3.4 / 3.5 / tree-ssa comparisons

[Richard Guenther <rguenth at tat dot physik dot uni-tuebingen dot de>]

The automated tester at 
http://www.tat.physik.uni-tuebingen.de/~rguenth/gcc/monitor-summary.html
completed its first 3.5 build.  I never checked 3.5, and so I'm 
surprised on the numbers it got:

bootstrap time (52min) is inbetween 3.4 (50min) and tree-ssa (62min), 
build times for the tramp3d-v3 test, too(!), I did expect them to 
improve compared to 3.4, not already regress again..., they are now
2.43min vs. 2.28min (3.4) and 2.75min (tree-ssa).  Also performance of 
the resulting binary is better(!) for 3.5 (6.9s/it) than for tree-ssa 
(7.68s/it) and of course 3.4 is slowest (8.85s/it).  This means we'll 
regress in both compile and runtime if merging tree-ssa now, but we 
won't have a runtime regression towards 3.4 then, only a compile time 
performance regression.

The obvious question is, why is 3.5 so much better than 3.4?  And of 
course, why is tree-ssa not better than 3.5 for C++ expression template 
numeric code?

All compilers built with checking disabled, tramp3d.cpp is build with
-O2 -funroll-loops -ffast-math, testing machine is a 1.3GHz Itanium2 
machine.

Richard.