[Bug middle-end/21628] GCC much slower than ICL. Lack of inlining?

[laurent at ient dot rwth-aachen dot de]

------- Comment #6 from laurent at ient dot rwth-aachen dot de  2007-11-16 20:42 -------
> Note that for completely inlining kernels you can use the
> __attribute__((flatten))
> on the *calling* function.  Usually with expression templates that is the
> function
> containing the loops, like
> void __attribute__((flatten)) doit()
> {
>   for (;;)
>     lots_of_calls_to_inline ();
> }
> and it will make sure to inline all calls done in doit (recursively, so no
> calls
> will be left in the final version).  Also starting with GCC 4.2 (and much
> improved on trunk which will become 4.3) using profile-feedback will
> improve inline performance a lot (use -fprofile-generate, run, -fprofile-use).
Good to know! Thanks for the advices!

> But we are better in freedom. ;-)
Much better!

> OK. So this is fixed. Thanks for the report nonetheless. And sorry for the
> delay.
No problemo. Thank to all of you.

> I don't think all the inlining improvements (many) can be traced back to any
> specific individual complaining (not even Linus Torvalds ;)
(Ups! sorry for having misspelled the name of Linus Torvalds!)
You are most probably right. I was nevertheless happy to notice I was not alone
to complain about the problem. 

> Details would be certainly welcome. Ideally, a reduced snippet, to pursue the
> optimization people to take action reasonably quickly...
Hmm, difficult. 
I just sometimes compare execution speed of numerical calculations from
different compilers (ICL,VC2005,GCC), and ICL is often quicker by maybe 10%.
If I have more specific and easier examples, I'll post them.

I especially appreciate the way GCC notifies the compilation errors from deep
nested templates. I could not have programmed deep nested template expression
with the complicated error messages form ICL or VC2005.

I have to say that ICL has obviously not respected the __forceinline directive
any more since the version 9 and 10, this is for me a clear regression.
I do not know exactly the changes in these latest versions, but I do not want
to exchange with my good old version 8.1.

Thanks


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21628