------- Comment #16 from steven at gcc dot gnu dot org 2010-02-05 13:33 -------
I'm trying to coerce IVOPTSs into producing the following, optimal code in the
GIMPLE optimizers (without much luck, so far):
<bb 2>:
pretmp.11_26 = (int) s_11(D);
ivtmp.20_28 = (long unsigned int) b_inout_5(D);
D.1848_31 = (long unsigned int) b_inout_5(D);
D.1849_32 = D.1848_31 + 256;
<bb 3>:
# ivtmp.20_25 = PHI <ivtmp.20_24(4), ivtmp.20_28(2)>
D.1846_29 = (void *) ivtmp.20_25;
D.1814_10 = MEM[base: D.1846_29]{*D.1813};
D.1816_13 = D.1814_10 * pretmp.11_26;
D.1847_30 = (void *) ivtmp.20_25;
MEM[base: D.1847_30]{*D.1813} = D.1816_13;
ivtmp.20_24 = ivtmp.20_25 + 4;
if (ivtmp.20_24 != D.1849_32)
goto <bb 4>;
else
goto <bb 5>;
<bb 4>:
goto <bb 3>;
<bb 5>:
return;
If we can get the compiler to generate the above code in IVOPTS, then we should
get the same code as the hand-unrolled example (although it also looks like a
bit more scheduler look-ahead freedom is necessary).
I asked Zdenek for help. For ARM9 he found the following problem:
/quote/
Address costs:
index costs 6
cst + index costs 2
arm_arm_address_cost pretends that having reg + cst as an address is cheaper
than having reg by itself. Ivopts are happy to make this happen :-)
There used to be the same problem on x86, which was eventually fixed by making
address_cost reflect the real cost of addresses, rather than "which addressing
modes should CSE prefer" metrics.
/quote/
Of course all cases with cost of "index" > cost of "index + cst" results in the
code like that of an unpatched compiler. But if I adjust the cost to make
"index" cost only 1 or 2, I get this:
<bb 2>:
pretmp.11_26 = (int) s_11(D);
ivtmp.25_28 = (long unsigned int) b_inout_5(D);
<bb 3>:
# i_20 = PHI <i_14(4), 0(2)>
# ivtmp.25_25 = PHI <ivtmp.25_24(4), ivtmp.25_28(2)>
D.1846_29 = (void *) ivtmp.25_25;
D.1814_10 = MEM[base: D.1846_29]{*D.1813};
D.1816_13 = D.1814_10 * pretmp.11_26;
D.1847_30 = (void *) ivtmp.25_25;
MEM[base: D.1847_30]{*D.1813} = D.1816_13;
ivtmp.25_24 = ivtmp.25_25 + 4;
i_14 = i_20 + 1;
if (i_14 != 64)
goto <bb 4>;
else
goto <bb 5>;
<bb 4>:
goto <bb 3>;
<bb 5>:
return;
That is still not optimal: We get an extra IV for some reason.
--
steven at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rakdver at gcc dot gnu dot
| |org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36712
