------- Comment #28 from amonakov at gcc dot gnu dot org 2010-03-16 15:26
-------
To provide an update of the situation on 4.5 trunk:
AFAIK the situation has been generally improved with Zdenek's second commit (in
comment #23) and auto-inc-dec improvements in 4.5. However, on the particular
testcase discussed here (comment #20), we still don't DTRT on ia64 (powerpc is
OK, don't know about arm). There is unfortunate interplay between IVOPTS, RTL
PRE, RTL loop analysis and the auto-inc-dec pass.
First, ivopts produce harder-to-grok sequence in loop preheader, transforming
<bb 3>:
pretmp.2_8 = (short int) v_4(D);
<bb 4>:
# i_13 = PHI <i_6(5), 0(3)>
a[i_13] = pretmp.2_8;
i_6 = i_13 + 1;
if (len_3(D) > i_6)
goto <bb 5>;
else
goto <bb 6>;
<bb 5>:
goto <bb 4>;
to
<bb 3>:
pretmp.2_8 = (short int) v_4(D);
ivtmp.7_11 = (long unsigned int) &a[0];
D.2006_17 = (unsigned int) len_3(D);
D.2007_18 = D.2006_17 + 4294967295;
D.2008_19 = (long unsigned int) D.2007_18;
D.2009_20 = D.2008_19 * 2;
a.13_21 = (long unsigned int) &a;
D.2011_22 = a.13_21 + 2;
D.2012_23 = D.2009_20 + D.2011_22;
<bb 4>:
# ivtmp.7_1 = PHI <ivtmp.7_12(5), ivtmp.7_11(3)>
D.2005_16 = (void *) ivtmp.7_1;
MEM[base: D.2005_16]{a[i]} = pretmp.2_8;
ivtmp.7_12 = ivtmp.7_1 + 2;
if (ivtmp.7_12 != D.2012_23)
goto <bb 5>;
else
goto <bb 6>;
<bb 5>:
goto <bb 4>;
The preheader is not cleaned up until RTL PRE. Then, PRE transforms
L54:
51 NOTE_INSN_BASIC_BLOCK
52 [r371:DI]=r373:SI#0
53 r371:DI=r371:DI+0x2
55 r392:BI=r371:DI!=r381:DI
56 pc={(r392:BI!=0x0)?L54:pc}
to
L83:
82 NOTE_INSN_BASIC_BLOCK
77 r397:DI=r371:DI+0x2
L54:
51 NOTE_INSN_BASIC_BLOCK
52 [r371:DI]=r373:SI#0
75 r371:DI=r397:DI
REG_EQUAL: r371:DI+0x2
55 r392:BI=r371:DI!=r381:DI
56 pc={(r392:BI!=0x0)?L83:pc}
REG_DEAD: r392:BI
REG_BR_PROB: 0x26ac
... which is something auto-inc-dec pass is not able to handle. If I disable
rtl pre with -fdbg-cnt=pre:0, auto-inc is generated, but doloop pass is
confused instead:
Loop 1 is simple:
simple exit 4 -> 5
infinite if: (expr_list:REG_DEP_TRUE (subreg:SI (and:DI (plus:DI (minus:DI
(ashift:DI (reg:DI 390)
(const_int 1 [0x1]))
(reg:DI 371 [ ivtmp.7 ]))
(const:DI (plus:DI (symbol_ref:DI ("a") [flags 0x2] <var_decl
0x7ffff7968000 a>)
(const_int 2 [0x2]))))
(const_int 1 [0x1])) 0)
(nil))
number of iterations: (lshiftrt:DI (plus:DI (minus:DI (reg:DI 381 [ D.2012 ])
(reg:DI 371 [ ivtmp.7 ]))
(const_int -2 [0xfffffffffffffffe]))
(const_int 1 [0x1]))
upper bound: -2
Doloop: Possible infinite iteration case.
Doloop: The loop is not suitable.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32283
