x87 register clobber returns "unknown register name" error

Segher Boessenkool segher@kernel.crashing.org
Thu Oct 15 09:29:00 GMT 2015


On Thu, Oct 15, 2015 at 09:59:03AM +0200, Hadrien Grasland wrote:
> I am trying to write inline assembly code that uses the x86 floating
> point stack.
> 
> As far as I understood
> https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Clobbers , the
> proper register clobber to use in this case is "st(x)". For example,
> this example from the documentation uses the "st(1)" clobber :
> 
> asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
> 
> However, if I try it on the trivial example attached, it fails with the
> following error.
> 
> $ gcc-4.8 display_representation.c
> display_representation.c: In function ‘main’:
> display_representation.c:11:5: error: unknown register name ‘st(0)’ in ‘asm’
>      asm volatile ("fstps (%1)" :
>      ^
> 
> What am I doing wrong ?

I'm not sure you can clobber stack regs at all.  You could add it as
an output and just not use the result?  Like

asm ("fyl2xp1" : "=t" (result), "=u" (dummy) : "0" (x), "1" (y));


Segher



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