1 files changed, 30 insertions, 1 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 1881f78255b..8f2b44036f6 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -52,6 +52,7 @@
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/CFG.h"
+#include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/GetElementPtrTypeIterator.h"
@@ -91,6 +92,16 @@ static cl::opt<unsigned>
 MaxArraySize("instcombine-maxarray-size", cl::init(1024),
              cl::desc("Maximum array size considered when doing a combine"));
 
+// FIXME: Remove this flag when it is no longer necessary to convert
+// llvm.dbg.declare to avoid inaccurate debug info. Setting this to false
+// increases variable availability at the cost of accuracy. Variables that
+// cannot be promoted by mem2reg or SROA will be described as living in memory
+// for their entire lifetime. However, passes like DSE and instcombine can
+// delete stores to the alloca, leading to misleading and inaccurate debug
+// information. This flag can be removed when those passes are fixed.
+static cl::opt<unsigned> ShouldLowerDbgDeclare("instcombine-lower-dbg-declare",
+                                               cl::Hidden, cl::init(true));
+
 Value *InstCombiner::EmitGEPOffset(User *GEP) {
   return llvm::EmitGEPOffset(&Builder, DL, GEP);
 }
@@ -2092,6 +2103,16 @@ Instruction *InstCombiner::visitAllocSite(Instruction &MI) {
   // to null and free calls, delete the calls and replace the comparisons with
   // true or false as appropriate.
   SmallVector<WeakTrackingVH, 64> Users;
+
+  // If we are removing an alloca with a dbg.declare, insert dbg.value calls
+  // before each store.
+  DbgDeclareInst *DDI = nullptr;
+  std::unique_ptr<DIBuilder> DIB;
+  if (isa<AllocaInst>(MI)) {
+    DDI = FindAllocaDbgDeclare(&MI);
+    DIB.reset(new DIBuilder(*MI.getModule(), /*AllowUnresolved=*/false));
+  }
+
   if (isAllocSiteRemovable(&MI, Users, &TLI)) {
     for (unsigned i = 0, e = Users.size(); i != e; ++i) {
       // Lowering all @llvm.objectsize calls first because they may
@@ -2124,6 +2145,8 @@ Instruction *InstCombiner::visitAllocSite(Instruction &MI) {
       } else if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I) ||
                  isa<AddrSpaceCastInst>(I)) {
         replaceInstUsesWith(*I, UndefValue::get(I->getType()));
+      } else if (DDI && isa<StoreInst>(I)) {
+        ConvertDebugDeclareToDebugValue(DDI, cast<StoreInst>(I), *DIB);
       }
       eraseInstFromFunction(*I);
     }
@@ -2135,6 +2158,10 @@ Instruction *InstCombiner::visitAllocSite(Instruction &MI) {
       InvokeInst::Create(F, II->getNormalDest(), II->getUnwindDest(),
                          None, "", II->getParent());
     }
+
+    if (DDI)
+      eraseInstFromFunction(*DDI);
+
     return eraseInstFromFunction(MI);
   }
   return nullptr;
@@ -3188,7 +3215,9 @@ static bool combineInstructionsOverFunction(
 
   // Lower dbg.declare intrinsics otherwise their value may be clobbered
   // by instcombiner.
-  bool MadeIRChange = LowerDbgDeclare(F);
+  bool MadeIRChange = false;
+  if (ShouldLowerDbgDeclare)
+    MadeIRChange = LowerDbgDeclare(F);
 
   // Iterate while there is work to do.
   int Iteration = 0;