From: "David S." <[email protected]>
Sent: Tuesday, December 15, 2015 5:42 AM
Why do you use LM and STM to load the fields in to registers
and then out to memory again, rather than just using MVC for
a “memory to memory” move? ...
[eg] MVC ARG1(ARG1_L),CONSTS
MVC will work. And all else being equal, fewer instructions usually are
better.
And what's more, they don't modify registers.
But! In performance discussions on LinkeIn's "Mainframe Assembler
Professionals" group, like this one:
http://www.linkedin.com/groups/1462937/1462937-91082881 ...
...I've learned that LOAD/STORE sequences (IC/STC, L/ST, LM/STM) are much
faster than an equivalent length MVC. See comments in the above discussion
such as:
"CLC (like MVC) is very fast - but only after a start-up penalty. In the
MVC case using two LG/STG is much faster than a 16 byte MVC..."
and
"There is a good reason why small MVCs run slower than LG/STG pairs. There
is a bunch of overhead at the beginning of a MVC. Just one example: you
have to check if the operands are on "good" boundaries. If not, you tiptoe
your way to a good boundary. Then you can rip. Have to check for operand
overlap conditions. There is a length check involved. During this time the
LG/STG is far on its way. Some IBM machines (if I remember correctly) also
had a bigger fetch/store path for MVC. Again, once it got going. I can
assure you that a LG/STG pair is faster than a 16 byte MVC. 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.
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