviews.llvm.org/D27361
rdar://problem/29335809

llvm-svn: 288683

1 files changed, 9 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 01a5579e1ed..6de0f34e94c 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1102,26 +1102,26 @@ void llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
   if (SExtInst *SExt = dyn_cast<SExtInst>(SI->getOperand(0)))
     ExtendedArg = dyn_cast<Argument>(SExt->getOperand(0));
   if (ExtendedArg) {
-    // We're now only describing a subset of the variable. The piece we're
+    // We're now only describing a subset of the variable. The fragment we're
     // describing will always be smaller than the variable size, because
     // VariableSize == Size of Alloca described by DDI. Since SI stores
     // to the alloca described by DDI, if it's first operand is an extend,
     // we're guaranteed that before extension, the value was narrower than
     // the size of the alloca, hence the size of the described variable.
     SmallVector<uint64_t, 3> Ops;
-    unsigned PieceOffset = 0;
-    // If this already is a bit piece, we drop the bit piece from the expression
-    // and record the offset.
-    if (DIExpr->isBitPiece()) {
+    unsigned FragmentOffset = 0;
+    // If this already is a bit fragment, we drop the bit fragment from the
+    // expression and record the offset.
+    if (DIExpr->isFragment()) {
       Ops.append(DIExpr->elements_begin(), DIExpr->elements_end()-3);
-      PieceOffset = DIExpr->getBitPieceOffset();
+      FragmentOffset = DIExpr->getFragmentOffsetInBits();
     } else {
       Ops.append(DIExpr->elements_begin(), DIExpr->elements_end());
     }
-    Ops.push_back(dwarf::DW_OP_bit_piece);
-    Ops.push_back(PieceOffset); // Offset
+    Ops.push_back(dwarf::DW_OP_LLVM_fragment);
+    Ops.push_back(FragmentOffset);
     const DataLayout &DL = DDI->getModule()->getDataLayout();
-    Ops.push_back(DL.getTypeSizeInBits(ExtendedArg->getType())); // Size
+    Ops.push_back(DL.getTypeSizeInBits(ExtendedArg->getType()));
     auto NewDIExpr = Builder.createExpression(Ops);
     if (!LdStHasDebugValue(DIVar, NewDIExpr, SI))
       Builder.insertDbgValueIntrinsic(ExtendedArg, 0, DIVar, NewDIExpr,