diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 97b8737df83e4..a5a9f00f29362 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -952,8 +952,13 @@ static void addMask(SmallVectorImpl<int> &Mask, ArrayRef<int> SubMask,
                     bool ExtendingManyInputs = false) {
   if (SubMask.empty())
     return;
-  assert((!ExtendingManyInputs || SubMask.size() > Mask.size()) &&
-         "SubMask with many inputs support must be larger than the mask.");
+  assert(
+      (!ExtendingManyInputs || SubMask.size() > Mask.size() ||
+       // Check if input scalars were extended to match the size of other node.
+       (SubMask.size() == Mask.size() &&
+        std::all_of(std::next(Mask.begin(), Mask.size() / 2), Mask.end(),
+                    [](int Idx) { return Idx == PoisonMaskElem; }))) &&
+      "SubMask with many inputs support must be larger than the mask.");
   if (Mask.empty()) {
     Mask.append(SubMask.begin(), SubMask.end());
     return;
diff --git a/llvm/test/Transforms/SLPVectorizer/X86/pr63668.ll b/llvm/test/Transforms/SLPVectorizer/X86/pr63668.ll
new file mode 100644
index 0000000000000..391771e06cab8
--- /dev/null
+++ b/llvm/test/Transforms/SLPVectorizer/X86/pr63668.ll
@@ -0,0 +1,54 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt -passes=slp-vectorizer -mtriple=x86_64-unknown-linux-gnu -mcpu=znver4 -S < %s | FileCheck %s
+
+define internal i32 @testfunc() {
+; CHECK-LABEL: define internal i32 @testfunc
+; CHECK-SAME: () #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    br label [[TMP1:%.*]]
+; CHECK:       1:
+; CHECK-NEXT:    [[TMP2:%.*]] = phi float [ 0.000000e+00, [[TMP0:%.*]] ], [ 0.000000e+00, [[TMP8:%.*]] ]
+; CHECK-NEXT:    [[TMP3:%.*]] = phi float [ 0.000000e+00, [[TMP0]] ], [ 0.000000e+00, [[TMP8]] ]
+; CHECK-NEXT:    [[TMP4:%.*]] = phi float [ 0.000000e+00, [[TMP0]] ], [ 0.000000e+00, [[TMP8]] ]
+; CHECK-NEXT:    br i1 false, label [[TMP8]], label [[TMP5:%.*]]
+; CHECK:       5:
+; CHECK-NEXT:    br i1 false, label [[TMP6:%.*]], label [[TMP8]]
+; CHECK:       6:
+; CHECK-NEXT:    [[TMP7:%.*]] = extractelement <8 x float> zeroinitializer, i64 0
+; CHECK-NEXT:    br label [[TMP8]]
+; CHECK:       8:
+; CHECK-NEXT:    [[TMP9:%.*]] = phi float [ [[TMP7]], [[TMP6]] ], [ 0.000000e+00, [[TMP1]] ], [ 0.000000e+00, [[TMP5]] ]
+; CHECK-NEXT:    [[TMP10:%.*]] = phi float [ [[TMP2]], [[TMP6]] ], [ 0.000000e+00, [[TMP1]] ], [ [[TMP2]], [[TMP5]] ]
+; CHECK-NEXT:    [[TMP11:%.*]] = phi float [ [[TMP7]], [[TMP6]] ], [ 0.000000e+00, [[TMP1]] ], [ 0.000000e+00, [[TMP5]] ]
+; CHECK-NEXT:    [[TMP12:%.*]] = phi float [ [[TMP7]], [[TMP6]] ], [ 0.000000e+00, [[TMP1]] ], [ 0.000000e+00, [[TMP5]] ]
+; CHECK-NEXT:    [[TMP13:%.*]] = phi float [ [[TMP7]], [[TMP6]] ], [ 0.000000e+00, [[TMP1]] ], [ 0.000000e+00, [[TMP5]] ]
+; CHECK-NEXT:    [[TMP14:%.*]] = phi float [ [[TMP3]], [[TMP6]] ], [ 0.000000e+00, [[TMP1]] ], [ 0.000000e+00, [[TMP5]] ]
+; CHECK-NEXT:    [[TMP15:%.*]] = phi float [ [[TMP4]], [[TMP6]] ], [ 0.000000e+00, [[TMP1]] ], [ 0.000000e+00, [[TMP5]] ]
+; CHECK-NEXT:    [[TMP16:%.*]] = phi float [ [[TMP4]], [[TMP6]] ], [ 0.000000e+00, [[TMP1]] ], [ [[TMP3]], [[TMP5]] ]
+; CHECK-NEXT:    br label [[TMP1]]
+;
+  br label %1
+
+1:                                                ; preds = %8, %0
+  %2 = phi float [ 0.000000e+00, %0 ], [ 0.000000e+00, %8 ]
+  %3 = phi float [ 0.000000e+00, %0 ], [ 0.000000e+00, %8 ]
+  %4 = phi float [ 0.000000e+00, %0 ], [ 0.000000e+00, %8 ]
+  br i1 false, label %8, label %5
+
+5:                                                ; preds = %1
+  br i1 false, label %6, label %8
+
+6:                                                ; preds = %5
+  %7 = extractelement <8 x float> zeroinitializer, i64 0
+  br label %8
+
+8:                                                ; preds = %6, %5, %1
+  %9 = phi float [ %7, %6 ], [ 0.000000e+00, %1 ], [ 0.000000e+00, %5 ]
+  %10 = phi float [ %2, %6 ], [ 0.000000e+00, %1 ], [ %2, %5 ]
+  %11 = phi float [ %7, %6 ], [ 0.000000e+00, %1 ], [ 0.000000e+00, %5 ]
+  %12 = phi float [ %7, %6 ], [ 0.000000e+00, %1 ], [ 0.000000e+00, %5 ]
+  %13 = phi float [ %7, %6 ], [ 0.000000e+00, %1 ], [ 0.000000e+00, %5 ]
+  %14 = phi float [ %3, %6 ], [ 0.000000e+00, %1 ], [ 0.000000e+00, %5 ]
+  %15 = phi float [ %4, %6 ], [ 0.000000e+00, %1 ], [ 0.000000e+00, %5 ]
+  %16 = phi float [ %4, %6 ], [ 0.000000e+00, %1 ], [ %3, %5 ]
+  br label %1
+}