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how to fix anti-optimization?
- To: gcc at gcc dot gnu dot org
- Subject: how to fix anti-optimization?
- From: dalej at apple dot com
- Date: Wed, 12 Sep 2001 11:35:00 -0700
const double Prescale[] = { 3.0, 5.0 };
extern void bar(double, double);
void foo () { bar(Prescale[0], Prescale[1]); }
With -O1 or higher, gcc substitutes constants 3.0 and 5.0 for the array
references. This isn't a win on targets where the extra copies of these
constants have to go in memory; you get an extra copy of the constants in
data space. Furthermore, on many targets, loading two FP constants
individually requires computing two addresses, while loading them from the
array requires only one address computation. I see that this substitution
would gain if some other optimizations happened to be doable as a result,
e.g. constant folding, but I don't know how to determine that.
The substitution is done in the ARRAY_REF case of expr.c:expand_expr().
It looks like I could hack this by setting TREE_SIDE_EFFECTS on FP
constants, but surely that's not the right way to do it. Any better ideas?