[JumpThreading][DebugInfo] Propagate DebugLocs when simplifying loads

llvm/lib/Transforms/Scalar/JumpThreading.cpp

@@ -1437,9 +1437,18 @@ bool JumpThreadingPass::simplifyPartiallyRedundantLoad(LoadInst *LoadI) {
     // AvailablePreds vector as we go so that all of the PHI entries for this
     // predecessor use the same bitcast.
     Value *&PredV = I->second;
-    if (PredV->getType() != LoadI->getType())
+    if (PredV->getType() != LoadI->getType()) {
       PredV = CastInst::CreateBitOrPointerCast(
           PredV, LoadI->getType(), "", P->getTerminator()->getIterator());
+      // The new cast is producing the value used to replace the load
+      // instruction, so uses the load's debug location. If P does not always
+      // branch to the load BB however then the debug location must be dropped,
+      // as it is hoisted past a conditional branch.
+      DebugLoc DL = P->getTerminator()->getNumSuccessors() == 1
+                        ? LoadI->getDebugLoc()
+                        : DebugLoc::getDropped();
+      cast<CastInst>(PredV)->setDebugLoc(DL);
+    }
 
     PN->addIncoming(PredV, I->first);
   }

llvm/test/Transforms/JumpThreading/simplify-partially-redundant-load-debugloc.ll

@@ -0,0 +1,88 @@
+; RUN: opt -S -passes=jump-threading < %s | FileCheck %s
+
+;; Check that when we simplify the load of %p by replacing it with 0 when coming
+;; from %left, the inttoptr cast instruction is assigned the DILocation from the
+;; load instruction.
+
+; CHECK-LABEL: define void @foo(
+
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[P:%.*]] = alloca i32, align 8
+; CHECK-NEXT:    br i1 {{.*}}, label %[[LEFT:.*]], label %[[RIGHT:.*]]
+
+;; Cast in "left" should have the load's debug location.
+; CHECK:       [[LEFT]]:
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 8, i1 false)
+; CHECK-NEXT:    [[TMP0:%.*]] = inttoptr i64 0 to ptr, !dbg [[LOAD_LOC:![0-9]+]]
+; CHECK-NEXT:    br label %[[END:.*]]
+
+; CHECK:       [[RIGHT]]:
+; CHECK-NEXT:    br i1 {{.*}}, label %[[RIGHT_LEFT:.*]], label %[[ENDTHREAD_PRE_SPLIT:.*]]
+
+;; Cast in "right.left" should not have a debug location, as we are not
+;; guaranteed to reach the load's original position.
+; CHECK:       [[RIGHT_LEFT]]:
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr [[P]], i8 0, i64 8, i1 false)
+; CHECK-NEXT:    [[TMP1:%.*]] = inttoptr i64 0 to ptr
+; CHECK-NEXT:    br i1 {{.*}}, label %[[END]], label %[[EXIT:.*]]
+
+;; Load in "right.right" should have the load's debug location.
+; CHECK:       [[ENDTHREAD_PRE_SPLIT]]:
+; CHECK-NEXT:    [[DOTPR:%.*]] = load ptr, ptr [[P]], align 8, !dbg [[LOAD_LOC]]
+; CHECK-NEXT:    br label %[[END]], !dbg [[LOAD_LOC]]
+
+;; Finally, the PHI node should also have the load's debug location.
+; CHECK:       [[END]]:
+; CHECK-NEXT:    [[TMP2:%.*]] = phi ptr [ [[DOTPR]], %[[ENDTHREAD_PRE_SPLIT]] ], [ [[TMP1]], %[[RIGHT_LEFT]] ], [ [[TMP0]], %[[LEFT]] ], !dbg [[LOAD_LOC]]
+
+; CHECK: [[LOAD_LOC]] = !DILocation(line: 1, column: 1,
+
+define void @foo(i1 %b, i1 %c, i1 %d) !dbg !5 {
+entry:
+  %p = alloca i32, align 8
+  br i1 %b, label %left, label %right
+
+left:
+  call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 8, i1 false)
+  br label %end
+
+right:
+  br i1 %c, label %right.left, label %right.right
+
+right.left:
+  call void @llvm.memset.p0.i64(ptr %p, i8 0, i64 8, i1 false)
+  br i1 %d, label %end, label %exit
+
+right.right:
+  br label %end
+
+end:
+  %0 = load ptr, ptr %p, align 8, !dbg !8
+  %isnull = icmp eq ptr %0, null
+  br i1 %isnull, label %exit, label %notnull
+
+notnull:
+  br label %exit
+
+exit:
+  ret void
+}
+
+; Function Attrs: nocallback nofree nounwind willreturn memory(argmem: write)
+declare void @llvm.memset.p0.i64(ptr writeonly captures(none), i8, i64, i1 immarg) #0
+
+attributes #0 = { nocallback nofree nounwind willreturn memory(argmem: write) }
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "simplify-partially-redundant-load-debugloc.ll", directory: "/")
+!2 = !{i32 13}
+!3 = !{i32 0}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: !0, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!6 = !DISubroutineType(types: !7)
+!7 = !{}
+!8 = !DILocation(line: 1, column: 1, scope: !5)