131 lines
5.7 KiB
Text
131 lines
5.7 KiB
Text
First up, let me say I don't like writing in assembler. It is not portable,
|
|
dependant on the particular CPU architecture release and is generally a pig
|
|
to debug and get right. Having said that, the x86 architecture is probably
|
|
the most important for speed due to number of boxes and since
|
|
it appears to be the worst architecture to to get
|
|
good C compilers for. So due to this, I have lowered myself to do
|
|
assembler for the inner DES routines in libdes :-).
|
|
|
|
The file to implement in assembler is des_enc.c. Replace the following
|
|
4 functions
|
|
des_encrypt1(DES_LONG data[2],des_key_schedule ks, int encrypt);
|
|
des_encrypt2(DES_LONG data[2],des_key_schedule ks, int encrypt);
|
|
des_encrypt3(DES_LONG data[2],des_key_schedule ks1,ks2,ks3);
|
|
des_decrypt3(DES_LONG data[2],des_key_schedule ks1,ks2,ks3);
|
|
|
|
They encrypt/decrypt the 64 bits held in 'data' using
|
|
the 'ks' key schedules. The only difference between the 4 functions is that
|
|
des_encrypt2() does not perform IP() or FP() on the data (this is an
|
|
optimization for when doing triple DES and des_encrypt3() and des_decrypt3()
|
|
perform triple des. The triple DES routines are in here because it does
|
|
make a big difference to have them located near the des_encrypt2 function
|
|
at link time..
|
|
|
|
Now as we all know, there are lots of different operating systems running on
|
|
x86 boxes, and unfortunately they normally try to make sure their assembler
|
|
formating is not the same as the other peoples.
|
|
The 4 main formats I know of are
|
|
Microsoft Windows 95/Windows NT
|
|
Elf Includes Linux and FreeBSD(?).
|
|
a.out The older Linux.
|
|
Solaris Same as Elf but different comments :-(.
|
|
|
|
Now I was not overly keen to write 4 different copies of the same code,
|
|
so I wrote a few perl routines to output the correct assembler, given
|
|
a target assembler type. This code is ugly and is just a hack.
|
|
The libraries are x86unix.pl and x86ms.pl.
|
|
des586.pl, des686.pl and des-som[23].pl are the programs to actually
|
|
generate the assembler.
|
|
|
|
So to generate elf assembler
|
|
perl des-som3.pl elf >dx86-elf.s
|
|
For Windows 95/NT
|
|
perl des-som2.pl win32 >win32.asm
|
|
|
|
[ update 4 Jan 1996 ]
|
|
I have added another way to do things.
|
|
perl des-som3.pl cpp >dx86-cpp.s
|
|
generates a file that will be included by dx86unix.cpp when it is compiled.
|
|
To build for elf, a.out, solaris, bsdi etc,
|
|
cc -E -DELF asm/dx86unix.cpp | as -o asm/dx86-elf.o
|
|
cc -E -DSOL asm/dx86unix.cpp | as -o asm/dx86-sol.o
|
|
cc -E -DOUT asm/dx86unix.cpp | as -o asm/dx86-out.o
|
|
cc -E -DBSDI asm/dx86unix.cpp | as -o asm/dx86bsdi.o
|
|
This was done to cut down the number of files in the distribution.
|
|
|
|
Now the ugly part. I acquired my copy of Intels
|
|
"Optimization's For Intel's 32-Bit Processors" and found a few interesting
|
|
things. First, the aim of the exersize is to 'extract' one byte at a time
|
|
from a word and do an array lookup. This involves getting the byte from
|
|
the 4 locations in the word and moving it to a new word and doing the lookup.
|
|
The most obvious way to do this is
|
|
xor eax, eax # clear word
|
|
movb al, cl # get low byte
|
|
xor edi DWORD PTR 0x100+des_SP[eax] # xor in word
|
|
movb al, ch # get next byte
|
|
xor edi DWORD PTR 0x300+des_SP[eax] # xor in word
|
|
shr ecx 16
|
|
which seems ok. For the pentium, this system appears to be the best.
|
|
One has to do instruction interleaving to keep both functional units
|
|
operating, but it is basically very efficient.
|
|
|
|
Now the crunch. When a full register is used after a partial write, eg.
|
|
mov al, cl
|
|
xor edi, DWORD PTR 0x100+des_SP[eax]
|
|
386 - 1 cycle stall
|
|
486 - 1 cycle stall
|
|
586 - 0 cycle stall
|
|
686 - at least 7 cycle stall (page 22 of the above mentioned document).
|
|
|
|
So the technique that produces the best results on a pentium, according to
|
|
the documentation, will produce hideous results on a pentium pro.
|
|
|
|
To get around this, des686.pl will generate code that is not as fast on
|
|
a pentium, should be very good on a pentium pro.
|
|
mov eax, ecx # copy word
|
|
shr ecx, 8 # line up next byte
|
|
and eax, 0fch # mask byte
|
|
xor edi DWORD PTR 0x100+des_SP[eax] # xor in array lookup
|
|
mov eax, ecx # get word
|
|
shr ecx 8 # line up next byte
|
|
and eax, 0fch # mask byte
|
|
xor edi DWORD PTR 0x300+des_SP[eax] # xor in array lookup
|
|
|
|
Due to the execution units in the pentium, this actually works quite well.
|
|
For a pentium pro it should be very good. This is the type of output
|
|
Visual C++ generates.
|
|
|
|
There is a third option. instead of using
|
|
mov al, ch
|
|
which is bad on the pentium pro, one may be able to use
|
|
movzx eax, ch
|
|
which may not incur the partial write penalty. On the pentium,
|
|
this instruction takes 4 cycles so is not worth using but on the
|
|
pentium pro it appears it may be worth while. I need access to one to
|
|
experiment :-).
|
|
|
|
eric (20 Oct 1996)
|
|
|
|
22 Nov 1996 - I have asked people to run the 2 different version on pentium
|
|
pros and it appears that the intel documentation is wrong. The
|
|
mov al,bh is still faster on a pentium pro, so just use the des586.pl
|
|
install des686.pl
|
|
|
|
3 Dec 1996 - I added des_encrypt3/des_decrypt3 because I have moved these
|
|
functions into des_enc.c because it does make a massive performance
|
|
difference on some boxes to have the functions code located close to
|
|
the des_encrypt2() function.
|
|
|
|
9 Jan 1997 - des-som2.pl is now the correct perl script to use for
|
|
pentiums. It contains an inner loop from
|
|
Svend Olaf Mikkelsen <svolaf@inet.uni-c.dk> which does raw ecb DES calls at
|
|
273,000 per second. He had a previous version at 250,000 and the best
|
|
I was able to get was 203,000. The content has not changed, this is all
|
|
due to instruction sequencing (and actual instructions choice) which is able
|
|
to keep both functional units of the pentium going.
|
|
We may have lost the ugly register usage restrictions when x86 went 32 bit
|
|
but for the pentium it has been replaced by evil instruction ordering tricks.
|
|
|
|
13 Jan 1997 - des-som3.pl, more optimizations from Svend Olaf.
|
|
raw DES at 281,000 per second on a pentium 100.
|
|
|