Sorry: LHI in the below should be AHI, of course. Charles
-----Original Message----- From: IBM Mainframe Assembler List [mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On Behalf Of Charles Mills Sent: Wednesday, March 10, 2021 4:56 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. Charles -----Original Message----- From: IBM Mainframe Assembler List [mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On Behalf Of Schmitt, Michael Sent: Wednesday, March 10, 2021 3:26 PM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Subject: Add 1, Subtract 1 I was taught long ago to add 1 to a register using LA r#,1(,r#) and to subtract 1 using BCTR r#,0. Is the fastest way now to use AHI r#,1 and AHI r#,-1?
