[DebugInfo] Invalidate debug info in ReassociatePass::RewriteExprTree

Summary:
When Reassociate is rewriting an expression tree it may
reuse old binary expression nodes, for new expressions.
Whenever an expression node is reused, but with a non-trivial
change in the result, we need to invalidate any debug info
that is associated with the node.

If for example rewriting
  x = mul a, b
  y = mul c, x
into
  x = mul c, b
  y = mul a, x
we still get the same result for 'y', but 'x' is a new expression.
All debug info referring to 'x' must be invalidated (marked as
optimized out) since we no longer calculate the expected value.

As a side-effect this patch avoid (at least some) problems where
reassociate could end up creating IR with debug-use before def.
Earlier the dbg.value nodes where left untouched in the IR, while
the reused binary nodes where sinked to just before the root node
of the rewritten expression tree. See PR27273 for more info about
such problems.

Reviewers: dblaikie, aprantl, dexonsmith

Reviewed By: aprantl

Subscribers: JDevlieghere, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D45975

llvm-svn: 330804

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp
index 0317025e5d2..36f16618a75 100644
--- a/llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -43,6 +43,7 @@
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/IR/PatternMatch.h"
@@ -780,6 +781,18 @@ void ReassociatePass::RewriteExprTree(BinaryOperator *I,
 
       if (ExpressionChanged == I)
         break;
+
+      // Discard any debug info related to the expressions that has changed (we
+      // can leave debug infor related to the root, since the result of the
+      // expression tree should be the same even after reassociation).
+      SmallVector<DbgInfoIntrinsic *, 1> DbgUsers;
+      findDbgUsers(DbgUsers, ExpressionChanged);
+      for (auto *DII : DbgUsers) {
+        Value *Undef = UndefValue::get(ExpressionChanged->getType());
+        DII->setOperand(0, MetadataAsValue::get(DII->getContext(),
+                                                ValueAsMetadata::get(Undef)));
+      }
+
       ExpressionChanged->moveBefore(I);
       ExpressionChanged = cast<BinaryOperator>(*ExpressionChanged->user_begin());
     } while (true);