Index: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -6375,6 +6375,12 @@
     // Collect Uniform and Scalar instructions after vectorization with VF.
     CM.collectUniformsAndScalars(VF);
 
+    // collectUniformsAndScalars (and setCostBasedWideningDecision) change
+    // SCEV behaviour, which we haven't seen before, so check again if we need
+    // runtime checks.
+    if (CM.TheFunction->hasOptSize() && CM.runtimeChecksRequired())
+      return None;
+
     // Collect the instructions (and their associated costs) that will be more
     // profitable to scalarize.
     if (VF > 1)
Index: llvm/test/Transforms/LoopVectorize/pr43371-optsize-scevchecks.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopVectorize/pr43371-optsize-scevchecks.ll
@@ -0,0 +1,38 @@
+; RUN: opt -S -loop-vectorize  -pass-remarks-analysis='loop-vectorize' < %s 2>&1 | FileCheck %s
+
+@cm_array = external global [2592 x i16], align 1
+
+define void @pr43371() optsize {
+;
+; CHECK: remark: <unknown>:0:0: loop not vectorized: runtime SCEV checks needed. Enable vectorization of this loop with '#pragma clang loop vectorize(enable)' when compiling with -Os/-Oz
+;
+; CHECK-LABEL: @pr43371
+;
+; CHECK-NOT: vector.scevcheck:
+; CHECK-NOT: vector.body:
+;
+entry:
+  br label %for.body
+
+for.cond.for.cond.cleanup_crit_edge:
+  br label %for.body29
+
+for.body:
+  br label %for.cond.for.cond.cleanup_crit_edge
+
+for.cond.cleanup28:
+  unreachable
+
+for.body29:
+  %i24.0170 = phi i16 [ 0, %for.cond.for.cond.cleanup_crit_edge ], [ %inc37, %for.inc36 ]
+  %add33 = add i16 undef, %i24.0170
+  %idxprom34 = zext i16 %add33 to i32
+  %arrayidx35 = getelementptr [2592 x i16], [2592 x i16] * @cm_array, i32 0, i32 %idxprom34
+  store i16 0, i16 * %arrayidx35, align 1
+  br label %for.inc36
+
+for.inc36:
+  %inc37 = add i16 %i24.0170, 1
+  %cmp26 = icmp ult i16 %inc37, 756
+  br i1 %cmp26, label %for.body29, label %for.cond.cleanup28
+}