Date: Sun, 6 Jan 2019 08:54:08 -0800 From: Brian Neal <brian@aceshardware.com> To: Alan Somers <asomers@freebsd.org> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>, Garance A Drosehn <drosih@rpi.edu> Subject: Re: Speculative: Rust for base system components Message-ID: <26325c0b-4960-7739-72aa-c31c4e0638d3@aceshardware.com> In-Reply-To: <CAOtMX2gPrpO2O70pnwpU80bXMXo6LB9PCkU3bYw2ToMMnhf%2Bww@mail.gmail.com> References: <ca76e5f7-6e59-bd67-144a-90ad66f0252e@metricspace.net> <7d7bc47d-04cf-2f9b-00a3-e3d9d92b3623@aceshardware.com> <72922F2C-9D27-47AA-BB1C-2DA8589CF008@rpi.edu> <92bd5362-d898-aa12-8f3d-9fbe23f38e0c@aceshardware.com> <CAOtMX2gPrpO2O70pnwpU80bXMXo6LB9PCkU3bYw2ToMMnhf%2Bww@mail.gmail.com>
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I don't have it anymore, but yours is close enough. When compiling your
first example, I get 156 instructions, including map iterators and rust
result assert/unwrap logic. Your second example produces 21 lines,
including a loop iterator:
rustc 1.31.0 with -C opt-level=s:
<I as core::iter::traits::IntoIterator>::into_iter:
mov eax, edi
mov edx, esi
ret
example::oddcount2:
xor eax, eax
test edi, edi
jle .LBB1_3
xor ecx, ecx
.LBB1_2:
mov edx, ecx
and edx, 1
neg edx
and edx, ecx
lea esi, [rcx + 1]
add eax, edx
mov ecx, esi
cmp edi, esi
jne .LBB1_2
.LBB1_3:
ret
Converting to C:
int oddcount2(int num) {
int sum = 0;
for (int i = 0; i <= num; ++i)
{
if (i % 2 > 0)
{
sum += i;
}
}
return sum;
}
I get the following with gcc 8.2 using -Os:
oddcount2(int):
xor edx, edx
xor eax, eax
.L4:
cmp edx, edi
jg .L1
test dl, 1
je .L3
add eax, edx
.L3:
inc edx
jmp .L4
.L1:
ret
Cheers,
-Brian
On 1/6/2019 8:01 AM, Alan Somers wrote:
> I get different results on Godbolt. I don't know exactly what your
> program was, but I tried to recreate it from your description. I
> wrote it in two ways. With opt-level=s, I got 13 instructions for one
> version, and 16 for the other. With opt-level=3, I got vectorized
> code for both. Here's my code:
>
> pub fn oddcount(num: i32) -> i32 {
> (0..num).map(|i| {
> if i % 2 > 0 {
> i
> } else {
> 0
> }
> }).sum()
> }
>
> pub fn oddcount2(num: i32) -> i32 {
> let mut sum = 0;
> for i in 0..num {
> if i % 2 > 0 {
> sum += i;
> }
> }
> sum
> }
>
> -Alan
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