From 8dd6cf711f75f296cdeae2be7c6760b29cc68f57 Mon Sep 17 00:00:00 2001
From: Bjorn Pettersson <bjorn.a.pettersson@ericsson.com>
Date: Tue, 3 Jul 2018 11:29:00 +0000
Subject: [DebugInfo] Corrections for salvageDebugInfo

Summary:
When salvaging a dbg.declare/dbg.addr we should not add
DW_OP_stack_value to the DIExpression
(see test/Transforms/InstCombine/salvage-dbg-declare.ll).

Consider this example
  %vla = alloca i32, i64 2
  call void @llvm.dbg.declare(metadata i32* %vla, metadata !1, metadata !DIExpression())

Instcombine will turn it into
  %vla1 = alloca [2 x i32]
  %vla1.sub = getelementptr inbounds [2 x i32], [2 x i32]* %vla, i64 0, i64 0
  call void @llvm.dbg.declare(metadata [2 x i32]* %vla1.sub, metadata !19, metadata !DIExpression())

If the GEP can be eliminated, then the dbg.declare will be salvaged
and we should get
  %vla1 = alloca [2 x i32]
  call void @llvm.dbg.declare(metadata [2 x i32]* %vla1, metadata !19, metadata !DIExpression())

The problem was that salvageDebugInfo did not recognize dbg.declare
as being indirect (%vla1 points to the value, it does not hold the
value), so we incorrectly got
  call void @llvm.dbg.declare(metadata [2 x i32]* %vla1, metadata !19, metadata !DIExpression(DW_OP_stack_value))

I also made sure that llvm::salvageDebugInfo and
DIExpression::prependOpcodes do not add DW_OP_stack_value to
the DIExpression in case no new operands are added to the
DIExpression. That way we avoid to, unneccessarily, turn a
register location expression into an implicit location expression
in some situations (see test11 in test/Transforms/LICM/sinking.ll).

Reviewers: aprantl, vsk

Reviewed By: aprantl, vsk

Subscribers: JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D48837

llvm-svn: 336191
---
 llvm/lib/IR/DebugInfoMetadata.cpp | 3 +++
 1 file changed, 3 insertions(+)

(limited to 'llvm/lib/IR')

diff --git a/llvm/lib/IR/DebugInfoMetadata.cpp b/llvm/lib/IR/DebugInfoMetadata.cpp
index a9f96e2549d..217cec3e937 100644
--- a/llvm/lib/IR/DebugInfoMetadata.cpp
+++ b/llvm/lib/IR/DebugInfoMetadata.cpp
@@ -803,6 +803,9 @@ DIExpression *DIExpression::prepend(const DIExpression *Expr, bool DerefBefore,
 DIExpression *DIExpression::prependOpcodes(const DIExpression *Expr,
                                            SmallVectorImpl<uint64_t> &Ops,
                                            bool StackValue) {
+  // If there are no ops to prepend, do not even add the DW_OP_stack_value.
+  if (Ops.empty())
+    StackValue = false;
   if (Expr)
     for (auto Op : Expr->expr_ops()) {
       // A DW_OP_stack_value comes at the end, but before a DW_OP_LLVM_fragment.
-- 
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