Poor optimization of negative signs

[dtolnay]

#[inline(never)]
fn repro(buffer: &mut itoa::Buffer, n: i32) -> &str {
    buffer.format(n)
}

fn main() {
    println!("{}", repro(&mut itoa::Buffer::new(), i32::MIN));
}

The x86_64 assembly for repro is:

mov rax, rdi
mov edx, 11
movabs rcx, 3905527098775974194
mov word ptr [rdi + 9], 14388
mov qword ptr [rdi + 1], rcx
mov byte ptr [rdi], 45
ret

The string returned by repro is "-2147483648". The four immediates visible in the assembly are:

• 11: string length
• 3905527098775974194: first 8 digits 0x3633383437343132
• 14388: last 2 digits 0x3834
• 45: ASCII '-'

A better implementation might be able to compile to just 3 immediates.

[dtolnay]

Desired output:

mov rax, rdi
mov edx, 11
movabs rcx, 3690757306333606445
mov dword ptr [rdi + 7], 942945843
mov qword ptr [rdi], rcx
ret