On Thu, Jun 12, 2014 at 7:59 PM, Zdenek Dvorak <rakd...@iuuk.mff.cuni.cz> wrote: > Hi, > >> > I noticed there is below code/comments about may_be_zero field in loop >> > niter desc: >> > >> > tree may_be_zero; /* The boolean expression. If it evaluates to true, >> > the loop will exit in the first iteration (i.e. >> > its latch will not be executed), even if the niter >> > field says otherwise. */ >> > >> > I had difficulty in understanding this because I ran into some cases >> > in which it didn't behave as said. > > actually, in all the examples below, the field behaves as described, > i.e., > > the number of iterations = may_be_zero ? 0 : niter; > > In particular, the fact that may_be_zero is false *does not imply* > that the number of iterations as described by niter is non-zero. > >> > Example1, the dump of loop before sccp is like: >> > >> > <bb 2>: >> > bnd_4 = len_3(D) + 1; >> > >> > <bb 3>: >> > # ivtmp_1 = PHI <0(2), ivtmp_11(4)> >> > _6 = ivtmp_1 + len_3(D); >> > _7 = a[ivtmp_1]; >> > _8 = b[ivtmp_1]; >> > _9 = _7 + _8; >> > a[_6] = _9; >> > ivtmp_11 = ivtmp_1 + 1; >> > if (bnd_4 > ivtmp_11) >> > goto <bb 4>; >> > else >> > goto <bb 5>; >> > >> > <bb 4>: >> > goto <bb 3>; >> > >> > The loop niter information analyzed in sccp is like: >> > >> > Analyzing # of iterations of loop 1 >> > exit condition [1, + , 1] < len_3(D) + 1 >> > bounds on difference of bases: -1 ... 4294967294 >> > result: >> > zero if len_3(D) == 4294967295 >> > # of iterations len_3(D), bounded by 4294967294 >> > >> > Qeustion1, shouldn't it be like "len_3 +1 <= 1" because the latch >> > won't be executed when "len_3 == 0", right? > > the analysis determines the number of iterations as len_3, that is > 0 if len_3 == 0. So, the information is computed correctly here. > >> > But when boundary condition is the only case that latch get ZERO >> > executed, the may_be_zero info will not be computed. See example2, >> > with dump of loop before sccp like: >> > >> > foo (int M) >> > >> > <bb 2>: >> > if (M_4(D) > 0) >> > goto <bb 4>; >> > else >> > goto <bb 3>; >> > >> > <bb 3>: >> > return; >> > >> > <bb 4>: >> > >> > <bb 5>: >> > # i_13 = PHI <0(4), i_10(6)> >> > _5 = i_13 + M_4(D); >> > _6 = a[i_13]; >> > _7 = b[i_13]; >> > _8 = _6 + _7; >> > a[_5] = _8; >> > i_10 = i_13 + 1; >> > if (M_4(D) > i_10) >> > goto <bb 6>; >> > else >> > goto <bb 3>; >> > >> > <bb 6>: >> > goto <bb 5>; >> > >> > The niter information analyzed in sccp is like: >> > >> > Analyzing # of iterations of loop 1 >> > exit condition [1, + , 1](no_overflow) < M_4(D) >> > bounds on difference of bases: 0 ... 2147483646 >> > result: >> > # of iterations (unsigned int) M_4(D) + 4294967295, bounded by >> > 2147483646 >> > >> > So may_be_zero is always false here, but the latch may be ZERO >> > executed when "M_4 == 1". > > Again, this is correct, since then ((unsigned int) M_4) + 4294967295 == 0. > >> > Start from Example1, we can create Example3 which makes no sense to >> > me. Again, the dump of loop is like: >> > >> > <bb 2>: >> > bnd_4 = len_3(D) + 1; >> > >> > <bb 3>: >> > # ivtmp_1 = PHI <0(2), ivtmp_11(4)> >> > _6 = ivtmp_1 + len_3(D); >> > _7 = a[ivtmp_1]; >> > _8 = b[ivtmp_1]; >> > _9 = _7 + _8; >> > a[_6] = _9; >> > ivtmp_11 = ivtmp_1 + 4; >> > if (bnd_4 > ivtmp_11) >> > goto <bb 4>; >> > else >> > goto <bb 5>; >> > >> > <bb 4>: >> > goto <bb 3>; >> > >> > <bb 5>: >> > return 0; >> > >> > The niter info is like: >> > >> > Analyzing # of iterations of loop 1 >> > exit condition [4, + , 4] < len_3(D) + 1 >> > bounds on difference of bases: -4 ... 4294967291 >> > result: >> > under assumptions len_3(D) + 1 <= 4294967292 >> > zero if len_3(D) == 4294967295 >> > # of iterations len_3(D) / 4, bounded by 1073741823 >> > >> > The problem is: won't latch be ZERO executed when "len_3 == 0/1/2/3"? > > Again, in all these cases the number of iterations is len_3 / 4 == 0.
