RE: possible compiler optimization error

["Dave Korn" <dave dot korn at artimi dot com>]

On 28 June 2007 18:19, Frederich, Eric P21322 wrote:

> On Windows I have found that a program I wrote fails when compiled with
> -O1 and -O2 but runs fine with -O0.
> The program behaves correctly on Linux and Solaris with or without
> optimizations.

  Your code has a bug, most likely an uninitialised or otherwise stray
pointer, but could be anything that causes undefined behaviour.  It works on
Linux/Solaris by good fortune, because whatever memory addresses you're
stomping all over just happen to be free and safe to stomp on on those
platforms, whereas windows systems have different process space layout 

> If a program compiled with -O0 has different output than the same
> program compiled with -O1 or -O2, is that defiantly a compile error?

  No.  Very occasionally it is a genuine compiler bug in one of the
optimisation passes, but the *vast* majority of such 'bugs' are actually in
the source code fed to the compiler.  (Like many utilities, the compiler
cannot always detect when it is fed invalid input, and may sometimes silently
produce invalid output as a result.)

  Now, there /could/ be a compiler bug, but unless you can present us with a
testcase that we can compile ourselves and test out, it's not going to be easy
to help you.  Fragmentary code snippets aren't useful.  For advice on how to
write a really good compiler bug report, see the guidelines at
http://gcc.gnu.org/bugs.html


    cheers,
      DaveK
-- 
Can't think of a witty .sigline today....


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