diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index c2315d5de7041..d3c6a7151ec37 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -2085,6 +2085,10 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
       assert(ShadowPtr && "Could not find shadow for an argument");
       return ShadowPtr;
     }
+
+    // TODO: Partially undefined vectors are handled by the fall-through case
+    //       below (see partial-poison.ll); this causes false negatives.
+
     // For everything else the shadow is zero.
     return getCleanShadow(V);
   }
diff --git a/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll b/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll
new file mode 100644
index 0000000000000..5164441c17e10
--- /dev/null
+++ b/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll
@@ -0,0 +1,78 @@
+; 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
+;
+; Test case to show that MSan computes shadows for partially poisoned vectors
+; as fully initialized, resulting in false negatives.
+
+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"
+
+define <2 x i64> @left_poison(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @left_poison(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    call void @llvm.donothing()
+; CHECK-NEXT:    store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT:    ret <2 x i64> <i64 poison, i64 42>
+;
+  ret <2 x i64> <i64 poison, i64 42>
+}
+
+define <2 x i64> @right_poison(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @right_poison(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    call void @llvm.donothing()
+; CHECK-NEXT:    store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT:    ret <2 x i64> <i64 42, i64 poison>
+;
+  ret <2 x i64> <i64 42, i64 poison>
+}
+
+define <2 x i64> @full_poison(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @full_poison(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    call void @llvm.donothing()
+; CHECK-NEXT:    store <2 x i64> splat (i64 -1), ptr @__msan_retval_tls, align 8
+; CHECK-NEXT:    ret <2 x i64> poison
+;
+  ret <2 x i64> <i64 poison, i64 poison>
+}
+
+define <2 x i64> @no_poison_or_undef(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @no_poison_or_undef(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    call void @llvm.donothing()
+; CHECK-NEXT:    store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT:    ret <2 x i64> splat (i64 42)
+;
+  ret <2 x i64> <i64 42, i64 42>
+}
+
+define <2 x i64> @left_undef(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @left_undef(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    call void @llvm.donothing()
+; CHECK-NEXT:    store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT:    ret <2 x i64> <i64 undef, i64 42>
+;
+  ret <2 x i64> <i64 undef, i64 42>
+}
+
+define <2 x i64> @right_undef(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @right_undef(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    call void @llvm.donothing()
+; CHECK-NEXT:    store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT:    ret <2 x i64> <i64 42, i64 undef>
+;
+  ret <2 x i64> <i64 42, i64 undef>
+}
+
+define <2 x i64> @full_undef(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @full_undef(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    call void @llvm.donothing()
+; CHECK-NEXT:    store <2 x i64> splat (i64 -1), ptr @__msan_retval_tls, align 8
+; CHECK-NEXT:    ret <2 x i64> undef
+;
+  ret <2 x i64> <i64 undef, i64 undef>
+}