Re: GCC/G++ compiler: Error goes away when run through debugger

On Tue, 29 Mar 2011 21:42:12 +0000 (UTC), ike@localhost.claranet.nl
(Ike Naar) wrote:


>George Neuner <gneuner2@comcast.net> wrote:
>>GCC's -O3 optimization level is widely known to cause strange problems
>>... almost always because the program is violating assumptions made by
>>the more advanced optimizations.
>>
>>The -O2 level typically is safe. If you think you need the -O3
>>optimizations, you should individually enable them to see if any
>>breaks the program. Specify -O3 only if you find they all work.
>
>Sometimes using -O3 makes the program run slower than when using -O2.


Yes, inlining and function cloning can greatly increase the code size.
But in particular -O3 enables the "predictive-commoning" and
"tree-vectorize" optimizations. "predictive-commoning" is an
extension of loop unrolling which tries to identify and avoid
recomputing and/or reloading values that are common to two or more
(not necessarily consecutive) iterations of a loop but which overall
are not loop invariant. "tree-vectorize" tries to map a loop of
serial operations into an equivalent, but shorter, loop of SIMD
operations. Both of these optimizations can break badly - and in
quite unexpected ways - when indexed locations are aliased or when
there are iterative data dependencies. The compiler can't always
figure out when NOT to perform the optimization ... and the results
when the compiler guesses wrong are unpredictable.


George

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