diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -4392,7 +4392,6 @@
   // entire expression in the smaller type.
   if (VF.isVector() && PhiTy != RdxDesc.getRecurrenceType()) {
     assert(!IsInLoopReductionPhi && "Unexpected truncated inloop reduction!");
-    assert(!VF.isScalable() && "scalable vectors not yet supported.");
     Type *RdxVecTy = VectorType::get(RdxDesc.getRecurrenceType(), VF);
     Builder.SetInsertPoint(
         LI->getLoopFor(LoopVectorBody)->getLoopLatch()->getTerminator());
diff --git a/llvm/test/Transforms/LoopVectorize/scalable-reduction-inloop.ll b/llvm/test/Transforms/LoopVectorize/scalable-reduction-inloop.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/LoopVectorize/scalable-reduction-inloop.ll
@@ -0,0 +1,60 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -loop-vectorize -force-target-supports-scalable-vectors=true -S | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
+define i8 @reduction_add_trunc(i8* noalias nocapture %A) {
+; CHECK-LABEL: @reduction_add_trunc(
+; CHECK:       vector.body:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, %vector.ph ], [ [[INDEX_NEXT:%.*]], %vector.body ]
+; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <vscale x 8 x i32> [ insertelement (<vscale x 8 x i32> zeroinitializer, i32 255, i32 0), %vector.ph ], [ [[TMP34:%.*]], %vector.body ]
+; CHECK-NEXT:    [[VEC_PHI1:%.*]] = phi <vscale x 8 x i32> [ zeroinitializer, %vector.ph ], [ [[TMP36:%.*]], %vector.body ]
+; CHECK:         [[TMP14:%.*]] = and <vscale x 8 x i32> [[VEC_PHI]], shufflevector (<vscale x 8 x i32> insertelement (<vscale x 8 x i32> poison, i32 255, i32 0), <vscale x 8 x i32> poison, <vscale x 8 x i32> zeroinitializer)
+; CHECK-NEXT:    [[TMP15:%.*]] = and <vscale x 8 x i32> [[VEC_PHI1]], shufflevector (<vscale x 8 x i32> insertelement (<vscale x 8 x i32> poison, i32 255, i32 0), <vscale x 8 x i32> poison, <vscale x 8 x i32> zeroinitializer)
+; CHECK:         [[WIDE_LOAD:%.*]] = load <vscale x 8 x i8>, <vscale x 8 x i8>*
+; CHECK:         [[WIDE_LOAD2:%.*]] = load <vscale x 8 x i8>, <vscale x 8 x i8>*
+; CHECK-NEXT:    [[TMP26:%.*]] = zext <vscale x 8 x i8> [[WIDE_LOAD]] to <vscale x 8 x i32>
+; CHECK-NEXT:    [[TMP27:%.*]] = zext <vscale x 8 x i8> [[WIDE_LOAD2]] to <vscale x 8 x i32>
+; CHECK-NEXT:    [[TMP28:%.*]] = add <vscale x 8 x i32> [[TMP14]], [[TMP26]]
+; CHECK-NEXT:    [[TMP29:%.*]] = add <vscale x 8 x i32> [[TMP15]], [[TMP27]]
+; CHECK-NEXT:    [[TMP30:%.*]] = call i32 @llvm.vscale.i32()
+; CHECK-NEXT:    [[TMP31:%.*]] = mul i32 [[TMP30]], 16
+; CHECK-NEXT:    [[INDEX_NEXT]] = add i32 [[INDEX]], [[TMP31]]
+; CHECK-NEXT:    [[TMP32:%.*]] = icmp eq i32 [[INDEX_NEXT]], {{%.*}}
+; CHECK-NEXT:    [[TMP33:%.*]] = trunc <vscale x 8 x i32> [[TMP28]] to <vscale x 8 x i8>
+; CHECK-NEXT:    [[TMP34]] = zext <vscale x 8 x i8> [[TMP33]] to <vscale x 8 x i32>
+; CHECK-NEXT:    [[TMP35:%.*]] = trunc <vscale x 8 x i32> [[TMP29]] to <vscale x 8 x i8>
+; CHECK-NEXT:    [[TMP36]] = zext <vscale x 8 x i8> [[TMP35]] to <vscale x 8 x i32>
+; CHECK:       middle.block:
+; CHECK-NEXT:    [[TMP37:%.*]] = trunc <vscale x 8 x i32> [[TMP34]] to <vscale x 8 x i8>
+; CHECK-NEXT:    [[TMP38:%.*]] = trunc <vscale x 8 x i32> [[TMP36]] to <vscale x 8 x i8>
+; CHECK-NEXT:    [[BIN_RDX:%.*]] = add <vscale x 8 x i8> [[TMP38]], [[TMP37]]
+; CHECK-NEXT:    [[TMP39:%.*]] = call i8 @llvm.vector.reduce.add.nxv8i8(<vscale x 8 x i8> [[BIN_RDX]])
+; CHECK-NEXT:    [[TMP40:%.*]] = zext i8 [[TMP39]] to i32
+;
+entry:
+  br label %loop
+
+loop:                                           ; preds = %entry, %loop
+  %indvars.iv = phi i32 [ %indvars.iv.next, %loop ], [ 0, %entry ]
+  %sum.02p = phi i32 [ %l9, %loop ], [ 255, %entry ]
+  %sum.02 = and i32 %sum.02p, 255
+  %l2 = getelementptr inbounds i8, i8* %A, i32 %indvars.iv
+  %l3 = load i8, i8* %l2, align 4
+  %l3e = zext i8 %l3 to i32
+  %l9 = add i32 %sum.02, %l3e
+  %indvars.iv.next = add i32 %indvars.iv, 1
+  %exitcond = icmp eq i32 %indvars.iv.next, 256
+  br i1 %exitcond, label %exit, label %loop, !llvm.loop !0
+
+exit:                                      ; preds = %loop
+  %sum.0.lcssa = phi i32 [ %l9, %loop ]
+  %ret = trunc i32 %sum.0.lcssa to i8
+  ret i8 %ret
+}
+
+!0 = distinct !{!0, !1, !2, !3, !4}
+!1 = !{!"llvm.loop.vectorize.width", i32 8}
+!2 = !{!"llvm.loop.vectorize.scalable.enable", i1 true}
+!3 = !{!"llvm.loop.interleave.count", i32 2}
+!4 = !{!"llvm.loop.vectorize.enable", i1 true}