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

[Chris Lattner]

On May 13, 2009, at 5:26 AM, Duncan Sands wrote:

> Hi Richard,
>
>> -mpc64 sets the x87 floating point control register to not use the  
>> 80bit
>> extended precision.  This causes some x87 floating point operations
>> to operate faster and there are no issues with the extra roundings  
>> you
>> get when storing an 80bit precision register to a 64bit memory  
>> location.

However, this does break long double, right?

>> Does LLVM support x87 arithmetic at all or does it default to SSE
>> arithmetic in 32bits?  I guess using SSE math for both would be a  
>> more
>> fair comparison?
>
> LLVM does support the x86 floating point stack, though it doesn't  
> support
> all asm expressions for it (which is why llvm-gcc disables math  
> inlines).
> My understanding is that no effort has been made to produce optimal  
> code
> when using the x86 fp stack, and all the effort when into SSE instead.

As long as you configure llvm-gcc with --with-arch=pentium-4 or later,  
you should be fine.  I personally use --with-arch=nocona.

-Chris