Re: C++: EGCS perf. vs GCC

[Marc Lehmann <pcg at goof dot com>]

On Mon, Dec 07, 1998 at 02:14:07PM -0500, John S. Dyson wrote:
> When playing with GCC, PGCC, and EGCS, it appears that the "good" optimization
> options for FP codes (on the X86) are not the same as for non-FP.  I guess that
> is to be expected because of the necessarily different code generator for 
> X86 FP.

I think its because there are "more" fp registers than integer registers.
Loop unrolling and scheduling (first pass) often help alot, while they are
detrimental in integer code.

It might be a good idea to try to selectively unroll and schedule fp-loops,
while not touching the integer loops. (This would keep me from saying "use
-funroll-all-loops and -fschedule-insns for fpu-intensive code only" ;)

Particularly interesting could be to add additional dependencies to integer
instructions in the first scheduling pass (like pgcc once did), so it
schedules fp-instructions automatically.

Also, loop-unrolling could become more selective on which loops to unroll
(best would be only the loops that need it ;->>>), but I can't really
imagine an heuristic for this, although I think the distinction between
-funroll-loops and -funroll-all-loops isn't helpful.

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