[all-commits] [llvm/llvm-project] 7d81e0: [SimplifyCFG] When only one case value is missing, ...

[Quentin Dian via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 7d81e072712f4e6a150561b5538ccebda289aa13
      https://github.com/llvm/llvm-project/commit/7d81e072712f4e6a150561b5538ccebda289aa13
  Author: Quentin Dian <dianqk at dianqk.net>
  Date:   2024-01-03 (Wed, 03 Jan 2024)

  Changed paths:
    M llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    A llvm/test/Transforms/SimplifyCFG/switch-dead-default-lookup-table.ll
    M llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll

  Log Message:
  -----------
  [SimplifyCFG] When only one case value is missing, replace default with that case (#76669)

When the default branch is the last case, we can transform that branch
into a concrete branch with an unreachable default branch.

```llvm
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

define i64 @src(i64 %0) {
  %2 = urem i64 %0, 4
  switch i64 %2, label %5 [
    i64 1, label %3
    i64 2, label %3
    i64 3, label %4
  ]

3:                                                ; preds = %1, %1
  br label %5

4:                                                ; preds = %1
  br label %5

5:                                                ; preds = %1, %4, %3
  %.0 = phi i64 [ 2, %4 ], [ 1, %3 ], [ 0, %1 ]
  ret i64 %.0
}

define i64 @tgt(i64 %0) {
  %2 = urem i64 %0, 4
  switch i64 %2, label %unreachable [
    i64 0, label %5
    i64 1, label %3
    i64 2, label %3
    i64 3, label %4
  ]

unreachable:                              ; preds = %1
  unreachable

3:                                                ; preds = %1, %1
  br label %5

4:                                                ; preds = %1
  br label %5

5:                                                ; preds = %1, %4, %3
  %.0 = phi i64 [ 2, %4 ], [ 1, %3 ], [ 0, %1 ]
  ret i64 %.0
}
```

Alive2: https://alive2.llvm.org/ce/z/Y-PGXv

After transform to a lookup table, I believe `tgt` is better code.

The final instructions are as follows:

```asm
src:                                    # @src
        and     edi, 3
        lea     rax, [rdi - 1]
        cmp     rax, 2
        ja      .LBB0_1
        mov     rax, qword ptr [8*rdi + .Lswitch.table.src-8]
        ret
.LBB0_1:
        xor     eax, eax
        ret
tgt:                                    # @tgt
        and     edi, 3
        mov     rax, qword ptr [8*rdi + .Lswitch.table.tgt]
        ret
.Lswitch.table.src:
        .quad   1                               # 0x1
        .quad   1                               # 0x1
        .quad   2                               # 0x2

.Lswitch.table.tgt:
        .quad   0                               # 0x0
        .quad   1                               # 0x1
        .quad   1                               # 0x1
        .quad   2                               # 0x2
```

Godbolt: https://llvm.godbolt.org/z/borME8znd

Closes #73446.