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Re: Change x86 default arch for 4.5?


On Feb 21, 2010, at 17:42, Erik Trulsson wrote:
> Newer compilers usually have better generic optimizations that are not
> CPU-dependent.  Newer compilers also typically have improved support
> for new language-features (and new languages for that matter.)

This is exactly where CPU dependence comes into play. I'd like
for example take advantage of IEEE floating point semantics in
our Ada compiler. Fifteen years ago, we had to face the fact that
some systems were not compliant and therefore we developed
least-common-denominator libraries that catered to all.

However, nowadays even OpenVMS supports IEEE floating point.
I'd like to get rid of hacks in GNAT avoid relying on strict
IEEE compliance. Currently we end up doing some conversions
and computations in the widest floating-point type to avoid
double rounding.

So, I'd like to be able to put all this behind me, and only
have to think about fully IEEE compliant systems. Similarly,
there is no way to fully implement C99 Annex F without using
SSE2.

As a last point, because not many GCC users take advantage
of the hardware their systems have, there is less focus
and feedback on improving SSE2 code generation, vectorization 
and the like.

In conclusion, I think GCC is being held back by us sticking
to 20-year old hardware. For GCC to be the best compiler 
possible on current and future systems, we have to start
compiling for those systems by default.

This is essential for GCC's long term relevance.

  -Geert


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