Re: Using CPU flags in C

On the question about arm support I would suggest calling "info gcc" and searching 
for "@cc". It would be a real shame not to use the nice documentation the gcc 
developers provide.

regards, Matthias

On 2025-11-19 09:54, jh--- via Gcc-help wrote:
> Thank you for your answer.
> I'm not sure I understand it completely, could you give me some details?
> Le 2025-11-18 19:42, Florian Weimer a écrit :
>> * jh:
>>
>>> I don't really know where to look for this information.
>>> Is it possible to use the CPU status flags to communicate information
>>> between inline assembly and the normal C control flow?
>>> For example, an asm statement would set the Z flag and then the C code
>>> would jump (for example with beq or bne in the case of ARM) without
>>> using any additional variable and comparison instruction. Using a
>>> dummy variable is no issue if the generated code is just the
>>> conditional branch instruction.
>>
>> Do you mean something like this?
>>
>> commit 119d658ac2aad88e306b4a66c1717e5ebf86c73f
>> Author: Uros Bizjak <[email protected]>
>> Date:   Fri Aug 29 09:05:23 2025 +0200
>>
>>     x86: Use flag output operands for inline asm in atomic-machine.h
>>
>>     Use the flag output constraints feature available in gcc 6+
>>     ("=@cc<cond>") instead of explicitly setting a boolean variable
>>     with SETcc instruction.  This approach decouples the instruction
>>     that sets the flags from the code that consumes them, allowing
>>     the compiler to create better code when working with flags users.
> Does this "=@cc<cond>" also work for ARM? Is the usage of the conditions documented 
> somewhere (like Z is ==0, N==V is signed >=0). I know where to find the ARM 
> definitions, but is there a place where I can check gcc's interpretation (i.e. 
> which C code generates which flag access)?
>>     Instead of e.g.:
>>
>>        lock add %esi,(%rdi)
>>        sets   %sil
>>        test   %sil,%sil
>>        jne    <...>
>>
>>     the compiler now generates:
>>
>>        lock add %esi,(%rdi)
>>        js     <...>
> How is this code integrated into C? Would it be used like this? But can I get the 
> -6 value at the same time?
> if(asm_add(36, -42)<0){
>     printf("negative\n");
> }else{
>     printf("positive\n");
> }
> If yes, is it possible to at the same time set a register value and the flags and 
> how would that be written?
> For example, I would like to be able to set a register to any value, but when I set 
> it to 0, I would like to set the Z flag (to 1 or 0) and I would like the compiler 
> to jump depending on my Z flag, while being able to use the register value later in 
> the C code (not going through memory as is done in the commit).
>>
>>     No functional changes intended.
>>
>>     Signed-off-by: Uros Bizjak <[email protected]>
>>     Cc: H.J.Lu <[email protected]>
>>     Cc: Florian Weimer <[email protected]>
>>     Cc: Carlos O'Donell <[email protected]>
>>     Reviewed-by: Florian Weimer <[email protected]>
>>
>> <https://sourceware.org/cgit/glibc/commit/?id=119d658ac2aad88e306b4a66c1717e5ebf86c73f> 
>>
>>
>> Thanks,
>> Florian
> Best regards,
> JH


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