Re: compile with gcc option -O0 or -O

[David Brown <david at westcontrol dot com>]

On 16/09/2011 09:54, Miles Bader wrote:
> David Brown<david@westcontrol.com>  writes:
> > > > but one weird problem is that the following codes compiled with option -O0
> > >
> > > You need to show us a small complete self-contained example.
> > >
> > > Normally a program which works at -O0 and fails at -O1 has an
> > > uninitialized variable somewhere.
> >
> > Another common cause is aliasing - using pointer casts to access the
> > same data in different ways.
> >
> > And (especially if this is embedded programming), also check for missing
> > "volatile" qualifiers.
> >
> > Compile your code with lots of warnings - that will help spot
> > mistakes. The flags I often use are:
> >
> > -Wall
> > -Wextra
> > -Winit-self
> > -Wmissing-include-dirs
> ...
>
> ... but be a bit wary of any warning option which isn't included in
> -Wall or -Wextra -- they're usually omitted for a reason (typically
> because they yield tons of false positives on reasonable code).

That depends on your definition of "reasonable code" !  I don't mean to 
say that the warnings /I/ use will suit everyone - or every type of 
programming.  I often have to reduce them if I am working with other 
people's code, to avoid having lots of minor warning messages that can 
make real problems hard to spot.

An example here would be my insistence on "-Wmissing-declarations 
-Wnested-externs -Wmissing-prototypes -Wredundant-decls".  When I write 
code, an object (variable, function, etc.) is either exported by a 
module, or it is local to a module.  That means it is either declared 
"extern" in "module.h" and defined in "module.c" (which must #include 
"module.h"), or it is defined "static" in "module.c".  And any external 
objects that are used must be accessed from #include'ing the other 
module's header file.  I think these are good rules for the code I write 
(though I can see that some types of code need more flexibility), and 
these warnings help enforce them.  But if you take typical old-fashioned 
(IMHO, of course) code then most objects are defined without "static" 
qualifiers or "extern" definitions, leading to lots of warnings.

And there are other warning flags that will be useful to other people, 
but not to me - "-pedantic" would be useful for people writing code that 
must work with different compilers, for example.

gcc has excellent static error checking capabilities - I make as much 
use of them as I can.