[LLVMdev] Odd PPC inline asm constraint

[Hal Finkel]

Peter,

Could you please comment on:
http://llvm.org/bugs/show_bug.cgi?id=12757

Specifically, gcc seems to allow this:
int __flt_rounds() {
 unsigned long fpscr;
 __asm__ volatile("mffs %0" : "=f"(fpscr));
 return fpscr;
}

My reading of this is that gcc allocates a floating-point register to
hold the result of the mffs instruction, and then bit casts (and
truncates?) the result into the unsigned long variable. Is this
correct, and if so, is this a general gcc feature, or something PowerPC
specific?

Thanks again,
Hal

On Fri, 27 Apr 2012 19:33:58 -0500
Peter Bergner <bergner at vnet.ibm.com> wrote:

> On Fri, 2012-04-27 at 14:54 -0500, Hal Finkel wrote:
> > There is a comment in the file which reads:
> > 
> > /* The weird 'i#*X' constraints on the following suppress a gcc
> >    warning when __excepts is not a constant.  Otherwise, they mean
> > the same as just plain 'i'.  */
> [sinp]
> >  ("mtfsb0 %s0"  : : "i#*X"(__builtin_ffs (__excepts)));
> [snip]
> > Does anyone know what that "weird" asm constraint actually means?
> 
> 
> The "i" and "X" constraints are documented here:
> 
>   http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Simple-Constraints.html
> 
> `i'
>     An immediate integer operand (one with constant value) is allowed.
>     This includes symbolic constants whose values will be known only
>     at assembly time or later.
> 
> `X'
>     Any operand whatsoever is allowed.
> 
> 
> The # and * constraint modifiers are documented here:
> 
>   http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Modifiers.html
> 
> `#'
>     Says that all following characters, up to the next comma, are to
>     be ignored as a constraint. They are significant only for choosing
>     register preferences.
> 
> `*'
>     Says that the following character should be ignored when choosing
>     register preferences. `*' has no effect on the meaning of the
> constraint as a constraint, and no effect on reloading. 
> 
> For more info about PowerPC specific constraints, you'll want to look
> here:
> 
>   http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Machine-Constraints.html
> 
> 
> I'll note that the "s" in the %s0 means to only print the low 5 bits
> of operand 0.  I think that may only be documented in the src:
> 
>   gcc/config/rs6000/rs6000.c:print_operand()
> 
> 
> Peter
> 
> 
> 



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory