[llvm] [SimplifyCFG] Find the smallest table considering overflow in `switchToLookupTable` (PR #67885)

[Nikita Popov via llvm-commits]

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@@ -9,11 +9,17 @@ target triple = "x86_64-apple-darwin12.0.0"
 define i64 @test(i3 %arg) {
 ; CHECK-LABEL: @test(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = sub i3 [[ARG:%.*]], -4
+; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = sub i3 [[ARG:%.*]], 1
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i3 [[SWITCH_TABLEIDX]], -2
+; CHECK-NEXT:    br i1 [[TMP0]], label [[SWITCH_LOOKUP:%.*]], label [[DEFAULT:%.*]]
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nikic wrote:

This seems like a problematic regression, because it introduces a fallback branch. Why does this happen?

https://github.com/llvm/llvm-project/pull/67885