[Pw_forum] newbie - installation problem
paulatto at sissa.it
Wed Mar 4 10:55:03 CET 2009
On Tue, 03 Mar 2009 21:22:44 +0100, Axel Kohlmeyer <akohlmey at cmm.chem.upenn.edu> wrote:
> (like IPO, PGO, SSE/MMX-vectorization) are working best with small
> test cases, but for any larger, more complex code, they add
> overhead about as often as they don't. the compiler doesn't
> really know which parts of a code are executed a lot and which
> are not, so it may put a lot of effort into optimizing the
> wrong parts, and - as i wrote before - aggressive optimization
> has the high risk of miscompiled (overoptimized) code.
In this regard, some time ago I tryed to compile QE using PGO and IPO, I spent some time compiling the test code and running some extensive profiling. Then recompiling with aggressive optimization and profiling data. The final code was about 5% faster than the original, but it was numerically less stable (due to over-optimization), hence it took two more iterations to converge than the not-so-optimized code, rending the total speedup exactly zero.
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