Subject: Re: [PATCH 2/9] powerpc: Add macros to access floating point registers in thread_struct.

From: Scott Wood <[email-masked]>
Date: Wed, 25 Jun 2008 11:17:45 -0500

Gabriel Paubert wrote:
> On Wed, Jun 25, 2008 at 10:34:32AM -0500, Scott Wood wrote:
>> Kumar Gala wrote:
>>>> +/* Macros to workout the correct index for the FPR in the thread 
>>>> struct */
>>>> +#define FPRNUMBER(i) (((i) - PT_FPR0) >> 1)
>>>> +#define FPRHALF(i) (((i) - PT_FPR0) % 2)
>>> Have you looked at what the compiler spits out here to make sure we 
>>> aren't getting a divide?  Seems like we could use '& 0x1'.
>> GCC's not *that* dumb.  However, you may get some unnecessary 
>> sign-twiddling if "i" is signed.
> 
> Not for modulo 2, it's only an even/odd choice and GCC 
> implements that efficiently IIRC. For other powers of 2,
> making the left hand side unsigned helps the compiler.

 From this:

int foo(int x)
{
	return x % 2;
}

I get this with -O3:

foo:
         mr 0,3
         srawi 3,3,1
         addze 3,3
         slwi 3,3,1
         subf 3,3,0
         blr
         .size   foo, .-foo
         .ident  "GCC: (GNU) 4.1.2"

Changing it to "x & 1", or to unsigned, gives this:

foo:
         rlwinm 3,3,0,31,31
         blr
         .size   foo, .-foo
         .ident  "GCC: (GNU) 4.1.2"

Maybe newer GCCs are better?

-Scott