https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107879
--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:4500baaccb6e4d696e223c338bbdf7705c3646dd commit r13-4492-g4500baaccb6e4d696e223c338bbdf7705c3646dd Author: Jakub Jelinek <ja...@redhat.com> Date: Mon Dec 5 11:17:42 2022 +0100 range-op-float: Fix up multiplication and division reverse operation [PR107879] While for the normal cases it seems to be correct to implement reverse multiplication (op1_range/op2_range) through division with float_binary_op_range_finish, reverse division (op1_range) through multiplication with float_binary_op_range_finish or (op2_range) through division with float_binary_op_range_finish, as e.g. following testcase shows for the corner cases it is incorrect. Say on the testcase we are doing reverse multiplication, we have [-0., 0.] range (no NAN) on lhs and VARYING on op1 (or op2). We implement that through division, because x from lhs = x * op2 is x = lhs / op2 For the division, [-0., 0.] / VARYING is computed (IMHO correctly) as [-0., 0.] +-NAN, because 0 / anything but 0 or NAN is still 0 and 0 / 0 is NAN and ditto 0 / NAN. And then we just float_binary_op_range_finish, which figures out that because lhs can't be NAN, neither operand can be NAN. So, the end range is [-0., 0.]. But that is not correct for the reverse multiplication. When the result is 0, if op2 can be zero, then x can be anything (VARYING), to be precise anything but INF (unless result can be NAN), because anything * 0 is 0 (or NAN for INF). While if op2 must be non-zero, then x must be 0. Of course the sign logic (signbit(x) = signbit(lhs) ^ signbit(op2)) still holds, so it actually isn't full VARYING if both lhs and op2 have known sign bits. And going through other corner cases one by one shows other differences between what we compute for the corresponding forward operation and what we should compute for the reverse operations. The following patch is slightly conservative and includes INF (in case of result including 0 and not NAN) in the ranges or 0 in the ranges (in case of result including INF and not NAN). The latter is what happens anyway because we flush denormals to 0, and the former just not to deal with all the corner cases. So, the end test is that for reverse multiplication and division op2_range the cases we need to adjust to VARYING or VARYING positive or VARYING negative are if lhs and op? ranges both contain 0, or both contain some infinity, while for division op1_range the corner case is if lhs range contains 0 and op2 range contains INF or vice versa. Otherwise I believe ranges from the corresponding operation are ok, or could be slightly more conservative (e.g. for reverse multiplication, if op? range is singleton INF and lhs range doesn't include any INF, then x's range should be UNDEFINED or known NAN (depending on if lhs can be NAN), while the division computes [-0., 0.] +-NAN; or similarly if op? range is only 0 and lhs range doesn't include 0, division would compute +INF +-NAN, or -INF +-NAN, or (for lack of multipart franges -INF +INF +-NAN just VARYING +-NAN), while again it is UNDEFINED or known NAN. Oh, and I found by code inspection wrong condition for the division's known NAN result, due to thinko it would trigger not just when both operands are known to be 0 or both are known to be INF, but when either both are known to be 0, or at least one is known to be INF. 2022-12-05 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/107879 * range-op-float.cc (foperator_mult::op1_range): If both lhs and op2 ranges contain zero or both ranges contain some infinity, set r range to zero_to_inf_range depending on signbit_known_p. (foperator_div::op2_range): Similarly for lhs and op1 ranges. (foperator_div::op1_range): If lhs range contains zero and op2 range contains some infinity or vice versa, set r range to zero_to_inf_range depending on signbit_known_p. (foperator_div::rv_fold): Fix up condition for returning known NAN.
