91f7c60d2d
By putting an "unreachable" instruction into the default arm of a switch
instruction we can let LLVM know that the match is exhaustive, allowing
for better optimizations.
For example, this match:
```rust
pub enum Enum {
One,
Two,
Three,
}
impl Enum {
pub fn get_disc(self) -> u8 {
match self {
Enum::One => 0,
Enum::Two => 1,
Enum::Three => 2,
}
}
}
```
Currently compiles to this on x86_64:
```asm
.cfi_startproc
movzbl %dil, %ecx
cmpl $1, %ecx
setne %al
testb %cl, %cl
je .LBB0_2
incb %al
movb %al, %dil
.LBB0_2:
movb %dil, %al
retq
.Lfunc_end0:
```
But with this change we get:
```asm
.cfi_startproc
movb %dil, %al
retq
.Lfunc_end0:
```
30 lines
813 B
Rust
30 lines
813 B
Rust
// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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// compile-flags: -C no-prepopulate-passes
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#![crate_type = "lib"]
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pub enum E {
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A,
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B,
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}
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// CHECK-LABEL: @exhaustive_match
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#[no_mangle]
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pub fn exhaustive_match(e: E) {
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// CHECK: switch{{.*}}, label %[[DEFAULT:[a-zA-Z0-9_]+]]
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// CHECK: [[DEFAULT]]:
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// CHECK-NEXT: unreachable
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match e {
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E::A => (),
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E::B => (),
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}
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}
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