From: "David S." <[email protected]>
Sent: Thursday, December 17, 2015 6:10 AM
> "... a good reason why small MVCs run slower than LG/STG pairs ...
> overhead at the beginning of an MVC ... check if the operands are on 'good'
> boundaries ... check for operand overlap ... length check ...
[meanwhile] the
> LG/STG is far on its way. Anyone writing a length-1 MVC instead of an
> IC/STC would be laughed out of their code review.
Rubbish.
KISS.
A move of length 1 is clear and unequivocal.
And it's only one line.
Nevertheless, IC/STC is what the newer mainframe compilers generate for a
one-byte move. For the reasons given, it's less work for the system than
an MVC.
If that was the only criterion ("less work for the system") we'd never
use any of the more complex instructions that were designed to
simplify the task of the programmer and which help him/her to
write and maintain programs easily.
Most of the time, you wouldn't notice any time difference.
But using two instructions (where one would suffice) increases
memory traffic and increases program bulk.
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