Description
I was experimenting with some code on godbolt, when encountering this:
pub fn u16_be_to_arch(mut data: [u8;2]) -> [u8;2] {
data.reverse();
data
}with -C opt-level=3 produces the expected assembly:
example::u16_be_to_arch:
mov eax, edi
rol ax, 8
retHowever, when adding (into the same file) this function:
pub fn u32_be_to_arch(mut data: [u8;4]) -> [u8;4] {
data.reverse();
data
}The previous function's codegen changes to
example::u16_be_to_arch:
sub rsp, 2
mov eax, edi
rol ax, 8
mov word ptr [rsp], ax
add rsp, 2
rethttps://godbolt.org/z/MWTar9xa3
https://godbolt.org/z/sKTznrKh9
The issue: it stores the result on the stack, and then immediately discards that. (which it didn't do previously)
The second potential issue I'm unsure about is, that here a function's codegen changes by just adding removing seemingly unrelated functions. I was told on discord this is because the two functions use the same reverse, and adding the second one forces reverse to be generalized to [u8;2] and [u8;4] as opposed to just [u8;2]. But I by far don't know enough to judge whether this is a problem or not, or what it's caused by, but I wanted to mention it.