https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94793
--- Comment #5 from CVS Commits ---
The master branch has been updated by Andrew Carlotti :
https://gcc.gnu.org/g:d347fbf774dc50bf7511f4dc6bc74547ed364995
commit r13-5193-gd347fbf774dc50bf7511f4dc6bc74547ed364995
Author: Andrew Carlotti
Date: Thu Nov 10 15:56:51 2022 +
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
cselw0, 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.
