Index: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -7397,8 +7397,17 @@
       // Look for the next elements with the same type.
       SmallVector<Value *, 4>::iterator SameTypeIt = IncIt;
       Type *EltTy = (*IncIt)->getType();
-      unsigned EltSize = EltTy->isSized() ? DL->getTypeSizeInBits(EltTy)
-                                          : MaxVecRegSize;
+
+      assert(EltTy->isSized() &&
+             "Instructions should all be sized at this point");
+      TypeSize EltTS = DL->getTypeSizeInBits(EltTy);
+      if (EltTS.isScalable()) {
+        // For now, just ignore vectorizing scalable types.
+        ++IncIt;
+        continue;
+      }
+
+      unsigned EltSize = EltTS.getFixedSize();
       unsigned MaxNumElts = MaxVecRegSize / EltSize;
       if (MaxNumElts < 2) {
         ++IncIt;
Index: llvm/test/Transforms/SLPVectorizer/AArch64/scalable-vector.ll
===================================================================
--- llvm/test/Transforms/SLPVectorizer/AArch64/scalable-vector.ll
+++ llvm/test/Transforms/SLPVectorizer/AArch64/scalable-vector.ll
@@ -1,5 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -slp-vectorizer -S | FileCheck %s
+; RUN: opt < %s -slp-vectorizer -S 2>%t | FileCheck %s
+; RUN: FileCheck --check-prefix=WARN --allow-empty %s <%t
+
+; WARN-NOT: warning
 
 target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
 target triple = "aarch64-unknown-linux-gnu"
@@ -21,5 +24,28 @@
   ret void
 }
 
+define <vscale x 4 x i32> @scalable_phi(<vscale x 4 x i32> %a, i32 %b) {
+; CHECK-LABEL: @scalable_phi(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[B:%.*]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[END:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[RETVAL:%.*]] = phi <vscale x 4 x i32> [ [[A:%.*]], [[ENTRY:%.*]] ], [ zeroinitializer, [[IF_THEN]] ]
+; CHECK-NEXT:    ret <vscale x 4 x i32> [[RETVAL]]
+;
+entry:
+  %cmp = icmp eq i32 %b, 0
+  br i1 %cmp, label %if.then, label %end
+
+if.then:
+  br label %end
+
+end:
+  %retval = phi <vscale x 4 x i32> [ %a, %entry ], [ zeroinitializer, %if.then ]
+  ret <vscale x 4 x i32> %retval
+}
+
 declare <vscale x 16 x i8> @llvm.masked.load.nxv16i8.p0nxv16i8(<vscale x 16 x i8>*, i32 immarg, <vscale x 16 x i1>, <vscale x 16 x i8>)
 declare void @llvm.masked.store.nxv16i8.p0nxv16i8(<vscale x 16 x i8>, <vscale x 16 x i8>*, i32 immarg, <vscale x 16 x i1>)