Re: Optimization Defaults

[Robert Dewar <robert_dewar@my-deja.com>]

In article <38553cf0.22235803@newsnew.draper.com>,
  rracine@myremarq.com (Roger Racine) wrote:
> Based on Robert Dewar's strong statement to trust the GCC
optimizer,
> and based on my own knowledge of the use of the optimizer when
> building GCC itself, I suggested that there should be no
problem using
> -O2 for GCC on a current project.  Immediately someone said
that he
> had gotten burned using GCC a couple years ago (version 2.7.2,
I
> believe he said he was using) in two different ways (using C,
but the
> optimizer is common, right?).



No one claimed that there was no possibility of bugs that
cause the compiler to malfunction at one optimization level
where it succeeds at another. But the point is:

1) There are relatively few such problems because the gcc
backend is pretty mature at this stage.

2) Going from -O0 to -O2 is by no means a guarantee that the
number of such bugs will increase, we have seen (rarely, but
then optimization bugs are rare in general) cases in which
a bug appeared only at -O0 and disappeared at -O2.

One thing to be aware of is that the punishment for erroneous
code is often more severe at -O2, so if you write junk code,
you may well find that the code appears to "work" at -O0 and
appears to "fail" at -O2. In such cases, using -O2 is actually
helpful in helping to smoke out such bugs


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