Open
Description
I tried this code (godbolt):
extern "Rust" {
fn foo() -> u32;
fn bar() -> u32;
fn baz() -> u32;
}
pub fn src(x: u32, y: u32) -> u32 {
unsafe {
match (x, y) {
(1, 10) => foo(),
(2, 10) => bar(),
(3, 10) => baz(),
_ => 0,
}
}
}
pub fn tgt(x: u32, y: u32) -> u32 {
unsafe {
match (y, x) {
(10, 1) => foo(),
(10, 2) => bar(),
(10, 3) => baz(),
_ => 0,
}
}
}I expected to see this happen:
src should produce assembly as efficient as tgt, since they are both equivalent on all inputs. tgt first checks y against 10, then checks x against 1, 2 and 3:
example::tgt:
cmp w1, #10
b.ne .LBB1_5
cmp w0, #3
b.eq .LBB1_6
cmp w0, #2
b.eq .LBB1_7
cmp w0, #1
b.ne .LBB1_5
b foo
.LBB1_5:
mov w0, wzr
ret
.LBB1_6:
b baz
.LBB1_7:
b bar
Instead, this happened:
src checks x against 3, then checks y against 10, then checks x against 2, then checks y against 10, then checks x against 1, then checks y against 10:
example::src:
cmp w0, #3
b.eq .LBB0_5
cmp w0, #2
b.eq .LBB0_7
cmp w0, #1
b.ne .LBB0_9
cmp w1, #10
b.ne .LBB0_9
b foo
.LBB0_5:
cmp w1, #10
b.ne .LBB0_9
b baz
.LBB0_7:
cmp w1, #10
b.ne .LBB0_9
b bar
.LBB0_9:
mov w0, wzr
ret
Meta
rustc --version --verbose:
rustc 1.76.0-nightly (6b771f6b5 2023-11-15)
binary: rustc
commit-hash: 6b771f6b5a6c8b03b6322a9c77ac77cb346148f0
commit-date: 2023-11-15
host: x86_64-unknown-linux-gnu
release: 1.76.0-nightly
LLVM version: 17.0.5
Backtrace
<backtrace>