[DebugInfo] Avoid adding too much indirection to pointer-valued variables

This patch addresses PR41675, where a stack-pointer variable is dereferenced
too many times by its location expression, presenting a value on the stack as
the pointer to the stack.

The difference between a stack *pointer* DBG_VALUE and one that refers to a
value on the stack, is currently the indirect flag. However the DWARF backend
will also try to guess whether something is a memory location or not, based
on whether there is any computation in the location expression. By simply
prepending the stack offset to existing expressions, we can accidentally
convert a register location into a memory location, which introduces a
suprise (and unintended) dereference.

The solution is to add DW_OP_stack_value whenever we add a DIExpression
computation to a stack *pointer*. It's an implicit location computed on the
expression stack, thus needs to be flagged as a stack_value.

For the edge case where the offset is zero and the location could be a register
location, DIExpression::prepend will still generate opcodes, and thus
DW_OP_stack_value must still be added.

Differential Revision: https://reviews.llvm.org/D63429

llvm-svn: 364736

1 files changed, 21 insertions, 0 deletions

diff --git a/llvm/lib/IR/DebugInfoMetadata.cpp b/llvm/lib/IR/DebugInfoMetadata.cpp
index 1607cf5104f..900df27d1d3 100644
--- a/llvm/lib/IR/DebugInfoMetadata.cpp
+++ b/llvm/lib/IR/DebugInfoMetadata.cpp
@@ -926,6 +926,27 @@ bool DIExpression::isImplicit() const {
   return false;
 }
 
+bool DIExpression::isComplex() const {
+  if (!isValid())
+    return false;
+
+  if (getNumElements() == 0)
+    return false;
+
+  // If there are any elements other than fragment or tag_offset, then some
+  // kind of complex computation occurs.
+  for (const auto &It : expr_ops()) {
+    switch (It.getOp()) {
+      case dwarf::DW_OP_LLVM_tag_offset:
+      case dwarf::DW_OP_LLVM_fragment:
+        continue;
+      default: return true;
+    }
+  }
+
+  return false;
+}
+
 Optional<DIExpression::FragmentInfo>
 DIExpression::getFragmentInfo(expr_op_iterator Start, expr_op_iterator End) {
   for (auto I = Start; I != End; ++I)