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4cc67b0484
Follow an observation that (n ^ (n - 1)) will only ever retain the most significant bit set in the word operated on if that is the only bit set in the first place, and use it to determine whether a number is a whole power of 2, avoiding the need for an explicit check for nonzero. This reduces the sequence produced to 3 instructions only across Alpha, MIPS, and RISC-V targets, down from 4, 5, and 4 respectively, removing a branch in the two latter cases. And it's 5 instructions on POWER and x86-64 vs 8 and 9 respectively. There are no branches now emitted here for targets that have a suitable conditional set operation, although an inline expansion will often end with one, depending on what code a call to this function is used in. Credit goes to GCC authors for coming up with this optimisation used as the fallback for (__builtin_popcountl(n) == 1), equivalent to this code, for targets where the hardware population count operation is considered expensive. Link: https://lkml.kernel.org/r/alpine.DEB.2.21.2601111836250.30566@angie.orcam.me.uk Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk> Cc: Jens Axboe <axboe@kernel.dk> Cc: John Garry <john.g.garry@oracle.com> Cc: "Martin K. Petersen" <martin.petersen@oracle.com> Cc: Su Hui <suhui@nfschina.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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