[llvm] [SimplifyCFG] Add optimization for switches of powers of two (PR #70977)

[via llvm-commits]

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@@ -0,0 +1,139 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt %s -passes='simplifycfg<switch-to-lookup>' -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; Check that the range of switch of powers of two is reduced and switch itself is lowered to jump-table
+define i32 @switch_of_powers(i32 %x) {
+; CHECK-LABEL: define i32 @switch_of_powers(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.cttz.i32(i32 [[X]], i1 true)
+; CHECK-NEXT:    [[SWITCH_MASKINDEX:%.*]] = trunc i32 [[TMP0]] to i8
+; CHECK-NEXT:    [[SWITCH_SHIFTED:%.*]] = lshr i8 121, [[SWITCH_MASKINDEX]]
+; CHECK-NEXT:    [[SWITCH_LOBIT:%.*]] = trunc i8 [[SWITCH_SHIFTED]] to i1
+; CHECK-NEXT:    call void @llvm.assume(i1 [[SWITCH_LOBIT]])
+; CHECK-NEXT:    [[SWITCH_GEP:%.*]] = getelementptr inbounds [7 x i32], ptr @switch.table.switch_of_powers, i32 0, i32 [[TMP0]]
+; CHECK-NEXT:    [[SWITCH_LOAD:%.*]] = load i32, ptr [[SWITCH_GEP]], align 4
+; CHECK-NEXT:    ret i32 [[SWITCH_LOAD]]
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DKay7 wrote:

In that particular case, `ReduceSwitchRange()` wouldn't do anything because it can't produce dense switch

https://github.com/llvm/llvm-project/pull/70977