[macstl-dev] CSE and the single programmer

[Glen Low]

Hi All

Apple gcc 4.0 is leaps and bounds better than 3.3 for CSE (common  
subexpression elimination). For example in macstl 0.3.x I can now  
reliably do:

valarray <float> a, b;
valarray <float> b = a + a + a;

and have one lvx (or lfs) within the inner loop, instead of 3 lvx  
when the compiler can't track the identical origins of a in the  
expression.

Unfortunately, the CSE is fairly finicky, I've been chasing better  
CSE for macstl and discovered any of the following will make it worse  
i.e. longer expressions will cause all loads and code  to be taken/ 
generated, but shorter expressions will cause only the minimal loads/ 
code implied by CSE:

*    -maltivec, -mcpu=G5 even on scalar code (possibly because - 
faltivec without -maltivec affects how temporaries are generated??)
*    the # of temporaries generated within the expression, even if  
the load/store to memory is eventually optimized out.
*    the phase of the moon

Since it looks like the compiler gives up CSE at a certain length of  
expression rather than with a definite combination of options/usage,  
it feels like there's some sort of "maximum length of CSE'able  
expression" flag in gcc.

To all you gcc gurus, is there such a flag?

Cheers, Glen Low


---
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