diff --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -371,6 +371,29 @@
   // FIXME: Vet that this works for size upper-bounds. Seems unlikely that we'll
   // get imprecise values here, though (except for unknown sizes).
   if (!Later.Size.isPrecise() || !Earlier.Size.isPrecise()) {
+    // In case no constant size is known, try to an IR values for the number
+    // of bytes written and check if they match.
+    auto GetSizeFromInstr = [](const Instruction *I) -> Value * {
+      if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
+        switch (II->getIntrinsicID()) {
+        default:
+          return nullptr;
+        case Intrinsic::memcpy:
+        case Intrinsic::memset:
+          return II->getArgOperand(2);
+        }
+      }
+      return nullptr;
+    };
+    Value *LaterV = GetSizeFromInstr(LaterI);
+    Value *EarlierV = GetSizeFromInstr(EarlierI);
+    if (LaterV && LaterV == EarlierV) {
+      const Value *P1 = Earlier.Ptr->stripPointerCasts();
+      const Value *P2 = Later.Ptr->stripPointerCasts();
+      if (P1 == P2 || AA.isMustAlias(P1, P2))
+        return OW_Complete;
+    }
+
     // Masked stores have imprecise locations, but we can reason about them
     // to some extent.
     return isMaskedStoreOverwrite(LaterI, EarlierI, AA);
diff --git a/llvm/test/Transforms/DeadStoreElimination/memory-intrinsics-sizes.ll b/llvm/test/Transforms/DeadStoreElimination/memory-intrinsics-sizes.ll
--- a/llvm/test/Transforms/DeadStoreElimination/memory-intrinsics-sizes.ll
+++ b/llvm/test/Transforms/DeadStoreElimination/memory-intrinsics-sizes.ll
@@ -4,7 +4,6 @@
 define void @memset_equal_size_values(i8* %ptr, i64 %len) {
 ; CHECK-LABEL: @memset_equal_size_values(
 ; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* align 1 [[PTR:%.*]], i8 0, i64 [[LEN:%.*]], i1 false)
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* align 1 [[PTR]], i8 0, i64 [[LEN]], i1 false)
 ; CHECK-NEXT:    ret void
 ;
   call void @llvm.memset.p0i8.i64(i8* align 1 %ptr, i8 0, i64 %len, i1 false)
@@ -48,7 +47,6 @@
 define void @memcpy_equal_size_values(i8* noalias %src, i8* noalias %dst, i64 %len) {
 ; CHECK-LABEL: @memcpy_equal_size_values(
 ; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[DST:%.*]], i8* [[SRC:%.*]], i64 [[LEN:%.*]], i1 false)
-; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[DST]], i8* [[SRC]], i64 [[LEN]], i1 false)
 ; CHECK-NEXT:    ret void
 ;
   call void @llvm.memcpy.p0i8.p0i8.i64(i8* %dst, i8* %src, i64 %len, i1 false)