Missed optimization of switch to arithmetic

[ojeda]

From: rust-lang/rust#116272

target datalayout = "n8:16:32:64"

define i32 @src(i8 %0) {
start:
  switch i8 %0, label %bb2 [
    i8 0, label %bb5
    i8 1, label %bb4
    i8 -1, label %bb1
  ]

bb2:                                              ; preds = %start
  unreachable

bb4:                                              ; preds = %start
  br label %bb5

bb1:                                              ; preds = %start
  br label %bb5

bb5:                                              ; preds = %start, %bb1, %bb4
  %.0 = phi i32 [ 255, %bb1 ], [ 1, %bb4 ], [ 0, %start ]
  ret i32 %.0
}

define i32 @tgt(i8) {
  %_2 = zext i8 %0 to i32
  ret i32 %_2
}

https://alive2.llvm.org/ce/z/NvmXJk

[nikic]

I'll close this as won't fix for the time being, as this should be addressed during the initial switch transform instead. It's of course possible to optimize the lookup table as well, but we shouldn't do so unless and until there is cause to do so.

[nikic]

Eh, I updated the test case to start with the switch instead.

[ojeda]

Makes sense -- thanks! Yeah, rustc uses a switch originally.