[llvm] [SimplifyCFG]: Switch on umin replaces default (PR #164097)

[via llvm-commits]

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@@ -0,0 +1,246 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -S -passes=simplifycfg < %s | FileCheck %s
+
+declare void @a()
+declare void @b()
+declare void @c()
+declare void @d()
+
+define void @switch_replace_default(i32 %x) {
+; CHECK-LABEL: define void @switch_replace_default(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 3)
+; CHECK-NEXT:    switch i32 [[X]], label %[[COMMON_RET:.*]] [
+; CHECK-NEXT:      i32 0, label %[[CASE0:.*]]
+; CHECK-NEXT:      i32 1, label %[[CASE1:.*]]
+; CHECK-NEXT:      i32 2, label %[[CASE2:.*]]
+; CHECK-NEXT:    ], !prof [[PROF0:![0-9]+]]
+; CHECK:       [[COMMON_RET]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[CASE0]]:
+; CHECK-NEXT:    call void @a()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[CASE1]]:
+; CHECK-NEXT:    call void @b()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+; CHECK:       [[CASE2]]:
+; CHECK-NEXT:    call void @c()
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+;
+  %min = call i32 @llvm.umin.i32(i32 %x, i32 3)
+  switch i32 %min, label %unreachable [
+  i32 0, label %case0
+  i32 1, label %case1
+  i32 2, label %case2
+  i32 3, label %case3
+  ], !prof !0
+
+case0:
+  call void @a()
+  ret void
+
+case1:
+  call void @b()
+  ret void
+
+case2:
+  call void @c()
+  ret void
+
+case3:
+  ret void
+
+unreachable:
+  unreachable
+}
+
+define void @switch_replace_default_and_remove_dead_cases(i32 %x) {
+; CHECK-LABEL: define void @switch_replace_default_and_remove_dead_cases(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    [[MIN:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 3)
----------------
kper wrote:

We already update the condition:
`SIW->setCondition(A)`
So, we already do what the proof shows. The only thing left is to remove the umin.

I think, we just update and the original "umin" becomes orphaned. Because the .ll already has the updated condition.
When I run `opt -O3`, the orphan instruction gets removed. However, the .ll runs `RUN: opt -S -passes=simplifycfg < %s | FileCheck %s` which doesn't remove it.
I wonder how to remove it manually? I tried `SIW->getCondition()->replaceAllUsesWith(A);` but the result remains the same.

https://github.com/llvm/llvm-project/pull/164097