diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index 1a76898bd61c6..ca655b3597671 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -282,6 +282,12 @@ static cl::opt<bool> ClPoisonUndefVectors(
              "unaffected by this flag (see -msan-poison-undef)."),
     cl::Hidden, cl::init(false));
 
+static cl::opt<bool> ClPreciseDisjointOr(
+    "msan-precise-disjoint-or",
+    cl::desc("Precisely poison disjoint OR. If false (legacy behavior), "
+             "disjointedness is ignored (i.e., 1|1 is initialized)."),
+    cl::Hidden, cl::init(false));
+
 static cl::opt<bool>
     ClHandleICmp("msan-handle-icmp",
                  cl::desc("propagate shadow through ICmpEQ and ICmpNE"),
@@ -2497,11 +2503,16 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
 
   void visitOr(BinaryOperator &I) {
     IRBuilder<> IRB(&I);
-    //  "Or" of 1 and a poisoned value results in unpoisoned value.
-    //  1|1 => 1;     0|1 => 1;     p|1 => 1;
-    //  1|0 => 1;     0|0 => 0;     p|0 => p;
-    //  1|p => 1;     0|p => p;     p|p => p;
-    //  S = (S1 & S2) | (~V1 & S2) | (S1 & ~V2)
+    //  "Or" of 1 and a poisoned value results in unpoisoned value:
+    //    1|1 => 1;     0|1 => 1;     p|1 => 1;
+    //    1|0 => 1;     0|0 => 0;     p|0 => p;
+    //    1|p => 1;     0|p => p;     p|p => p;
+    //
+    //    S = (S1 & S2) | (~V1 & S2) | (S1 & ~V2)
+    //
+    //  Addendum if the "Or" is "disjoint":
+    //    1|1 => p;
+    //    S = S | (V1 & V2)
     Value *S1 = getShadow(&I, 0);
     Value *S2 = getShadow(&I, 1);
     Value *V1 = IRB.CreateNot(I.getOperand(0));
@@ -2513,7 +2524,14 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     Value *S1S2 = IRB.CreateAnd(S1, S2);
     Value *V1S2 = IRB.CreateAnd(V1, S2);
     Value *S1V2 = IRB.CreateAnd(S1, V2);
-    setShadow(&I, IRB.CreateOr({S1S2, V1S2, S1V2}));
+
+    Value *S = IRB.CreateOr({S1S2, V1S2, S1V2});
+    if (ClPreciseDisjointOr && cast<PossiblyDisjointInst>(&I)->isDisjoint()) {
+      Value *V1V2 = IRB.CreateAnd(V1, V2);
+      S = IRB.CreateOr({S, V1V2});
+    }
+
+    setShadow(&I, S);
     setOriginForNaryOp(I);
   }
 
diff --git a/llvm/test/Instrumentation/MemorySanitizer/or.ll b/llvm/test/Instrumentation/MemorySanitizer/or.ll
index 6570b6d9d91ca..2d51de13a8ebb 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/or.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/or.ll
@@ -1,8 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
-; RUN: opt < %s -S -passes=msan 2>&1 | FileCheck %s
+; RUN: opt < %s -S -passes=msan -msan-precise-disjoint-or=false 2>&1 | FileCheck %s --check-prefixes=CHECK,CHECK-IMPRECISE
+; RUN: opt < %s -S -passes=msan -msan-precise-disjoint-or=true  2>&1 | FileCheck %s --check-prefixes=CHECK,CHECK-PRECISE
 ;
-; Test bitwise OR instructions, especially the "disjoint OR", which is
-; currently handled incorrectly by MSan (as if it was a regular OR).
+; Test bitwise OR instructions, including "disjoint OR".
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
@@ -41,8 +41,15 @@ define i8 @test_disjoint_or(i8 %a, i8 %b) sanitize_memory {
 ; CHECK-NEXT:    [[TMP7:%.*]] = and i8 [[TMP1]], [[TMP4]]
 ; CHECK-NEXT:    [[TMP8:%.*]] = or i8 [[TMP5]], [[TMP6]]
 ; CHECK-NEXT:    [[TMP11:%.*]] = or i8 [[TMP8]], [[TMP7]]
-; CHECK-NEXT:    [[C:%.*]] = or disjoint i8 [[A]], [[B]]
-; CHECK-NEXT:    store i8 [[TMP11]], ptr @__msan_retval_tls, align 8
+;
+; 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 [[TMP3]], [[TMP4]]
+; CHECK-PRECISE-NEXT:    [[TMP12:%.*]] = or i8 [[TMP11]], [[TMP10]]
+; 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]]
 ;
   %c = or disjoint i8 %a, %b