[SafeStack] Insert the deref after the offset

While debugging code that uses SafeStack, we've noticed that LLVM
produces an invalid DWARF. Concretely, in the following example:

  int main(int argc, char* argv[]) {
    std::string value = "";
    printf("%s\n", value.c_str());
    return 0;
  }

DWARF would describe the value variable as being located at:

  DW_OP_breg14 R14+0, DW_OP_deref, DW_OP_constu 0x20, DW_OP_minus

The assembly to get this variable is:

  leaq    -32(%r14), %rbx

The order of operations in the DWARF symbols is incorrect in this case.
Specifically, the deref is incorrect; this appears to be incorrectly
re-inserted in repalceOneDbgValueForAlloca.

With this change which inserts the deref after the offset instead of
before it, LLVM produces correct DWARF:

  DW_OP_breg14 R14-32

Differential Revision: https://reviews.llvm.org/D64971

llvm-svn: 366726

1 files changed, 2 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index c2d4303ecb0..be160541117 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1597,13 +1597,13 @@ static void replaceOneDbgValueForAlloca(DbgValueInst *DVI, Value *NewAddress,
       DIExpr->getElement(0) != dwarf::DW_OP_deref)
     return;
 
-  // Insert the offset immediately after the first deref.
+  // Insert the offset before the first deref.
   // We could just change the offset argument of dbg.value, but it's unsigned...
   if (Offset) {
     SmallVector<uint64_t, 4> Ops;
-    Ops.push_back(dwarf::DW_OP_deref);
     DIExpression::appendOffset(Ops, Offset);
     Ops.append(DIExpr->elements_begin() + 1, DIExpr->elements_end());
+    Ops.push_back(dwarf::DW_OP_deref);
     DIExpr = Builder.createExpression(Ops);
   }