Optimization: Conclusions from Evolutionary Analysis

[Scott Robert Ladd <coyote at coyotegulch dot com>]

For those who haven't the time to read my full article, I thought I'd 
pass along the conclusions section. One of my goals with Acovea was to 
provide some empirical evidence of how optimizations behave, and to 
dispell some common misconceptions about optimization.

The settings for -O1, -O2, and -O3 are valid. Every option implied by 
-O3 may not be applicable to every program -- but enabling all those 
options does not seem to be detrimental, except in the case of 
huffbench. Several of the recently-added flags (-ftracer in particular) 
may be candidates for inclusion in -O2 or -O3 for future versions of the 
compiler.

Processor-specific options do not appear to be a major factor in 
performance on these benchmarks. I don't know if this is due to the 
nature of the processor, or if GCC can't take advantage of 
processor-specific instructions. I have double-checked my results; 
adding -mfpmath=sse (or any of its variants, or -msse) to a compile of 
almabench does not make the code run any faster. The only ia32-specific 
option that showed consistent value was -momit-leaf-frame-pointer.

The genetic algorithm was able to find sets of flags that produced 
faster code than that emitted by the default -O1, -O2, and -O3 options 
(with the exception of almabench, which is largely unoptimizable by 
nature). In many cases, this was due to the inclusion of "new" flags 
introduced with more recent version of GCC. Acovea discovers additional 
options that improve performance over that provided by the default 
settings; in essence, the genetic algorithm determines which of the 
non-standard and processor- specific options are most effective for a 
given algorithm.

A well-designed program will encapsulate algorithms. For most programs, 
performance is predicated on specific algorithms that can be identified 
via profiling; Acovea can then be applied to those algorithms for 
finding the set of optimizations that produce the fastest code. Only 
rarely does an entire program need to be fully optimized; instead, 
optimization should be applied to specific, critical routines that 
perform concise tasks.

The full article can be found at:

http://www.coyotegulch.com/acovea/index.html


--
Scott Robert Ladd
Coyote Gulch Productions (http://www.coyotegulch.com)
Software Invention for High-Performance Computing
In development: Alex, a database for common folk