Closed
Description
This is a kind of enhancement suggestion. Given this function:
pub fn foo7(n: u32, v: u32) -> Option<u32> {
n.checked_div(v + 1)
}Compiled wih rustc 1.46.0-nightly (67100f6 2020-06-24) with good optimization flags gives:
example::foo7:
inc esi
jne .LBB0_3
xor eax, eax
ret
.LBB0_3:
mov eax, edi
xor edx, edx
div esi
mov edx, eax
mov eax, 1
retBut I think the happy path should be with the denominator != 0 (that hopefully is the most common case), instead of the case with a division by zero.
So I've done few more experiments:
#![feature(core_intrinsics)]
use std::intrinsics::{unchecked_div, unlikely, likely};
pub fn checked_div1(this: u32, rhs: u32) -> Option<u32> {
match rhs {
0 => None,
rhs => Some(unsafe { unchecked_div(this, rhs) }),
}
}
pub fn foo1(n: u32, v: u32) -> Option<u32> {
checked_div1(n, v + 1)
}
pub fn checked_div2(this: u32, rhs: u32) -> Option<u32> {
if rhs != 0 {
Some(unsafe { unchecked_div(this, rhs) })
} else {
None
}
}
pub fn foo2(n: u32, v: u32) -> Option<u32> {
checked_div2(n, v + 1)
}
pub fn checked_div3(this: u32, rhs: u32) -> Option<u32> {
if unlikely(rhs == 0) {
None
} else {
Some(unsafe { unchecked_div(this, rhs) })
}
}
pub fn foo3(n: u32, v: u32) -> Option<u32> {
checked_div3(n, v + 1)
}
pub fn foo4(n: u32, v: u32) -> Option<u32> {
if likely(v + 1 != 0) {
Some(unsafe { unchecked_div(n, v + 1) })
} else {
None
}
}The asm:
example::checked_div1:
test esi, esi
je .LBB0_1
mov eax, edi
xor edx, edx
div esi
mov edx, eax
mov eax, 1
ret
.LBB0_1:
xor eax, eax
ret
example::foo1:
inc esi
jne .LBB1_2
xor eax, eax
ret
.LBB1_2:
mov eax, edi
xor edx, edx
div esi
mov edx, eax
mov eax, 1
retThis issue has been assigned to @nbdd0121 via this comment.