Compilation time in gcc-4.4.0

[Richard Guenther]

On Tue, Jul 13, 2010 at 5:26 PM, Paolo Bonzini <bonzini@gnu.org> wrote:
> On 07/13/2010 04:53 PM, Richard Guenther wrote:
>>
>> On Tue, Jul 13, 2010 at 4:50 PM, Paolo Bonzini<bonzini@gnu.org>  wrote:
>>>
>>> On 07/08/2010 10:58 PM, Maxiwell Garcia wrote:
>>>>
>>>> Hi,
>>>>
>>>> I am writing a paper about instruction-set architecture simulators. In
>>>> first time, I used gcc-4.4.0 and the compilation time reached 33
>>>> minutes (with -O3) for my simulator and the performance reached 270
>>>> MIPS (Million instruction per second). When I used the gcc-4.4.4, in
>>>> the same code, the compilation time reached 39 seconds and the
>>>> performance reached 600 MIPS. My code have many "switchs" with 512
>>>> "cases" and the library<systemc.h>    is in use.
>>>> How to explain this behavior in the compilation and performance in my
>>>> paper?
>>>
>>> Try -ftime-report, it should explain the compilation time change.  Such a
>>> dramatic performance change is usually due to better register allocation,
>>> but this is just a guess.
>>
>> I would guess it was the various backports of tree PRE fixes actually.
>
> Sorry, I was referring to the change in performance which I justified by
> better register allocation.  PRE is indeed a good candidate for the
> compilation time fixes.  Could the changes to PRE cause different code to be
> generated?

Yes, they will generally cause more PRE to be performed.  What ends up
generating the noticeable performance difference depends on the specific
testcase though - and it might very well be RA (and 4.4.0 to 4.4.4 is a
significant amount of changes).

Richard.

> Paolo
>