| This directory contains mpn functions optimized for Intel Pentium |
| processors. |
| |
| RELEVANT OPTIMIZATION ISSUES |
| |
| 1. Pentium doesn't allocate cache lines on writes, unlike most other modern |
| processors. Since the functions in the mpn class do array writes, we have to |
| handle allocating the destination cache lines by reading a word from it in the |
| loops, to achieve the best performance. |
| |
| 2. Pairing of memory operations requires that the two issued operations refer |
| to different cache banks. The simplest way to insure this is to read/write |
| two words from the same object. If we make operations on different objects, |
| they might or might not be to the same cache bank. |
| |
| STATUS |
| |
| 1. mpn_lshift and mpn_rshift run at about 6 cycles/limb, but the Pentium |
| documentation indicates that they should take only 43/8 = 5.375 cycles/limb, |
| or 5 cycles/limb asymptotically. |
| |
| 2. mpn_add_n and mpn_sub_n run at asymptotically 2 cycles/limb. Due to loop |
| overhead and other delays (cache refill?), they run at or near 2.5 cycles/limb. |
| |
| 3. mpn_mul_1, mpn_addmul_1, mpn_submul_1 all run 1 cycle faster than they |
| should... |
