Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Evan Cheng <evan.cheng@apple.com>]

On May 13, 2009, at 4:51 AM, Duncan Sands wrote:

> Hi,
>
>> Sorry, I missed to mention that I used an additional option -mpc64  
>> for
>> 32-bit GCC4.4.  It is not possible to generate SPECFP2000 expected
>> results  by GCC4.4 without this option. LLVM does not support this
>> option.  And this option can significantly improve the  
>> performance.  So
>> 32-bit comparison of SPECFP2000 should be taken with a grain of salt.
>
> what does -mpc64 do exactly?  The gcc docs say:
>  `-mpc64' rounds the the significands of results of floating-point  
> operations to 53 bits (double precision)
> Does this mean that a rounding operation is performed after each fp
> operation, or that optimizations are permitted that don't result in
> accurate extended double precision values as long as they are correct
> to 53 bits, or something else?
>
> The LLVM code generators have an option called -limit-float-precision:
>  -limit-float-precision=<uint>       - Generate low-precision inline  
> sequences for some float libcalls
> I'm not sure what it does exactly, but perhaps it is similar to - 
> mpc64?

No, that inline a small set of libcalls into sequences code that  
implement low precision math (6, 8, 12 bits).

Evan

>
> Ciao,
>
> Duncan.