diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index f2f11c2d142c0e..5c1c6b2a2d8f24 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1253,6 +1253,18 @@ static bool hasUndefContentsMSSA(MemorySSA *MSSA, AliasAnalysis *AA, Value *V,
       if (AA->isMustAlias(V, II->getArgOperand(1)) &&
           LTSize->getZExtValue() >= Size->getZExtValue())
         return true;
+
+      // If the lifetime.start covers a whole alloca (as it almost always does)
+      // and we're querying a pointer based on that alloca, then we know the
+      // memory is definitely undef, regardless of how exactly we alias. The
+      // size also doesn't matter, as an out-of-bounds access would be UB.
+      AllocaInst *Alloca = dyn_cast<AllocaInst>(getUnderlyingObject(V));
+      if (getUnderlyingObject(II->getArgOperand(1)) == Alloca) {
+        DataLayout DL = Alloca->getModule()->getDataLayout();
+        if (Optional<TypeSize> AllocaSize = Alloca->getAllocationSizeInBits(DL))
+          if (*AllocaSize == LTSize->getValue() * 8)
+            return true;
+      }
     }
   }
 
diff --git a/llvm/test/Transforms/MemCpyOpt/memcpy-undef.ll b/llvm/test/Transforms/MemCpyOpt/memcpy-undef.ll
index 561c4cc3d645b5..983aba204af450 100644
--- a/llvm/test/Transforms/MemCpyOpt/memcpy-undef.ll
+++ b/llvm/test/Transforms/MemCpyOpt/memcpy-undef.ll
@@ -67,20 +67,12 @@ define void @test_lifetime_may_alias(i8* %lifetime, i8* %src, i8* %dst) {
 
 ; lifetime.start on full alloca size, copy in range.
 define void @test_lifetime_partial_alias_1(i8* noalias %dst) {
-; NO-MSSA-LABEL: @test_lifetime_partial_alias_1(
-; NO-MSSA-NEXT:    [[A:%.*]] = alloca [16 x i8], align 1
-; NO-MSSA-NEXT:    [[A_I8:%.*]] = bitcast [16 x i8]* [[A]] to i8*
-; NO-MSSA-NEXT:    call void @llvm.lifetime.start.p0i8(i64 16, i8* [[A_I8]])
-; NO-MSSA-NEXT:    [[GEP:%.*]] = getelementptr i8, i8* [[A_I8]], i64 8
-; NO-MSSA-NEXT:    ret void
-;
-; MSSA-LABEL: @test_lifetime_partial_alias_1(
-; MSSA-NEXT:    [[A:%.*]] = alloca [16 x i8], align 1
-; MSSA-NEXT:    [[A_I8:%.*]] = bitcast [16 x i8]* [[A]] to i8*
-; MSSA-NEXT:    call void @llvm.lifetime.start.p0i8(i64 16, i8* [[A_I8]])
-; MSSA-NEXT:    [[GEP:%.*]] = getelementptr i8, i8* [[A_I8]], i64 8
-; MSSA-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[DST:%.*]], i8* [[GEP]], i64 8, i1 false)
-; MSSA-NEXT:    ret void
+; CHECK-LABEL: @test_lifetime_partial_alias_1(
+; CHECK-NEXT:    [[A:%.*]] = alloca [16 x i8], align 1
+; CHECK-NEXT:    [[A_I8:%.*]] = bitcast [16 x i8]* [[A]] to i8*
+; CHECK-NEXT:    call void @llvm.lifetime.start.p0i8(i64 16, i8* [[A_I8]])
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, i8* [[A_I8]], i64 8
+; CHECK-NEXT:    ret void
 ;
   %a = alloca [16 x i8]
   %a.i8 = bitcast [16 x i8]* %a to i8*
@@ -92,20 +84,12 @@ define void @test_lifetime_partial_alias_1(i8* noalias %dst) {
 
 ; lifetime.start on full alloca size, copy out of range.
 define void @test_lifetime_partial_alias_2(i8* noalias %dst) {
-; NO-MSSA-LABEL: @test_lifetime_partial_alias_2(
-; NO-MSSA-NEXT:    [[A:%.*]] = alloca [16 x i8], align 1
-; NO-MSSA-NEXT:    [[A_I8:%.*]] = bitcast [16 x i8]* [[A]] to i8*
-; NO-MSSA-NEXT:    call void @llvm.lifetime.start.p0i8(i64 16, i8* [[A_I8]])
-; NO-MSSA-NEXT:    [[GEP:%.*]] = getelementptr i8, i8* [[A_I8]], i64 8
-; NO-MSSA-NEXT:    ret void
-;
-; MSSA-LABEL: @test_lifetime_partial_alias_2(
-; MSSA-NEXT:    [[A:%.*]] = alloca [16 x i8], align 1
-; MSSA-NEXT:    [[A_I8:%.*]] = bitcast [16 x i8]* [[A]] to i8*
-; MSSA-NEXT:    call void @llvm.lifetime.start.p0i8(i64 16, i8* [[A_I8]])
-; MSSA-NEXT:    [[GEP:%.*]] = getelementptr i8, i8* [[A_I8]], i64 8
-; MSSA-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[DST:%.*]], i8* [[GEP]], i64 16, i1 false)
-; MSSA-NEXT:    ret void
+; CHECK-LABEL: @test_lifetime_partial_alias_2(
+; CHECK-NEXT:    [[A:%.*]] = alloca [16 x i8], align 1
+; CHECK-NEXT:    [[A_I8:%.*]] = bitcast [16 x i8]* [[A]] to i8*
+; CHECK-NEXT:    call void @llvm.lifetime.start.p0i8(i64 16, i8* [[A_I8]])
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, i8* [[A_I8]], i64 8
+; CHECK-NEXT:    ret void
 ;
   %a = alloca [16 x i8]
   %a.i8 = bitcast [16 x i8]* %a to i8*