[Debug] Add dbg.value intrinsics for PHIs created during LCSSA.

Summary:
This patch is an enhancement to propagate dbg.value information when Phis are created on behalf of LCSSA.
I noticed a case where a value carried across a loop was reported as <optimized out>.

Specifically this case:
```
int bar(int x, int y) {
  return x + y;
}

int foo(int size) {
  int val = 0;
  for (int i = 0; i < size; ++i) {
    val = bar(val, i);  // Both val and i are correct
  }
  return val; // <optimized out>
}
```

In the above case, after all of the interesting computation completes our value
is reported as "optimized out." This change will add a dbg.value to correct this.

This patch also moves the dbg.value insertion routine from LoopRotation.cpp 
into Local.cpp, so that we can share it in both places (LoopRotation and LCSSA).

Reviewers: mzolotukhin, aprantl, vsk, davide

Reviewed By: aprantl, vsk

Subscribers: dberlin, llvm-commits

Differential Revision: https://reviews.llvm.org/D42551

llvm-svn: 325926

2 files changed, 12 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Utils/LCSSA.cpp b/llvm/lib/Transforms/Utils/LCSSA.cpp
index ae0e2bb6c28..a68448bb25a 100644
--- a/llvm/lib/Transforms/Utils/LCSSA.cpp
+++ b/llvm/lib/Transforms/Utils/LCSSA.cpp
@@ -43,6 +43,7 @@
 #include "llvm/IR/PredIteratorCache.h"
 #include "llvm/Pass.h"
 #include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/SSAUpdater.h"
 using namespace llvm;
@@ -214,11 +215,15 @@ bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
         Worklist.push_back(PostProcessPN);
 
     // Keep track of PHI nodes that we want to remove because they did not have
-    // any uses rewritten.
+    // any uses rewritten. If the new PHI is used, store it so that we can
+    // try to propagate dbg.value intrinsics to it.
+    SmallVector<PHINode *, 2> NeedDbgValues;
     for (PHINode *PN : AddedPHIs)
       if (PN->use_empty())
         PHIsToRemove.insert(PN);
-
+      else
+        NeedDbgValues.push_back(PN);
+    insertDebugValuesForPHIs(InstBB, NeedDbgValues);
     Changed = true;
   }
   // Remove PHI nodes that did not have any uses rewritten.
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 7573fc39b9d..fa059a11295 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1365,14 +1365,17 @@ void llvm::insertDebugValuesForPHIs(BasicBlock *BB,
   // propagate the info through the new PHI.
   LLVMContext &C = BB->getContext();
   for (auto PHI : InsertedPHIs) {
+    BasicBlock *Parent = PHI->getParent();
+    // Avoid inserting an intrinsic into an EH block.
+    if (Parent->getFirstNonPHI()->isEHPad())
+      continue;
+    auto PhiMAV = MetadataAsValue::get(C, ValueAsMetadata::get(PHI));
     for (auto VI : PHI->operand_values()) {
       auto V = DbgValueMap.find(VI);
       if (V != DbgValueMap.end()) {
         auto *DbgII = cast<DbgInfoIntrinsic>(V->second);
         Instruction *NewDbgII = DbgII->clone();
-        auto PhiMAV = MetadataAsValue::get(C, ValueAsMetadata::get(PHI));
         NewDbgII->setOperand(0, PhiMAV);
-        BasicBlock *Parent = PHI->getParent();
         auto InsertionPt = Parent->getFirstInsertionPt();
         assert(InsertionPt != Parent->end() && "Ill-formed basic block");
         NewDbgII->insertBefore(&*InsertionPt);