diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index 592db79be0070..7b5831652aa6e 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -2509,9 +2509,9 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     //
     //    S = (S1 & S2) | (~V1 & S2) | (S1 & ~V2)
     //
-    //  Addendum if the "Or" is "disjoint":
-    //    1|1 => p;
-    //    S = S | (V1 & V2)
+    //  If the "disjoint OR" property is violated, the result is poison, and
+    //  hence the entire shadow is uninitialized:
+    //    S = S | SignExt(V1 & V2 != 0)
     Value *S1 = getShadow(&I, 0);
     Value *S2 = getShadow(&I, 1);
     Value *V1 = I.getOperand(0);
@@ -2532,7 +2532,9 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
 
     if (ClPreciseDisjointOr && cast<PossiblyDisjointInst>(&I)->isDisjoint()) {
       Value *V1V2 = IRB.CreateAnd(V1, V2);
-      S = IRB.CreateOr(S, V1V2, "_ms_disjoint");
+      Value *DisjointOrShadow = IRB.CreateSExt(
+          IRB.CreateICmpNE(V1V2, getCleanShadow(V1V2)), V1V2->getType());
+      S = IRB.CreateOr(S, DisjointOrShadow, "_ms_disjoint");
     }
 
     setShadow(&I, S);
diff --git a/llvm/test/Instrumentation/MemorySanitizer/or.ll b/llvm/test/Instrumentation/MemorySanitizer/or.ll
index 27a1800aa495b..20993a54187ac 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/or.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/or.ll
@@ -29,28 +29,41 @@ define i8 @test_or(i8 %a, i8 %b) sanitize_memory {
 }
 
