How do you get the benefit of -fstrict-aliasing?

[Bradley Lucier <lucier at math dot purdue dot edu>]

I've decided to try to contribute modifications to the the C code  
that is generated by the Gambit Scheme->C compiler so that (a) it  
doesn't have any aliasing violations and (b) more aliasing  
distinctions can be made (the car and cdr of a pair don't overlap  
with the entries of a vector, etc.).  This was in response to a  
measured 20% speedup with some numerical code with -fstrict-aliasing  
instead of -fno-strict-aliasing, nearly all of which came because gcc  
then knew that stores to a vector of doubles didn't change the values  
of variables on the stack.

Part (a) is essentially a non-issue for user-written code, since the  
only aliasing problems of which I am aware are in the bignum library,  
so as a preliminary test I added -fstrict-aliasing to the gcc command  
line and reran the benchmark suite on a 2GHz G5.  To my surprise,  
while there were some improvements, the -fstrict-aliasing option led  
to slower code overall, in some cases quite severely (7.014 seconds  
to 11.794 seconds, for example), and, perhaps not surprisingly,  
compilation times were significantly longer.  This was true both with  
Apple's 4.0.1 and FSF 4.1.2.

So I'm wondering whether certain options have to be included on the  
command line to get the benefits of -fstrict-aliasing.  The current  
command line is

gcc -mcpu=970 -m64  -no-cpp-precomp -Wall -W -Wno-unused -O1 -fno- 
math-errno -fschedule-insns2 -fno-trapping-math -fno-strict-aliasing - 
fwrapv -fexpensive-optimizations -fforce-addr -fpeephole2 -falign- 
jumps -falign-functions -fno-function-cse -ftree-copyrename -ftree- 
fre -ftree-dce -fregmove -fgcse-las -freorder-functions -fcaller- 
saves -fno-if-conversion2 -foptimize-sibling-calls -fcse-skip-blocks - 
funit-at-a-time -finline-functions -fomit-frame-pointer -fPIC -fno- 
common -bundle -flat_namespace -undefined suppress -fstrict-aliasing

where the optimizations between -fwrapv (which is no longer  
necessary, I should remove that) and -fstrict-aliasing were chosen by  
some experiments with genetic algorithms.

I didn't think that adding aliasing information could lead to worse  
code.  So I'm wondering how to use that aliasing information more  
effectively to get better code.

Brad