https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101950
--- Comment #2 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:301dc6011cbceb7ea9debd86aaec7cadb37213c8 commit r12-3017-g301dc6011cbceb7ea9debd86aaec7cadb37213c8 Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Aug 19 11:00:27 2021 +0200 expand: Add new clrsb fallback expansion [PR101950] As suggested in the PR, the following patch adds two new clrsb expansion possibilities if target doesn't have clrsb_optab for the requested nor wider modes, but does have clz_optab for the requested mode. One expansion is clrsb (op0) expands as clz (op0 ^ (((stype)op0) >> (prec-1))) - 1 which is usable if CLZ_DEFINED_VALUE_AT_ZERO is 2 with value of prec, because the clz argument can be 0 and clrsb should give prec-1 in that case. The other expansion is clz (((op0 << 1) ^ (((stype)op0) >> (prec-1))) | 1) where the clz argument is never 0, but it is one operation longer. E.g. on x86_64-linux with -O2 -mno-lzcnt, this results for int foo (int x) { return __builtin_clrsb (x); } in - subq $8, %rsp - movslq %edi, %rdi - call __clrsbdi2 - addq $8, %rsp - subl $32, %eax + leal (%rdi,%rdi), %eax + sarl $31, %edi + xorl %edi, %eax + orl $1, %eax + bsrl %eax, %eax + xorl $31, %eax and with -O2 -mlzcnt: + movl %edi, %eax + sarl $31, %eax + xorl %edi, %eax + lzcntl %eax, %eax + subl $1, %eax On armv7hl-linux-gnueabi with -O2: - push {r4, lr} - bl __clrsbsi2 - pop {r4, pc} + @ link register save eliminated. + eor r0, r0, r0, asr #31 + clz r0, r0 + sub r0, r0, #1 + bx lr As it (at least usually) will make code larger, it is disabled for -Os or cold instructions. 2021-08-19 Jakub Jelinek <ja...@redhat.com> PR middle-end/101950 * optabs.c (expand_clrsb_using_clz): New function. (expand_unop): Use it as another clrsb expansion fallback. * gcc.target/i386/pr101950-1.c: New test. * gcc.target/i386/pr101950-2.c: New test.
