Regarding -fno optimization options

Ian Lance Taylor iant@google.com
Thu Aug 2 08:21:00 GMT 2012


On Thu, Aug 2, 2012 at 1:15 AM, Ralf Jahr
<ralf.jahr@informatik.uni-augsburg.de> wrote:
> Hi,
>
> I tried in the last days to reconstruct an optimization level by just using
> the individual optimization flags. Well, it did not work at all.
>
> My basic idea was to take the list from "-Q -v" and use it. But there still
> were some differences where comparing the list from a basic optimization
> level with the list from the flags.
>
> Also, as I am working with a processor simulator (gem5), I was able to
> compare the performance of both programs. There was a big difference between
> using e.g. O3 and setting the flags.
>
> Also I found this FAQ entry:
>
> http://gcc.gnu.org/wiki/FAQ#optimization-options
>
> Ian, is this still valid?

Yes.

The -O option is not simply the sum of -f options.  Different -O
levels enable optimizations that are not controlled by any -f option.
Also, some -f options depend on a specific -O level.

Ian



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