[InstCombine] Preserve debug values for eliminable casts

A cast from A to B is eliminable if its result is casted to C, and if
the pair of casts could just be expressed as a single cast. E.g here,
%c1 is eliminable:

  %c1 = zext i16 %A to i32
  %c2 = sext i32 %c1 to i64

InstCombine optimizes away eliminable casts. This patch teaches it to
insert a dbg.value intrinsic pointing to the final result, so that local
variables pointing to the eliminable result are preserved.

Differential Revision: https://reviews.llvm.org/D42566

llvm-svn: 323570

1 files changed, 15 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 178c8eaf250..74d026e641e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -16,6 +16,7 @@
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Support/KnownBits.h"
 using namespace llvm;
@@ -265,7 +266,20 @@ Instruction *InstCombiner::commonCastTransforms(CastInst &CI) {
     if (Instruction::CastOps NewOpc = isEliminableCastPair(CSrc, &CI)) {
       // The first cast (CSrc) is eliminable so we need to fix up or replace
       // the second cast (CI). CSrc will then have a good chance of being dead.
-      return CastInst::Create(NewOpc, CSrc->getOperand(0), CI.getType());
+      auto *Res = CastInst::Create(NewOpc, CSrc->getOperand(0), CI.getType());
+
+      // If the eliminable cast has debug users, insert a debug value after the
+      // cast pointing to the new Value.
+      SmallVector<DbgInfoIntrinsic *, 1> CSrcDbgInsts;
+      findDbgUsers(CSrcDbgInsts, CSrc);
+      if (CSrcDbgInsts.size()) {
+        DIBuilder DIB(*CI.getModule());
+        for (auto *DII : CSrcDbgInsts)
+          DIB.insertDbgValueIntrinsic(
+              Res, DII->getVariable(), DII->getExpression(),
+              DII->getDebugLoc().get(), &*std::next(CI.getIterator()));
+      }
+      return Res;
     }
   }