On Fri, Jul 8, 2011 at 4:35 PM, Kai Tietz <ktiet...@googlemail.com> wrote: > 2011/7/8 Richard Guenther <richard.guent...@gmail.com>: >> On Thu, Jul 7, 2011 at 6:28 PM, Kai Tietz <ktiet...@googlemail.com> wrote: >>> 2011/7/7 Paolo Bonzini <bonz...@gnu.org>: >>>> On 07/07/2011 06:07 PM, Kai Tietz wrote: >>>>> >>>>> + /* We redo folding here one time for allowing to inspect more >>>>> + complex reductions. */ >>>>> + substitute_and_fold (op_with_constant_singleton_value_range, >>>>> + vrp_fold_stmt, false); >>>>> + /* We need to mark this second pass to avoid re-entering of same >>>>> + edges for switch statments. */ >>>>> + in_second_pass = true; >>>>> substitute_and_fold (op_with_constant_singleton_value_range, >>>>> vrp_fold_stmt, false); >>>>> + in_second_pass = false; >>>> >>>> This needs a much better explanation. >>>> >>>> Paolo >>> >>> Well, I can work on a better comment. The complex reduction I mean >>> here are cases like >>> >>> int x; >>> int y; >>> _Bool D1; >>> _Bool D2; >>> _Bool D3; >>> int R; >>> >>> D1 = x[0..1] != 0; >>> D2 = y[0..1] != 0; >>> D3 = D1 & D2 >>> R = (int) D3 >>> >>> (testcase is already present. See tree-ssa/vrp47.c). >>> >>> As VRP in first pass produces (and replaces) to: >>> >>> D1 = (_Bool) x[0..1]; >>> D2 = (_Bool) y[0..1]; >>> D3 = D1 & D2 >>> R = (int) D3 >>> >>> Just in the second pass the reduction >>> >>> R = x[0..1] & y[0..1] >> >> So why wouldn't that happen during the first pass? The first >> pass could change the IL to >> >> D1 = x[0..1] != 0; >> D2 = y[0..1] != 0; >> D3 = D1 & D2; >> R = x & y; >> >> if D3 only has a single use. > No, as D3 would need a type change, and this isn't possible. If it > wasn't absolutely clear, this patch to VRP is necessary after patch 2, > as here D1, D2, and D3 have bool-type, and just R is of type int.
In your example x,y and R are int, so it works with re-using R. >>> can happen. In general it is sad that VRP can't insert during pass >>> new statements right now. This would cause issues in range-tables, >>> which aren't designed for insertations. As otherwise, we could do >>> also simplify things like >>> >>> D1 = x[0..1] != 0; >>> D2 = y[0..1] == 0; >>> D3 = D1 & D2 >>> R = (int) D3 >>> >>> to >>> R = x[0..1] & (y[0..1] ^ 1) >> >> Why that ^ 1? And why does that confuse the range tables >> if you re-use R? > Because we would need to insert a new statement and this isn't allowed > in VRP. See the comments in VRP and substitute_and_fold. VRP > disallows to remove statements or to insert new ones. That's not a hard limitation. >>> Regards, >>> Kai >
