Re: [PATCH] flex_array: Avoid divisions when accessing elements.

[Dave Hansen]

On Tue, 2011-04-26 at 13:41 -0700, Jesse Gross wrote:
> On most architectures division is an expensive operation and
> accessing an element currently requires four of them.  This
> performance penalty effectively precludes flex arrays from
> being used on any kind of fast path.  However, two of these
> divisions can be handled at creation time and the others can
> be replaced by a reciprocal divide, completely avoiding real
> divisions on access. 

flex_array.c has a nice table for how many objects can be allocated:

 * Element size | Objects | Objects |
 * PAGE_SIZE=4k |  32-bit |  64-bit |
 * ---------------------------------|
 *      1 bytes | 4186112 | 2093056 |
 *      2 bytes | 2093056 | 1046528 |
 *      3 bytes | 1395030 |  697515 |
 *      4 bytes | 1046528 |  523264 |
 *     32 bytes |  130816 |   65408 |
 *     33 bytes |  126728 |   63364 |
 *   2048 bytes |    2044 |    1022 |
 *   2049 bytes |    1022 |     511 |
 *       void * | 1046528 |  261632 |

This patch changes that a bit.  Would you mind updating it?

-- Dave

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