From mboxrd@z Thu Jan  1 00:00:00 1970
From: dalej@apple.com
To: gcc@gcc.gnu.org
Subject: how to fix anti-optimization?
Date: Wed, 12 Sep 2001 11:34:00 -0000
Message-id: <200109121834.LAA21595@scv1.apple.com>
X-SW-Source: 2001-09/msg00478.html

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?