Re: [PATCH net-next v5 12/21] bitmap: introduce generic optimized bitmap_size()

[Yury Norov <yury.norov@gmail.com>]

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@intel.com>
> Acked-by: Yury Norov <yury.norov@gmail.com>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>

Signed-off-by: Yury Norov <yury.norov@gmail.com>