Hello,
I am working on GCC4.2.1 porting to our VLIW processor. Our No. 1
priority is code size. I noticed the following code generation:
Source code:
if (a == 0x1ff )
c = a + b;
return c;
After tree copy propagation:
foo (a, b, c)
{
<bb 2>:
if (a_2 == 511) goto <L0>; else goto <L1>;
<L0>:;
c_5 = b_4 + 511;
# c_1 = PHI <c_3(2), c_5(3)>;
<L1>:;
return c_1;
}
It will generate the following assembly code for our processor
tstieqw p0, r0, #0x1ff //Compare r0 with 0x1ff and
write result to a predicate
p0. addwi r2, r1, #0x1ff //Predicated add
sbl [link] : movw r8, r2
In our processor, p0. addwi r2, r1, #0x1ff is a long instruction
(64-bit)
Ideally, I don't want this copy propagation if the immediate is out of
certain range. Then it will generate the following code
tstieqw p0, r0, #0x1ff //Compare r0 with 0x1ff and
write result to predicate
p0. addw r2, r1, r0 //Predicated add (32-bit
instruciton)
sbl [link] : movw r8, r2
It is going to save us four bytes.
Of couse, for processors without long/short instructions, this copy
propagation is benefiical for performance by reducing unnecessary
dependency. Therefore, whether to apply this copy propagation is machine
dependent to some degree.
What I do now is to add some check in tree-ssa-copy.c and tree-ssa-dom.c
for our target. But this is not very clean. My question is whether there
is better way to implement such machine-dependent tree-level
optimization (like hooks in RTL level). I believe there are other
processors that have the similar problem. What is common solution?
Thanks,
Bingfeng Mei
Broadcom UK
