Re: Using CPU flags in C

I don't think the OP is subscribed, so if you don't use Reply-All they
might not see your replies.

On Wed, 19 Nov 2025 at 10:31, Matthias Pfaller <[email protected]> wrote:
>
> How about googling for "=@ccz" __asm__? One of the top hits is a six year old
> stackoverflow thread. Its funny, Florian participated in that thread if I'm not
> mistaken...
>
> 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
>