Pessimization from reduce-all-givs and move-all-movables

[Jerry Quinn]

I'm trying to see how much speed I can squeeze out of a 1.8GHz
Pentium4 laptop.  I've been playing with the c4 benchmark
(http://www.cwl.nl/~tromp/c4/fhour.html) and have been trying out
various optimization combos.

One of the combos I tried was:

gcc-dev -fmove-all-movables -freduce-all-givs  -O2  -mcpu=pentium4
-march=pentium4 -msse2 -DUNIX -DTRANSIZE=1050011 -DPROBES=8
-DREPORTPLY=8 *.c

which gives a score of 976 Kpos/sec.

Without -fmove-all-movables -freduce-all-givs, performance improves to
1048 Kpos/sec, a 7% difference.

The docs ask for examples of code that is pessimized by these two
options, so here is some.

Also, -O3 loses about 1% on this code.  And a 700MHz Pentium3 laptop
can be tuned to a score of 812 with the mainline.  While the best I've
been able to get out of this machine is 1100.  There are a bunch of
shifts, so that may be the main source of slowness.


Jerry Quinn