Instruction speed is still a concept, but it is much less relevant to
predicting performance than it used to be, and even in the old days the
execution time of an instruction could differ based on its data. Pipeling and
caches are subject to various kinds of flushes and stalls. Predicting the
performance of an instruction mix on a particular processor is far too
complicated to be practical.
LA behaves differently depending on the addressing mode.
The importance of code maintainability and readability cannot be overstated. If
a code sequence is not crystal clear, add a comment to explain what, how and
why it does what it does, in terms of the application. Pillory the programmer
that writes
L R7,=A(MAGIC_NUMBER) Put magic number in register 7
Note: you can also use equated symbols in LA, and should, for the same reasons
that you should use them on AHI.
--
Shmuel (Seymour J.) Metz
http://mason.gmu.edu/~smetz3
________________________________________
From: IBM Mainframe Assembler List [ASSEMBLER-LIST@LISTSERV.UGA.EDU] on behalf
of Charles Mills [charl...@mcn.org]
Sent: Wednesday, March 10, 2021 7:56 PM
To: ASSEMBLER-LIST@LISTSERV.UGA.EDU
Subject: Re: Add 1, Subtract 1
1. "Instruction speed" is not exactly a concept anymore due to pipelining.
An instruction can take literally zero effective time because it overlaps
with something else such as a wait for cache. "Does an AHI or an LA take
longer?" is no longer a question that has an answer.
2. AHI is wonderful but not exactly the functional equivalent of LA and
BCTR. LA can add two registers plus an offset, AHI only one register. AHI
sets the condition code, which may be good for your situation, or bad. BCTR
is a branch instruction, which may affect cache performance, although modern
CPUs are probably smart enough to realize that BCTR x,0 is not really a
branch.
3. Code readability is much more important than instruction speed. CPUs are
fast; programmers are relatively very, very slow and error-prone. I like AHI
for its readability. LHI says what it does: adds an immediate value to a
register. Would a novice read BCTR as subtracting one? LHI also takes
equates, which improve maintainability. Prefix_Offset EQU
TablePrefix-TableStart / AHI R1,Prefix_Offset is a lot clearer than BCTR
R1,0 / BCTR R1,0.