 define i8 @test_disjoint_or(i8 %a, i8 %b) sanitize_memory {
-; CHECK-LABEL: define i8 @test_disjoint_or(
-; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) #[[ATTR0]] {
-; CHECK-NEXT:    [[TMP1:%.*]] = load i8, ptr @__msan_param_tls, align 8
-; CHECK-NEXT:    [[TMP2:%.*]] = load i8, ptr inttoptr (i64 add (i64 ptrtoint (ptr @__msan_param_tls to i64), i64 8) to ptr), align 8
-; CHECK-NEXT:    call void @llvm.donothing()
-; CHECK-NEXT:    [[TMP3:%.*]] = xor i8 [[A]], -1
-; CHECK-NEXT:    [[TMP4:%.*]] = xor i8 [[B]], -1
-; CHECK-NEXT:    [[TMP5:%.*]] = and i8 [[TMP1]], [[TMP2]]
-; CHECK-NEXT:    [[TMP6:%.*]] = and i8 [[TMP3]], [[TMP2]]
-; CHECK-NEXT:    [[TMP7:%.*]] = and i8 [[TMP1]], [[TMP4]]
-; CHECK-NEXT:    [[TMP8:%.*]] = or i8 [[TMP5]], [[TMP6]]
-; CHECK-NEXT:    [[TMP11:%.*]] = or i8 [[TMP8]], [[TMP7]]
+; CHECK-IMPRECISE-LABEL: define i8 @test_disjoint_or(
+; CHECK-IMPRECISE-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) #[[ATTR0]] {
+; CHECK-IMPRECISE-NEXT:    [[TMP1:%.*]] = load i8, ptr @__msan_param_tls, align 8
+; CHECK-IMPRECISE-NEXT:    [[TMP2:%.*]] = load i8, ptr inttoptr (i64 add (i64 ptrtoint (ptr @__msan_param_tls to i64), i64 8) to ptr), align 8
+; CHECK-IMPRECISE-NEXT:    call void @llvm.donothing()
+; CHECK-IMPRECISE-NEXT:    [[TMP3:%.*]] = xor i8 [[A]], -1
+; CHECK-IMPRECISE-NEXT:    [[TMP4:%.*]] = xor i8 [[B]], -1
+; CHECK-IMPRECISE-NEXT:    [[TMP5:%.*]] = and i8 [[TMP1]], [[TMP2]]
+; CHECK-IMPRECISE-NEXT:    [[TMP6:%.*]] = and i8 [[TMP3]], [[TMP2]]
+; CHECK-IMPRECISE-NEXT:    [[TMP7:%.*]] = and i8 [[TMP1]], [[TMP4]]
+; CHECK-IMPRECISE-NEXT:    [[TMP8:%.*]] = or i8 [[TMP5]], [[TMP6]]
+; CHECK-IMPRECISE-NEXT:    [[TMP9:%.*]] = or i8 [[TMP8]], [[TMP7]]
+; CHECK-IMPRECISE-NEXT:    [[C:%.*]] = or disjoint i8 [[A]], [[B]]
+; CHECK-IMPRECISE-NEXT:    store i8 [[TMP9]], ptr @__msan_retval_tls, align 8
+; CHECK-IMPRECISE-NEXT:    ret i8 [[C]]
 ;
-; CHECK-IMPRECISE:      [[C:%.*]] = or disjoint i8 [[A]], [[B]]
-; CHECK-IMPRECISE-NEXT: store i8 [[TMP11]], ptr @__msan_retval_tls, align 8
-;
-; CHECK-PRECISE:         [[TMP10:%.*]] = and i8 [[A]], [[B]]
-; CHECK-PRECISE-NEXT:    [[TMP12:%.*]] = or i8 [[TMP11]], [[TMP10]]
+; CHECK-PRECISE-LABEL: define i8 @test_disjoint_or(
+; CHECK-PRECISE-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) #[[ATTR0]] {
+; CHECK-PRECISE-NEXT:    [[TMP1:%.*]] = load i8, ptr @__msan_param_tls, align 8
+; CHECK-PRECISE-NEXT:    [[TMP2:%.*]] = load i8, ptr inttoptr (i64 add (i64 ptrtoint (ptr @__msan_param_tls to i64), i64 8) to ptr), align 8
+; CHECK-PRECISE-NEXT:    call void @llvm.donothing()
+; CHECK-PRECISE-NEXT:    [[TMP3:%.*]] = xor i8 [[A]], -1
+; CHECK-PRECISE-NEXT:    [[TMP4:%.*]] = xor i8 [[B]], -1
+; CHECK-PRECISE-NEXT:    [[TMP5:%.*]] = and i8 [[TMP1]], [[TMP2]]
+; CHECK-PRECISE-NEXT:    [[TMP6:%.*]] = and i8 [[TMP3]], [[TMP2]]
+; CHECK-PRECISE-NEXT:    [[TMP7:%.*]] = and i8 [[TMP1]], [[TMP4]]
+; CHECK-PRECISE-NEXT:    [[TMP8:%.*]] = or i8 [[TMP5]], [[TMP6]]
+; CHECK-PRECISE-NEXT:    [[TMP9:%.*]] = or i8 [[TMP8]], [[TMP7]]
+; CHECK-PRECISE-NEXT:    [[TMP10:%.*]] = and i8 [[A]], [[B]]
+; CHECK-PRECISE-NEXT:    [[TMP11:%.*]] = icmp ne i8 [[TMP10]], 0
+; CHECK-PRECISE-NEXT:    [[TMP12:%.*]] = sext i1 [[TMP11]] to i8
+; CHECK-PRECISE-NEXT:    [[_MS_DISJOINT:%.*]] = or i8 [[TMP9]], [[TMP12]]
 ; CHECK-PRECISE-NEXT:    [[C:%.*]] = or disjoint i8 [[A]], [[B]]
-; CHECK-PRECISE-NEXT:    store i8 [[TMP12]], ptr @__msan_retval_tls, align 8
-;
-; CHECK-NEXT:    ret i8 [[C]]
+; CHECK-PRECISE-NEXT:    store i8 [[_MS_DISJOINT]], ptr @__msan_retval_tls, align 8
+; CHECK-PRECISE-NEXT:    ret i8 [[C]]
 ;
   %c = or disjoint i8 %a, %b
   ret i8 %c