Another way to clear the register (not really recommended but it works :-)
SRL R15,32
Mike Walter <[EMAIL PROTECTED]>
Sent by: The IBM z/VM Operating System <IBMVM@LISTSERV.UARK.EDU>
12/04/2006 03:01 PM
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The IBM z/VM Operating System <IBMVM@LISTSERV.UARK.EDU>
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Re: CMSCALL return code
Sheesh, this goes way back to my good old Assembler diaper days when
programmers really cared about performance instead of drag and drop
solutions.
Slightly off-topic: if I remember correctly, we argued intensely about
zeroing a GPR and the performance differences between:
- SR R15,R15
- XR R15,R15
- LA R15,0 (not seriously considered by performance geeks)
- L R15,=F'0' (considered for use only by amateur programmers coming from
a BASIC or COBOL background and otherwise held in low esteem by "real
programmers"). ;-)
IIRC, the actual performance difference between SR and XR was different
based more on specific processor models that anything else.
Mike Walter
Hewitt Associates
Any opinions expressed herein are mine alone and do not necessarily
represent the opinions or policies of Hewitt Associates.
"Schuh, Richard" <[EMAIL PROTECTED]>
Sent by: "The IBM z/VM Operating System" <IBMVM@LISTSERV.UARK.EDU>
12/04/2006 11:37 AM
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"The IBM z/VM Operating System" <IBMVM@LISTSERV.UARK.EDU>
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Re: CMSCALL return code
True, and it is undoubtedly faster to use SR R15,R15 than it is to use LA
R15,0 to zero the register - there are no storage fetches and real
subtraction is not needed if the result can be predicted, as it can in
this case. However, the discussion had more to do with fetches of
boundary-aligned vs. non-aligned data. There was no mention of the optimum
speed for getting either a specific or an arbitrary value loaded into a
register. In this day of pipelined machines
This is sort of reminiscent of the good old days, programming in 7080
Autocoder. Boeing insisted that the programmers use a MOVE macro because
there were 26 different ways to move data from one storage location to
another. It was expected that most programmers would use either their
favorite way or the first one that popped into their heads if left on
their own. The macro chose the optimal way, depending on the operand
definitions.
From: The IBM z/VM Operating System [mailto:[EMAIL PROTECTED] On
Behalf Of Stanley Rarick
Sent: Friday, December 01, 2006 10:37 PM
To: IBMVM@LISTSERV.UARK.EDU
Subject: Re: CMSCALL return code
For a return code, LA R15,value is *much* faster than a L - only one
storage fetch.
Schuh, Richard wrote:
I really would not have left it to chance, I would have defined a
word-aligned constant rather than using a literal. However, it might not
have been as chancy as it may seem. The literal pool is doubleword
aligned and boundary alignment may have been a factor in determining
where the literal resided. I would like to think that the 8-byte
multiples are put at the front, the 4-byters next, then the twos
followed by everybody else. In looking at an assembly listing, that
seems to be the sequence. The first two literals in the program are
=x'0000A00', the next =x'FF', etc. In the literal pool, all 4 byte
entries (there were no 8 byte literals) precede the two byte literals
and then come the ones of only 1 byte. Within each of these groups, the
literals appear in the order in which they were defined. There were no
long strings defined as literals in the particular listing.
-----Original Message-----
From: The IBM z/VM Operating System [mailto:[EMAIL PROTECTED] On
Behalf Of Don Russell
Sent: Tuesday, November 21, 2006 3:46 PM
To: IBMVM@LISTSERV.UARK.EDU
Subject: Re: CMSCALL return code
Schuh, Richard wrote:
I agree, it does seem non-intuitive. The initial SR R15,R15 was
undoubtedly preparing for a default rc of zero. How the non-zero rc
gets put into the register later is largely a matter of taste. In this
case I
probably would have chosen L R15,=X'...' - a habit learned, when
machines were slower, based on the knowledge that they were mostly
optimized for the LOAD instruction vs. any other way of putting data
from memory into a register.
If your habit was to use L Rx,=X'...' you were probably lucky in the old
days.... the =X literal would not necessarily be word-aligned, causing
two fetches to load the register, or, in the days when alignment really
mattered... a program exception.
Better to use L R15,=A(X'...') if alignment is a concern and you want to
use literals.
Then the literal IS aligned on a fullword boundary.
The initial SR 15,15 is unlikely to be setting the default return code..
.it's clearing the register preparing for the different option bytes to
be OR'd in. I agree the macro could (should?) have generated a single L
instruction instead, but then what nits would we have to discuss? :-)
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