https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81635
--- Comment #14 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> --- Author: rsandifo Date: Thu Feb 8 15:16:29 2018 New Revision: 257491 URL: https://gcc.gnu.org/viewcvs?rev=257491&root=gcc&view=rev Log: Use nonzero bits to refine range in split_constant_offset (PR 81635) This patch is part 2 of the fix for PR 81635. It means that split_constant_offset can handle loops like: for (unsigned int i = 0; i < n; i += 4) { a[i] = ...; a[i + 1] = ...; } CCP records that "i" must have its low 2 bits clear, but we don't include this information in the range of "i", which remains [0, +INF]. I tried making set_nonzero_bits update the range info in the same way that set_range_info updates the nonzero bits, but it regressed cases like vrp117.c and made some other tests worse. vrp117.c has a multiplication by 10, so CCP can infer that the low bit of the result is clear. If we included that in the range, the range would go from [-INF, +INF] to [-INF, not-quite-+INF]. However, the multiplication is also known to overflow in all cases, so VRP saturates the result to [INT_MAX, INT_MAX]. This obviously creates a contradiction with the nonzero bits, and intersecting the new saturated range with an existing not-quite-+INF range would make us drop to VR_UNDEFINED. We're prepared to fold a comparison with an [INT_MAX, INT_MAX] value but not with a VR_UNDEFINED value. The other problems were created when intersecting [-INF, not-quite-+INF] with a useful VR_ANTI_RANGE like ~[-1, 1]. The intersection would keep the former range rather than the latter. The patch therefore keeps the adjustment local to split_constant_offset for now, but adds a helper routine so that it's easy to move this later. 2018-02-08 Richard Sandiford <richard.sandif...@linaro.org> gcc/ PR tree-optimization/81635 * wide-int.h (wi::round_down_for_mask, wi::round_up_for_mask): Declare. * wide-int.cc (wi::round_down_for_mask, wi::round_up_for_mask) (test_round_for_mask): New functions. (wide_int_cc_tests): Call test_round_for_mask. * tree-vrp.h (intersect_range_with_nonzero_bits): Declare. * tree-vrp.c (intersect_range_with_nonzero_bits): New function. * tree-data-ref.c (split_constant_offset_1): Use it to refine the range returned by get_range_info. gcc/testsuite/ PR tree-optimization/81635 * gcc.dg/vect/bb-slp-pr81635-3.c: New test. * gcc.dg/vect/bb-slp-pr81635-4.c: Likewise. Added: trunk/gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-3.c trunk/gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-4.c Modified: trunk/gcc/ChangeLog trunk/gcc/testsuite/ChangeLog trunk/gcc/tree-data-ref.c trunk/gcc/tree-vrp.c trunk/gcc/tree-vrp.h trunk/gcc/wide-int.cc trunk/gcc/wide-int.h
