https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94793
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Andrew Carlotti <acarlo...@gcc.gnu.org>: https://gcc.gnu.org/g:d347fbf774dc50bf7511f4dc6bc74547ed364995 commit r13-5193-gd347fbf774dc50bf7511f4dc6bc74547ed364995 Author: Andrew Carlotti <andrew.carlo...@arm.com> Date: Thu Nov 10 15:56:51 2022 +0000 Add cltz_complement idiom recognition This recognises patterns of the form: while (n) { n >>= 1 } This patch results in improved (but still suboptimal) codegen: foo (unsigned int b) { int c = 0; while (b) { b >>= 1; c++; } return c; } foo: .LFB11: .cfi_startproc cbz w0, .L3 clz w1, w0 tst x0, 1 mov w0, 32 sub w0, w0, w1 csel w0, w0, wzr, ne ret The conditional is unnecessary. phiopt could recognise a redundant csel (using cond_removal_in_builtin_zero_pattern) when one of the inputs is a clz call, but it cannot recognise the redunancy when the input is (e.g.) (32 - clz). I could perhaps extend this function to recognise this pattern in a later patch, if this is a good place to recognise more patterns. gcc/ChangeLog: PR tree-optimization/94793 * tree-scalar-evolution.cc (expression_expensive_p): Add checks for c[lt]z optabs. * tree-ssa-loop-niter.cc (build_cltz_expr): New. (number_of_iterations_cltz_complement): New. (number_of_iterations_bitcount): Add call to the above. gcc/testsuite/ChangeLog: * lib/target-supports.exp (check_effective_target_clz) (check_effective_target_clzl, check_effective_target_clzll) (check_effective_target_ctz, check_effective_target_clzl) (check_effective_target_ctzll): New. * gcc.dg/tree-ssa/cltz-complement-max.c: New test. * gcc.dg/tree-ssa/clz-complement-char.c: New test. * gcc.dg/tree-ssa/clz-complement-int.c: New test. * gcc.dg/tree-ssa/clz-complement-long-long.c: New test. * gcc.dg/tree-ssa/clz-complement-long.c: New test. * gcc.dg/tree-ssa/ctz-complement-char.c: New test. * gcc.dg/tree-ssa/ctz-complement-int.c: New test. * gcc.dg/tree-ssa/ctz-complement-long-long.c: New test. * gcc.dg/tree-ssa/ctz-complement-long.c: New test.
