On 13-02-13 1:36 AM, Michael Eager wrote:
Hi --
I'm seeing register allocation problems and code size increases
with gcc-4.6.2 (and gcc-head) compared with older (gcc-4.1.2).
Both are compiled using -O3.
One test case that I have has a long series of nested if's
each with the same comparison and similar computation.
if (n<max_no){
n+=*(cp-*p++);
if (n<max_no){
n+=*(cp-*p);
if (n<max_no){
. . . ~20 levels of nesting
<more computations with 'cp' and 'p'>
. . . }}}
Gcc-4.6.2 generates many blocks like the following:
lwi r28,r1,68 -- load into dead reg
lwi r31,r1,140 -- load p from stack
lbui r28,r31,0
rsubk r31,r28,r19
lbui r31,r31,0
addk r29,r29,r31
swi r31,r1,308
lwi r31,r1,428 -- load of max_no from stack
cmp r28,r31,r29 -- n in r29
bgeid r28,$L46
gcc-4.1.2 generates the following:
lbui r3,r26,3
rsubk r3,r3,r19
lbui r3,r3,0
addk r30,r30,r3
swi r3,r1,80
cmp r18,r9,r30 -- max_no in r9, n in r30
bgei r18,$L6
gcc-4.6.2 (and gcc-head) load max_no from the stack in each block.
There also are extra loads into r28 (which is not used) and r31 at
the start of each block. Only r28, r29, and r31 are used.
I'm having a hard time telling what is happening or why. The
IRA dump has this line:
Ignoring reg 772, has equiv memory
where pseudo 772 is loaded with max_no early in the function.
The reload dump has
Reloads for insn # 254
Reload 0: reload_in (SI) = (reg/v:SI 722 [ max_no ])
GR_REGS, RELOAD_FOR_INPUT (opnum = 1)
reload_in_reg: (reg/v:SI 722 [ max_no ])
reload_reg_rtx: (reg:SI 31 r31)
and similar for each of the other insns using 722.
This is followed by
Spilling for insn 254.
Using reg 31 for reload 0
for each insn using pseudo 722.
Any idea what is going on?
So many changes happened since then (7 years ago), that it is very hard
to me to say something definitely. I also have no gcc-4.1 microblaze
(as I see microblaze was added to public gcc for 4.6 version) and it
makes me even more difficult to say something useful.
First of all, the new RA was introduced in gcc4.4 (IRA) which uses
different heuristics (Chaitin-Briggs graph coloring vs Chow's priority RA).
We could blame IRA when we have the same started conditions for it RA
gcc4.1 and gcc4.6-gcc-4.8. But I am sure it is not the same. More
aggressive optimizations creates higher register pressure. I compared
peak reg pressure in the test for gcc4.6 and gcc4.8. It became higher
(from 102 to 106). I guess the increase was even bigger since gcc4.1.
RA focused on generation of faster code. Looking at the fragment you
provided it, it is hard to say something about it. I tried -Os for
gcc4.8 and it generates desirable code for the fragment in question (by
the way the peak register pressure decreased to 66 in this case).
Any industrial RA uses heuristic algorithms, in some cases better
heuristics can work worse than worse heuristics. So you should probably
check is there any progress moving from gcc4.1 to gcc4.6 with
performance point of view for variety benchmarks. Introducing IRA
improves code for x86 4% on SPEC2000. Subsequent improving (like using
dynamic register classes) made further performance improvements.
Looking at the test code, I can make some conclusions for myself:
o We need a common pass decreasing reg pressure (I already expressed
this in the past) as optimizations become more aggressive. Some
progress was made to make few optimizations aware about RA (reg-pressure
scheduling, loop-invariant motions, and code hoisting) but there are too
many passes and it is wrong and impossible to make them all aware of
RA. Some register pressure decreasing heuristics are difficult to
implement in RA (like insn rearrangements or complex rematerialization)
and this pass could focus on them.
o Implement RA live range splitting in regions different from loops or
BB (now IRA makes splitting only on loop bounds and LRA in BB, the old
RA had no live range splitting at all).
I'd also recommend to try the following options concerning RA:
-fira-loop-pressure, -fsched-pressure, -fira-algorithm=CB|priority,
-fira-region=one,all,mixed. Actually -fira-algorithm=priority +
-fira-region=one is analog of what the old RA did.
I hope I answered to your question.
