diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -352,9 +352,9 @@
                                          const DataLayout &DL) {
   do {
     Type *SrcTy = C->getType();
-    uint64_t DestSize = DL.getTypeSizeInBits(DestTy);
-    uint64_t SrcSize = DL.getTypeSizeInBits(SrcTy);
-    if (SrcSize < DestSize)
+    TypeSize DestSize = DL.getTypeSizeInBits(DestTy);
+    TypeSize SrcSize = DL.getTypeSizeInBits(SrcTy);
+    if (!TypeSize::isKnownGE(SrcSize, DestSize))
       return nullptr;
 
     // Catch the obvious splat cases (since all-zeros can coerce non-integral
diff --git a/llvm/test/Transforms/InstCombine/vscale_load.ll b/llvm/test/Transforms/InstCombine/vscale_load.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/vscale_load.ll
@@ -0,0 +1,27 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S -verify | FileCheck %s
+
+define <2 x i8> @constprop_load_bitcast(<vscale x 16 x i1>* %ptr) {
+; CHECK-LABEL: @constprop_load_bitcast(
+; CHECK-NEXT:    store <vscale x 16 x i1> zeroinitializer, <vscale x 16 x i1>* [[PTR:%.*]], align 16
+; CHECK-NEXT:    ret <2 x i8> zeroinitializer
+;
+  store <vscale x 16 x i1> zeroinitializer, <vscale x 16 x i1>* %ptr, align 16
+  %cast_to_fixed = bitcast <vscale x 16 x i1>* %ptr to <2 x i8>*
+  %a = load <2 x i8>, <2 x i8>* %cast_to_fixed, align 16
+  ret <2 x i8> %a
+}
+
+; vscale-sized vec not guaranteed to fill destination.
+define <8 x i8> @constprop_load_bitcast_neg(<vscale x 16 x i1>* %ptr) {
+; CHECK-LABEL: @constprop_load_bitcast_neg(
+; CHECK-NEXT:    store <vscale x 16 x i1> zeroinitializer, <vscale x 16 x i1>* [[PTR:%.*]], align 16
+; CHECK-NEXT:    [[CAST_TO_FIXED:%.*]] = bitcast <vscale x 16 x i1>* [[PTR]] to <8 x i8>*
+; CHECK-NEXT:    [[A:%.*]] = load <8 x i8>, <8 x i8>* [[CAST_TO_FIXED]], align 16
+; CHECK-NEXT:    ret <8 x i8> [[A]]
+;
+  store <vscale x 16 x i1> zeroinitializer, <vscale x 16 x i1>* %ptr, align 16
+  %cast_to_fixed = bitcast <vscale x 16 x i1>* %ptr to <8 x i8>*
+  %a = load <8 x i8>, <8 x i8>* %cast_to_fixed, align 16
+  ret <8 x i8> %a
+}