> I need 27 instructions (maybe more because of two JO after TROT) see end of
> this post.
>
> But I am sure this code is faster than what was used before. The simple
> relation 256 input bytes processed by 27 instructions to create 192 bytes -
> should speed up the whole precess by a factor (even if the TROT is slowing
> stuff down).
I uploaded your code and tested it last weekend (lovely to have a the lpar all
to yourself). It seems you where somewhat overly optimistic about the effects
of 'using only 27 instructions to process 256 input bytes'. The code is more
than 4 times slower than the original 'process 8 bytes using RISBG' variant.
I think the memory accesses are killing you. The original code does one TROO to
convert the Base64 code to bytes with a 0...63 value and then every source byte
is read once and every result byte is written once. Assuming a translate
instruction does 2 reads and 1 write this results in 3 reads and 1.75 writes
for every source byte. Your code accesses memory a lot more. I tried a quick
tally:
Access
MVC INTER,TR1 256*RW
TR INTER,INPUT 256*RRW
MVC INTER2,TR2 256*RW
TR INTER2,INTER-1 256*RRW
MVC RESULT,INTER2 192*RW
NC RESULT,TR31+2 192*RRW
NC INTER2,TR31 192*RRW
TR INTER2,TR32 256*RRW
OC RESULT,INTER2 192*RRW
LA R1,TR4
LA R14,INTER
LA R15,64
LA R2,INTER+128
TROT R14,R2,1 64*RRRWW
JO *-4
MVC INTER2,TR6 256*RW
TR INTER2,INTER-1 256*RRW
OC RESULT,INTER2 256*RRW
LA R1,TR5
LA R14,INTER
LA R15,64
LA R2,INTER+192
TROT R14,R2,1 64*RRRWW
JO *-4
MVC INTER2,TR6-1 256*RW
TR INTER2,INTER-1 256*RRW
OC RESULT,INTER2 192*RRW
6208 bytes read and 3776 bytes read to process 256 bytes means 24.25 reads and
14.75 writes for every source byte (versus 3 reads and 1.75 writes).
We've seen simple 'rules of thumb' on writing faster code before in this group.
Maybe we should add something about 'minimizing memory access':
1. Select the best algorithm
2. Minimize the number of memory accesses
3. Minimize the number of instructions
We also had something about using 'simple' versus 'complex' (millicoded)
instructions but that also tends to invoke religious discussions about
maintainability and such...
Fred!
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