On Thu, Feb 01, 2024 at 01:22:07PM +0100, Alexander Lobakin wrote: > The number of times yet another open coded > `BITS_TO_LONGS(nbits) * sizeof(long)` can be spotted is huge. > Some generic helper is long overdue. > > Add one, bitmap_size(), but with one detail. > BITS_TO_LONGS() uses DIV_ROUND_UP(). The latter works well when both > divident and divisor are compile-time constants or when the divisor > is not a pow-of-2. When it is however, the compilers sometimes tend > to generate suboptimal code (GCC 13): > > 48 83 c0 3f add $0x3f,%rax > 48 c1 e8 06 shr $0x6,%rax > 48 8d 14 c5 00 00 00 00 lea 0x0(,%rax,8),%rdx > > %BITS_PER_LONG is always a pow-2 (either 32 or 64), but GCC still does > full division of `nbits + 63` by it and then multiplication by 8. > Instead of BITS_TO_LONGS(), use ALIGN() and then divide by 8. GCC: > > 8d 50 3f lea 0x3f(%rax),%edx > c1 ea 03 shr $0x3,%edx > 81 e2 f8 ff ff 1f and $0x1ffffff8,%edx > > Now it shifts `nbits + 63` by 3 positions (IOW performs fast division > by 8) and then masks bits[2:0]. bloat-o-meter: > > add/remove: 0/0 grow/shrink: 20/133 up/down: 156/-773 (-617) > > Clang does it better and generates the same code before/after starting > from -O1, except that with the ALIGN() approach it uses %edx and thus > still saves some bytes: > > add/remove: 0/0 grow/shrink: 9/133 up/down: 18/-538 (-520) > > Note that we can't expand DIV_ROUND_UP() by adding a check and using > this approach there, as it's used in array declarations where > expressions are not allowed. > Add this helper to tools/ as well. > > Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@xxxxxxxxx> > Acked-by: Yury Norov <yury.norov@xxxxxxxxx> > Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx> Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
