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diff --git a/gmp-6.3.0/mpn/x86/pentium/README b/gmp-6.3.0/mpn/x86/pentium/README new file mode 100644 index 0000000..305936b --- /dev/null +++ b/gmp-6.3.0/mpn/x86/pentium/README @@ -0,0 +1,181 @@ +Copyright 1996, 1999-2001, 2003 Free Software Foundation, Inc. + +This file is part of the GNU MP Library. + +The GNU MP Library is free software; you can redistribute it and/or modify +it under the terms of either: + + * the GNU Lesser General Public License as published by the Free + Software Foundation; either version 3 of the License, or (at your + option) any later version. + +or + + * the GNU General Public License as published by the Free Software + Foundation; either version 2 of the License, or (at your option) any + later version. + +or both in parallel, as here. + +The GNU MP Library is distributed in the hope that it will be useful, but +WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY +or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License +for more details. + +You should have received copies of the GNU General Public License and the +GNU Lesser General Public License along with the GNU MP Library. If not, +see https://www.gnu.org/licenses/. + + + + + + INTEL PENTIUM P5 MPN SUBROUTINES + + +This directory contains mpn functions optimized for Intel Pentium (P5,P54) +processors. The mmx subdirectory has additional code for Pentium with MMX +(P55). + + +STATUS + + cycles/limb + + mpn_add_n/sub_n 2.375 + + mpn_mul_1 12.0 + mpn_add/submul_1 14.0 + + mpn_mul_basecase 14.2 cycles/crossproduct (approx) + + mpn_sqr_basecase 8 cycles/crossproduct (approx) + or 15.5 cycles/triangleproduct (approx) + + mpn_l/rshift 5.375 normal (6.0 on P54) + 1.875 special shift by 1 bit + + mpn_divrem_1 44.0 + mpn_mod_1 28.0 + mpn_divexact_by3 15.0 + + mpn_copyi/copyd 1.0 + +Pentium MMX gets the following improvements + + mpn_l/rshift 1.75 + + mpn_mul_1 12.0 normal, 7.0 for 16-bit multiplier + + +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. + +mpn_mul_1, mpn_addmul_1, mpn_submul_1 all run 1 cycle faster than they +should. Intel documentation says a mul instruction is 10 cycles, but it +measures 9 and the routines using it run as 9. + + + +P55 MMX AND X87 + +The cost of switching between MMX and x87 floating point on P55 is about 100 +cycles (fld1/por/emms for instance). In order to avoid that the two aren't +mixed and currently that means using MMX and not x87. + +MMX offers a big speedup for lshift and rshift, and a nice speedup for +16-bit multipliers in mpn_mul_1. If fast code using x87 is found then +perhaps the preference for MMX will be reversed. + + + + +P54 SHLDL + +mpn_lshift and mpn_rshift run at about 6 cycles/limb on P5 and P54, but the +documentation indicates that they should take only 43/8 = 5.375 cycles/limb, +or 5 cycles/limb asymptotically. The P55 runs them at the expected speed. + +It seems that on P54 a shldl or shrdl allows pairing in one following cycle, +but not two. For example, back to back repetitions of the following + + shldl( %cl, %eax, %ebx) + xorl %edx, %edx + xorl %esi, %esi + +run at 5 cycles, as expected, but repetitions of the following run at 7 +cycles, whereas 6 would be expected (and is achieved on P55), + + shldl( %cl, %eax, %ebx) + xorl %edx, %edx + xorl %esi, %esi + xorl %edi, %edi + xorl %ebp, %ebp + +Three xorls run at 7 cycles too, so it doesn't seem to be just that pairing +inhibited is only in the second following cycle (or something like that). + +Avoiding this problem would bring P54 shifts down from 6.0 c/l to 5.5 with a +pattern of shift, 2 loads, shift, 2 stores, shift, etc. A start has been +made on something like that, but it's not yet complete. + + + + +OTHER NOTES + +Prefetching Destinations + + 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. + +Prefetching Sources + + Prefetching of sources is pointless since there's no out-of-order loads. + Any load instruction blocks until the line is brought to L1, so it may + as well be the load that wants the data which blocks. + +Data Cache Bank Clashes + + Pairing of memory operations requires that the two issued operations + refer to different cache banks (ie. different addresses modulo 32 + bytes). The simplest way to ensure 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. + +PIC %eip Fetching + + A simple call $+5 and popl can be used to get %eip, there's no need to + balance calls and returns since P5 doesn't have any return stack branch + prediction. + +Float Multiplies + + fmul is pairable and can be issued every 2 cycles (with a 4 cycle + latency for data ready to use). This is a lot better than integer mull + or imull at 9 cycles non-pairing. Unfortunately the advantage is + quickly eaten away by needing to throw data through memory back to the + integer registers to adjust for fild and fist being signed, and to do + things like propagating carry bits. + + + + + +REFERENCES + +"Intel Architecture Optimization Manual", 1997, order number 242816. This +is mostly about P5, the parts about P6 aren't relevant. Available on-line: + + http://download.intel.com/design/PentiumII/manuals/242816.htm + + + +---------------- +Local variables: +mode: text +fill-column: 76 +End: |