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.

0 files changed, 0 insertions, 0 deletions