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. Zdenek
