[Bug tree-optimization/94793] Failure to optimize clz idiom

cvs-commit at gcc dot gnu.org gcc-bugzilla@gcc.gnu.org
Mon Jan 16 10:48:04 GMT 2023


--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Andrew Carlotti <acarlotti@gcc.gnu.org>:


commit r13-5193-gd347fbf774dc50bf7511f4dc6bc74547ed364995
Author: Andrew Carlotti <andrew.carlotti@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;

        return c;

            cbz     w0, .L3
            clz     w1, w0
            tst     x0, 1
            mov     w0, 32
            sub     w0, w0, w1
            csel    w0, w0, wzr, ne

    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.


            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.


            * 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.

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