[DEBUGINFO] Prevent Instcombine from dropping debuginfo when removing zexts

Zexts can be treated like no-op casts when it comes to assessing whether their
removal affects debug info.

Reviewer: aprantl

Differential Revision: https://reviews.llvm.org/D60641

llvm-svn: 358431

2 files changed, 13 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 3110322b314..65d5928e99c 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1666,11 +1666,10 @@ DIExpression *llvm::salvageDebugInfoImpl(Instruction &I,
   };
 
   if (auto *CI = dyn_cast<CastInst>(&I)) {
-    if (!CI->isNoopCast(DL))
-      return nullptr;
-
-    // No-op casts are irrelevant for debug info.
-    return SrcDIExpr;
+    // No-op casts and zexts are irrelevant for debug info.
+    if (CI->isNoopCast(DL) || isa<ZExtInst>(&I))
+      return SrcDIExpr;
+    return nullptr;
   } else if (auto *GEP = dyn_cast<GetElementPtrInst>(&I)) {
     unsigned BitWidth =
         M.getDataLayout().getIndexSizeInBits(GEP->getPointerAddressSpace());
diff --git a/llvm/test/Transforms/InstCombine/cast-mul-select.ll b/llvm/test/Transforms/InstCombine/cast-mul-select.ll
index b140dfa2229..c501fd8d04c 100644
--- a/llvm/test/Transforms/InstCombine/cast-mul-select.ll
+++ b/llvm/test/Transforms/InstCombine/cast-mul-select.ll
@@ -170,3 +170,12 @@ exit:
   unreachable
 }
 
+; Check that we don't drop debug info when a zext is removed.
+define i1 @foo(i1 zeroext %b) {
+; DBGINFO-LABEL: @foo(
+; DBGINFO-NEXT:  call void @llvm.dbg.value(metadata i1 %b
+; DBGINFO-NEXT:  ret i1 %b
+
+  %frombool = zext i1 %b to i8 
+  ret i1 %b
+}