[JumpThreading][DebugInfo] Propagate DebugLocs when simplifying loads (#157683)

In simplifyPartiallyRedundantLoad we may replace a load with a PHI of
available values in predecessor blocks. As part of this process, we may
need to cast those values, which we do by inserting a new cast at the
end of the predecessor. These cast instructions should take their debug
location from the load instruction, just as the PHI does; we make an
exception if the predecessor does not unconditionally branch to the
load's block, as in that case we are not guaranteed to reach the load
and must therefore drop its debug location.

Found using llvm/llvm-project#107279.

Author: SLTozer

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)