Re: [PATCH 1/5] New Identical Code Folding IPA pass

[Jeff Law <law@redhat.com>]

On 06/13/14 04:24, mliska wrote:
>
> You may ask, why the GNU GCC does need such a new optimization. The
> compiler, having simply better knowledge of a compiled source file,
> is capable of reaching better results, especially if Link-Time
> optimization is enabled. Apart from that, GCC implementation adds
> support for read-only variables like construction vtables (mentioned
> in:
> http://hubicka.blogspot.cz/2014/02/devirtualization-in-c-part-3-building.html).
Can you outline at a high level cases where GCC's knowledge allows it to 
reach a better result?  Is it because you're not requiring bit for bit 
identical code, but that the code merely be semantically equivalent?

The GCC driven ICF seems to pick up 2X more opportunities than the gold 
driven ICF.  But if I'm reading everything correctly, that includes ICF 
of both functions and variables.

Do you have any sense of how those improvements break down?  ie, is it 
mostly more function's you're finding as identical, and if so what is it 
about the GCC implementation that allows us to find more ICF 
opportunities.  If it's mostly variables, that's fine too.  I'm just 
trying to understand where the improvements are coming from.

Jeff