1Copyright 2001 Free Software Foundation, Inc.
2
3This file is part of the GNU MP Library.
4
5The GNU MP Library is free software; you can redistribute it and/or modify
6it under the terms of either:
7
8  * the GNU Lesser General Public License as published by the Free
9    Software Foundation; either version 3 of the License, or (at your
10    option) any later version.
11
12or
13
14  * the GNU General Public License as published by the Free Software
15    Foundation; either version 2 of the License, or (at your option) any
16    later version.
17
18or both in parallel, as here.
19
20The GNU MP Library is distributed in the hope that it will be useful, but
21WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
22or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
23for more details.
24
25You should have received copies of the GNU General Public License and the
26GNU Lesser General Public License along with the GNU MP Library.  If not,
27see https://www.gnu.org/licenses/.
28
29
30
31
32                   INTEL PENTIUM-4 MPN SUBROUTINES
33
34
35This directory contains mpn functions optimized for Intel Pentium-4.
36
37The mmx subdirectory has routines using MMX instructions, the sse2
38subdirectory has routines using SSE2 instructions.  All P4s have these, the
39separate directories are just so configure can omit that code if the
40assembler doesn't support it.
41
42
43STATUS
44
45                                cycles/limb
46
47          mpn_add_n/sub_n            4 normal, 6 in-place
48
49          mpn_mul_1                  4 normal, 6 in-place
50          mpn_addmul_1               6
51          mpn_submul_1               7
52
53          mpn_mul_basecase           6 cycles/crossproduct (approx)
54
55          mpn_sqr_basecase           3.5 cycles/crossproduct (approx)
56                                   or 7.0 cycles/triangleproduct (approx)
57
58          mpn_l/rshift               1.75
59
60
61
62The shifts ought to be able to go at 1.5 c/l, but not much effort has been
63applied to them yet.
64
65In-place operations, and all addmul, submul, mul_basecase and sqr_basecase
66calls, suffer from pipeline anomalies associated with write combining and
67movd reads and writes to the same or nearby locations.  The movq
68instructions do not trigger the same hardware problems.  Unfortunately,
69using movq and splitting/combining seems to require too many extra
70instructions to help.  Perhaps future chip steppings will be better.
71
72
73
74NOTES
75
76The Pentium-4 pipeline "Netburst", provides for quite a number of surprises.
77Many traditional x86 instructions run very slowly, requiring use of
78alterative instructions for acceptable performance.
79
80adcl and sbbl are quite slow at 8 cycles for reg->reg.  paddq of 32-bits
81within a 64-bit mmx register seems better, though the combination
82paddq/psrlq when propagating a carry is still a 4 cycle latency.
83
84incl and decl should be avoided, instead use add $1 and sub $1.  Apparently
85the carry flag is not separately renamed, so incl and decl depend on all
86previous flags-setting instructions.
87
88shll and shrl have a 4 cycle latency, or 8 times the latency of the fastest
89integer instructions (addl, subl, orl, andl, and some more).  shldl and
90shrdl seem to have 13 and 15 cycles latency, respectively.  Bizarre.
91
92movq mmx -> mmx does have 6 cycle latency, as noted in the documentation.
93pxor/por or similar combination at 2 cycles latency can be used instead.
94The movq however executes in the float unit, thereby saving MMX execution
95resources.  With the right juggling, data moves shouldn't be on a dependent
96chain.
97
98L1 is write-through, but the write-combining sounds like it does enough to
99not require explicit destination prefetching.
100
101xmm registers so far haven't found a use, but not much effort has been
102expended.  A configure test for whether the operating system knows
103fxsave/fxrestor will be needed if they're used.
104
105
106
107REFERENCES
108
109Intel Pentium-4 processor manuals,
110
111          http://developer.intel.com/design/pentium4/manuals
112
113"Intel Pentium 4 Processor Optimization Reference Manual", Intel, 2001,
114order number 248966.  Available on-line:
115
116          http://developer.intel.com/design/pentium4/manuals/248966.htm
117
118
119
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