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Backend port: Minimizing register usage in favor of memory accesses
- From: xyzzy at hotpop dot com
- To: GCC list <gcc at gcc dot gnu dot org>
- Date: Tue, 30 Mar 2004 11:48:19 +0200
- Subject: Backend port: Minimizing register usage in favor of memory accesses
I understand that one of the goals of any optimizing compiler is to maximize
use of registers since this is the fastest and least space consuming type of
instruction on most processors.
Thus, C code like:
extern void bar();
int a,b;
foo()
{
a = 5;
if ( b > a )
bar();
}
... produces pseudo-code like:
.word a,b
move 5,tmp1reg
move tmp1reg,a
move b,tmp2reg
cmp tmp2reg,tmp1reg
bgt label1
call bar
label1:
ret
What I want to do is reverse this and minimize register usage as much as
possible, massively favoring memory accesses unless a register absolutely
MUST be used. Thus, I want my backend to produce code like this:
.word a,b
move 5,a
cmp b,a
bgt label1
call bar
label1:
ret
Is there any way to force the GCC backend to do this, if at all? Will
defining SMALL_REGISTER_CLASSES to 1 do the trick or is there any other magic
that can be invoked? Doing it in the optimizer stage is OK, too... if O2 or
less.
Many thanks for any help on this.