1 First up, let me say I don't like writing in assembler. It is not portable,
2 dependant on the particular CPU architecture release and is generally a pig
3 to debug and get right. Having said that, the x86 architecture is probably
4 the most important for speed due to number of boxes and since
5 it appears to be the worst architecture to to get
6 good C compilers for. So due to this, I have lowered myself to do
7 assembler for the inner DES routines in libdes :-).
9 The file to implement in assembler is des_enc.c. Replace the following
11 des_encrypt1(DES_LONG data[2],des_key_schedule ks, int encrypt);
12 des_encrypt2(DES_LONG data[2],des_key_schedule ks, int encrypt);
13 des_encrypt3(DES_LONG data[2],des_key_schedule ks1,ks2,ks3);
14 des_decrypt3(DES_LONG data[2],des_key_schedule ks1,ks2,ks3);
16 They encrypt/decrypt the 64 bits held in 'data' using
17 the 'ks' key schedules. The only difference between the 4 functions is that
18 des_encrypt2() does not perform IP() or FP() on the data (this is an
19 optimization for when doing triple DES and des_encrypt3() and des_decrypt3()
20 perform triple des. The triple DES routines are in here because it does
21 make a big difference to have them located near the des_encrypt2 function
24 Now as we all know, there are lots of different operating systems running on
25 x86 boxes, and unfortunately they normally try to make sure their assembler
26 formating is not the same as the other peoples.
27 The 4 main formats I know of are
28 Microsoft Windows 95/Windows NT
29 Elf Includes Linux and FreeBSD(?).
30 a.out The older Linux.
31 Solaris Same as Elf but different comments :-(.
33 Now I was not overly keen to write 4 different copies of the same code,
34 so I wrote a few perl routines to output the correct assembler, given
35 a target assembler type. This code is ugly and is just a hack.
36 The libraries are x86unix.pl and x86ms.pl.
37 des586.pl, des686.pl and des-som[23].pl are the programs to actually
38 generate the assembler.
40 So to generate elf assembler
41 perl des-som3.pl elf >dx86-elf.s
43 perl des-som2.pl win32 >win32.asm
46 I have added another way to do things.
47 perl des-som3.pl cpp >dx86-cpp.s
48 generates a file that will be included by dx86unix.cpp when it is compiled.
49 To build for elf, a.out, solaris, bsdi etc,
50 cc -E -DELF asm/dx86unix.cpp | as -o asm/dx86-elf.o
51 cc -E -DSOL asm/dx86unix.cpp | as -o asm/dx86-sol.o
52 cc -E -DOUT asm/dx86unix.cpp | as -o asm/dx86-out.o
53 cc -E -DBSDI asm/dx86unix.cpp | as -o asm/dx86bsdi.o
54 This was done to cut down the number of files in the distribution.
56 Now the ugly part. I acquired my copy of Intels
57 "Optimization's For Intel's 32-Bit Processors" and found a few interesting
58 things. First, the aim of the exersize is to 'extract' one byte at a time
59 from a word and do an array lookup. This involves getting the byte from
60 the 4 locations in the word and moving it to a new word and doing the lookup.
61 The most obvious way to do this is
62 xor eax, eax # clear word
63 movb al, cl # get low byte
64 xor edi DWORD PTR 0x100+des_SP[eax] # xor in word
65 movb al, ch # get next byte
66 xor edi DWORD PTR 0x300+des_SP[eax] # xor in word
68 which seems ok. For the pentium, this system appears to be the best.
69 One has to do instruction interleaving to keep both functional units
70 operating, but it is basically very efficient.
72 Now the crunch. When a full register is used after a partial write, eg.
74 xor edi, DWORD PTR 0x100+des_SP[eax]
78 686 - at least 7 cycle stall (page 22 of the above mentioned document).
80 So the technique that produces the best results on a pentium, according to
81 the documentation, will produce hideous results on a pentium pro.
83 To get around this, des686.pl will generate code that is not as fast on
84 a pentium, should be very good on a pentium pro.
85 mov eax, ecx # copy word
86 shr ecx, 8 # line up next byte
87 and eax, 0fch # mask byte
88 xor edi DWORD PTR 0x100+des_SP[eax] # xor in array lookup
89 mov eax, ecx # get word
90 shr ecx 8 # line up next byte
91 and eax, 0fch # mask byte
92 xor edi DWORD PTR 0x300+des_SP[eax] # xor in array lookup
94 Due to the execution units in the pentium, this actually works quite well.
95 For a pentium pro it should be very good. This is the type of output
98 There is a third option. instead of using
100 which is bad on the pentium pro, one may be able to use
102 which may not incur the partial write penalty. On the pentium,
103 this instruction takes 4 cycles so is not worth using but on the
104 pentium pro it appears it may be worth while. I need access to one to
109 22 Nov 1996 - I have asked people to run the 2 different version on pentium
110 pros and it appears that the intel documentation is wrong. The
111 mov al,bh is still faster on a pentium pro, so just use the des586.pl
114 3 Dec 1996 - I added des_encrypt3/des_decrypt3 because I have moved these
115 functions into des_enc.c because it does make a massive performance
116 difference on some boxes to have the functions code located close to
117 the des_encrypt2() function.
119 9 Jan 1997 - des-som2.pl is now the correct perl script to use for
120 pentiums. It contains an inner loop from
121 Svend Olaf Mikkelsen <svolaf@inet.uni-c.dk> which does raw ecb DES calls at
122 273,000 per second. He had a previous version at 250,000 and the best
123 I was able to get was 203,000. The content has not changed, this is all
124 due to instruction sequencing (and actual instructions choice) which is able
125 to keep both functional units of the pentium going.
126 We may have lost the ugly register usage restrictions when x86 went 32 bit
127 but for the pentium it has been replaced by evil instruction ordering tricks.
129 13 Jan 1997 - des-som3.pl, more optimizations from Svend Olaf.
130 raw DES at 281,000 per second on a pentium 100.