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
The issue is that substitute_and_fold runs within BBs statements folding from last to first. So most simplifications are done too late to recognize dependent one. Maybe it would be another way here to have a flag for substitute_and_fold to indicate that folding pass shall run first -> last or last->first? > D1 = x[0..1] != 0; > D2 = y[0..1] != 0; > D3 = D1 & D2; > R = x & y; > > if D3 only has a single use. Well, to change type of an SSA-name, if it has single-use might be another way here. To have the ability to enter new temp-registers would be better and avoids the dependency of single use, but well, range tables don't support that now.
