Re: identifying c++ aliasing violations

[Dale Johannesen <dalej at apple dot com>]

On Dec 5, 2005, at 12:03 AM, Giovanni Bajo wrote:
> Jack Howarth <howarth@bromo.msbb.uc.edu> wrote:
> > What exactly is the implication of having a hundred or more of this in
> > an application being built with gcc/g++ 4.x at -O3? Does it only risk
> > random crashes in the generated code or does it also impact the
> > quality
> > of the generated code in terms of execution speed?
>
> The main problem is wrong-code generation. Assuming the warning is 
> right and
> does not mark false positives, you should have those fixed. I don't 
> think
> quality of the generated code would be better with this change.
>
> However, it's pretty strange that C++ code generation is worse with 
> GCC 4: I
> saw many C++ programs which actually got much faster due to higher 
> lever
> optimazations (such as SRA). You should really try and identify inner 
> loops
> which might have been slowed down and submit those as bugreports in our
> Bugzilla.

Could also be inlining differences, and you might check out whether
-fno-threadsafe-statics is applicable; that can make a big difference.
Bottom line, you're going to have to do some analysis to figure out
why it got slower.  (It sounds like you're on a MacOSX system, in
which case Shark is a good tool for this.)