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


Paolo Bonzini wrote:

Rather, we should seriously understand what caused the compilation time jump in 4.2, and whether those are still a problem. We made a good job in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with speedups from other changes; and 4.4 while slower than 4.3 at least stays below 4.2. But, 4.2 was a disaster for compilation time.

It was interesting for me that GCC4.4 is faster than GCC4.3 on Intel Core I7. This is not true for most other processors (at least for P4, Core2, Power4/5/6 etc). Intel Core I7 has 3rd level big and fast cache and memory controller on the chip. I guess that slowdown in GCC is mostly because of data and/or code locality.

Fortunately, GCC4.5 will be 5% faster because of Richard Guenter's work on improving aliasing (of course if it will be not eaten by a new optimization). People are complaining about GCC compilation speed and of course we should work on its speedup. But GCC is not so bad, for example SUN Studio compiler is almost 2 times slower than GCC. IMHO, GCC performace is still #1 priority for us (although people working on embedded processors could disagree with me). I think that because in my experience the performance improvement is much harder to achieve than compilation speed and code size improvements.


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