online compiler assembly tricks for you

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sylware

Joined: 23 Oct 2020
Posts: 461
Location: Marseille/France

sylware 17 Oct 2022, 11:42

https://godbolt.org/noscript

You don't have to deal with the HORRIBLE AND INSANE SDK of those compilers.

17 Oct 2022, 11:42

Furs

Joined: 04 Mar 2016
Posts: 2564

Furs 17 Oct 2022, 13:18

They added noscript page, cool.

17 Oct 2022, 13:18

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 17 Oct 2022, 14:39

It works on my JS-free browser. That is good. Smile

17 Oct 2022, 14:39

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 17 Oct 2022, 14:45

Haha, I tried to default "return num * num;" code for ARM gcc trunk, and got this monstrosity

Code:

square(int):
        push    {r7}
        sub     sp, sp, #12
        add     r7, sp, #0
        str     r0, [r7, #4]
        ldr     r3, [r7, #4]
        mul     r3, r3, r3
        mov     r0, r3
        adds    r7, r7, #12
        mov     sp, r7
        ldr     r7, [sp], #4
        bx      lr

Why not simply

Code:

square(int):
        mul     r0, r0, r0
        bx      lr

Any normal person wouldn't even make it a function. It is just a single instruction.

17 Oct 2022, 14:45

sylware

Joined: 23 Oct 2020
Posts: 461
Location: Marseille/France

sylware 17 Oct 2022, 15:01

revolution wrote:

Any normal person wouldn't even make it a function. It is just a single instruction.

That makes me think about memcpy and memset which are now, on x86_64, accelerated using "rep movsX" and "rep stosX", then which should not be into the libc but generated inline by compilers.

17 Oct 2022, 15:01

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 17 Oct 2022, 15:08

Are there still HLL coders out there claiming the compilers can produce the best possible code ever, and how humans can't possibly do any better?

Just show them these results. Laughing

17 Oct 2022, 15:08

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 17 Oct 2022, 21:13

They’ll just say it’s a corner case. Religion and HLL-oriented propaganda are still things.

17 Oct 2022, 21:13

Furs

Joined: 04 Mar 2016
Posts: 2564

Furs 18 Oct 2022, 13:24

revolution wrote:

Haha, I tried to default "return num * num;" code for ARM gcc trunk, and got this monstrosity

Code:

square(int):
        push    {r7}
        sub     sp, sp, #12
        add     r7, sp, #0
        str     r0, [r7, #4]
        ldr     r3, [r7, #4]
        mul     r3, r3, r3
        mov     r0, r3
        adds    r7, r7, #12
        mov     sp, r7
        ldr     r7, [sp], #4
        bx      lr

Why not simply

Code:

square(int):
        mul     r0, r0, r0
        bx      lr

Any normal person wouldn't even make it a function. It is just a single instruction.

I don't know about ARM, but did you enable optimizations? Since this would be trivial for a compiler.

i.e. pass -Ofast (or more typically -O2, or -O3) on the command line.

18 Oct 2022, 13:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 18 Oct 2022, 13:31

Furs wrote:

... did you enable optimizations? Since this would be trivial for a compiler.

i.e. pass -Ofast (or more typically -O2, or -O3) on the command line.

I didn't. But I tried all three of your suggestions just now, and all three return exactly the two line code mul+bx.

18 Oct 2022, 13:31

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 19 Oct 2022, 06:12

Using -O3, this is suboptimal

Input:

Code:

int square(int num) {
    return num ^ (num << num);
}

Output

Code:

square(int):
        lsl     r3, r0, r0
        eors    r0, r0, r3
        bx      lr

Can be:

Code:

        eor     r0, r0, r0, lsl r0
        bx      lr

19 Oct 2022, 06:12

Furs

Joined: 04 Mar 2016
Posts: 2564

Furs 19 Oct 2022, 13:12

Sounds like a typical issue when it doesn't recognize more complicated patterns. At the instruction level passes (RTL stages in GCC) compiler optimizers are usually just pattern matching.

There's a lot of optimization passes before that (GIMPLE stages in GCC) which deal with overall code optimizations, not specific to a target instruction set, like removing redundant code and simplifying stuff, of course.

19 Oct 2022, 13:12

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 19 Oct 2022, 13:31

It knows the instruction with a fixed shift:

Code:

    return num ^ (num << 1);

Code:

        eor     r0, r0, r0, lsl #1
        bx      lr

Note the change from eors to eor. So it probably comes from a different part of the code.

19 Oct 2022, 13:31

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 25 Oct 2022, 11:57

This is also suboptimal.

Code:

int square(int num) {
    return (num << num) ^ 0xffffffff ;
}

Code:

square(int):
        lsls    r0, r0, r0
        mvns    r0, r0
        bx      lr

Can be

Code:

square(int):
        mvn     r0, r0, lsl r0
        bx      lr

Also, excessive and unnecessary updates to the flags can use more power, draining the battery faster.

This is why your "smart"phone needs recharging every 10 minutes. Bad code generation. Razz

25 Oct 2022, 11:57

Furs

Joined: 04 Mar 2016
Posts: 2564

Furs 25 Oct 2022, 13:03

revolution wrote:

This is why your "smart"phone needs recharging every 10 minutes. Bad code generation.

Facts.

25 Oct 2022, 13:03

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20450
Location: In your JS exploiting you and your system

revolution 26 Oct 2022, 05:38

It appears that arm gcc by default starts in thumb mode. So some of the above code might be because those instructions are not available.

So now using options -marm -O3 we can see this;

Code:

int square(int num) {
    return num * num > 0 ? num * num + 1 : num * num - 1 ;
}

Code:

square(int):
        mul     r0, r0, r0
        cmp     r0, #0
        addne   r0, r0, #1
        mvneq   r0, #0
        bx      lr

Can be

Code:

        muls    r0, r0, r0
        addne   r0, r0, #1
        mvneq   r0, #0
        bx      lr

26 Oct 2022, 05:38

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