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Re: Benchmarks gcc 3.0.4 (soon 3.1) vs. Intel C++ 6.0


On Thursday 09 May 2002 09:01, Daniel Berlin wrote:
> On Thu, 9 May 2002, Erik Schnetter wrote:
> > Tim Prince <tprince at computer dot org> wrote:
> > > icc's equivalent to -fstrict-aliasing is -ansi.  It's not a default.
> >
> > I couldn't find anything about aliasing in the description of "-ansi".
>
> It does what he says, however, i can prove it through option dumps (it
> turns on P2OPT_disam_assume_ansi_c)
>
> > However, icc does have a "-falias" option (which is also not the
> > default). The description of "-falias" is unfortunately unusable.
>
> You mean -fno-alias and -fno-fnalias, i thinks.
>
> If you want to see the aliasing/intermediate code dumps for intel's
> compiler, so you can watch what it does at each stage, "-mIPOPT_trace=-1
> -mP1OPT_il0_trace=-1 -mP2OPT_il0_list_source_lines=-1 -mP2OPT_il0_dump=0
> -mIPOPT_opt_trace=-1"  will suffice.
>
> If you want the debug options to print the mod-ref sets and whatnot, let
> me know.
>
> > -erik
Thanks for this information.  I was intending to test whether -ansi affected 
vectorization when I got a chance. Meanwhile, I've noticed that the latest 
Windows ICL documentation says -Qansi is on by default, which I disbelieve, 
since the defaults are often compatible with Microsoft's, and because even 
the Fortran 6.0 assumes argument aliasing, contrary to Fortran standards of 
36 years standing. Also, it would be mighty strange to have it on by default 
in Windows, which is notorious for dirty C, and off for linux, where the gcc 
struggles should have raised awareness of the issues. They agreed to correct 
the Fortran default for 7.0, but that shouldn't have a direct bearing on C.  
I'll be filing a documentation problem report with Intel, but I'd like to 
have as many facts at hand as possible.  The description of -ansi is 
certainly oblique.  It assumes [the program is in] conformance with ISO C89 
[the main consequence being that typed aliasing analysis can be employed, as 
with gcc -fstrict-aliasing].
-- 
Tim Prince


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