[LLVMdev] Odd PPC inline asm constraint

[Krzysztof Parzyszek]

The only way to access FPSCR it to move its contents to a FP register.  
Since PPC doesn't really have FPR-to-GPR instructions, you will end up 
with a store followed by a load.

-K


On 5/10/2012 7:45 AM, Hal Finkel wrote:
> 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
>>
>>
>>
>
>