Thanks, I realized that boundary condition is described by niter part of information and it's more conveniently handled in this way for consumer of may_be_zero like IVOPT. One problem is with below example: <bb 6>: vectp_a.8_57 = &a; vectp_b.11_61 = &b; _67 = _1 * 4; vectp_a.15_66 = &a + _67; <bb 7>: # vectp_a.7_58 = PHI <vectp_a.8_57(6), vectp_a.7_59(14)> # vectp_b.10_62 = PHI <vectp_b.11_61(6), vectp_b.10_63(14)> # vectp_a.14_68 = PHI <vectp_a.15_66(6), vectp_a.14_69(14)> # ivtmp_9 = PHI <0(6), ivtmp_71(14)> vect__6.9_60 = MEM[(float *)vectp_a.7_58]; vect__7.12_64 = MEM[(float *)vectp_b.10_62]; vect__8.13_65 = vect__6.9_60 + vect__7.12_64; MEM[(float *)vectp_a.14_68] = vect__8.13_65; vectp_a.7_59 = vectp_a.7_58 + 16; vectp_b.10_63 = vectp_b.10_62 + 16; vectp_a.14_69 = vectp_a.14_68 + 16; ivtmp_71 = ivtmp_9 + 1; if (ivtmp_71 < bnd.4_36) goto <bb 14>; else goto <bb 9>; <bb 14>: goto <bb 7>; After ivopt: <bb 6>: vectp_a.8_57 = &a; vectp_b.11_61 = &b; _67 = _1 * 4; vectp_a.15_66 = &a + _67; <bb 7>: # ivtmp.24_26 = PHI <0(6), ivtmp.24_33(14)> # ivtmp.27_88 = PHI <0(6), ivtmp.27_89(14)> vect__6.9_60 = MEM[symbol: a, index: ivtmp.27_88, offset: 0B]; vect__7.12_64 = MEM[symbol: b, index: ivtmp.27_88, offset: 0B]; vect__8.13_65 = vect__6.9_60 + vect__7.12_64; MEM[base: vectp_a.15_66, index: ivtmp.27_88, offset: 0B] = vect__8.13_65; ivtmp.24_33 = ivtmp.24_26 + 1; ivtmp.27_89 = ivtmp.27_88 + 16; if (ivtmp.24_33 < bnd.4_36) goto <bb 14>; else goto <bb 9>; <bb 14>: goto <bb 7>; Ideally, "(ivtmp.24_33 < bnd.4_36)" should be eliminated by using "ivtmp.27_89". One blocker is may_be_zero information like below: Analyzing # of iterations of loop 1 exit condition [1, + , 1] < _38 + 1 bounds on difference of bases: -1 ... 4294967294 result: zero if _38 == 4294967295 # of iterations _38, bounded by 4294967294 number of iterations _38; zero if _38 == 4294967295 "_38 == 4294967295" is folded from "_38 + 1 < 1", only the folded form can't be handled by iv_elimination_compare_lt. It seems I should investigate how to extend iv_elimination_compare_lt to handle more general may_be_zero information. Thanks, bin -- Best Regards.