"(working storage or stack storage)" I interpret this is mean storage that is being ALTERED, not CONSTANTS. I would think that duplicate unchanged cache lines in the instruction and data caches would not have the same SERIOUS penalty as altering data would. But I am not a hardware engineer nor do I know if this is true or not.
I've noticed that IBM has been changing many of their macros to generate fewer inline constants with branches around them and use more literals (which can sometime surprise you with unexpected addressability problems when the data suddenly move from being very local) presumably to reduce the double cache usage (with or without the move/copy penalty), but one of the most glaring mixture of instructions and data that is (potentially) updated are the CVTEXIT and CVTBRET instructions. Programs invoked via system linkage have Register 14 pointing to CVTEXIT. The CVT is in the read/write nucleus and is not even cache line aligned! Keith Moe BMC Software, Inc. -------------------------------------------- On Wed, 8/1/18, Charles Mills <charl...@mcn.org> wrote: Subject: Re: EQU * considered harmful To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Date: Wednesday, August 1, 2018, 1:41 PM "Avoid instructions (executable code) and operand data (working storage or stack storage) in the same cache lines; which can be costly due to moving cache lines between the separated (split) local caches (instruction/data L1/L2)" -- C. Kevin Shum, Distinguished Engineer, IBM z Systems Microprocessor Development (March 2016) Charles -----Original Message----- From: IBM Mainframe Assembler List [mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On Behalf Of Keith Moe Sent: Wednesday, August 1, 2018 1:27 PM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Subject: Re: EQU * considered harmful Inline data is only a killer if it is updated. It is merely less efficient if it is read only (same cache line in instruction and data caches). My example was merely to show that the "INSTRUCT" statement would force half word alignment. Aside from macros that expand inline CONSTANTS (not updatable areas), I generally avoid mixing instructions and data. Even in non-reentrant code (unfortunately there's a lot here that I have to maintain and it's not worth it to make it reentrant), I try to isolate code blocks and data blocks. Keith Moe BMC Software, Inc. -------------------------------------------- On Wed, 8/1/18, Charles Mills <charl...@mcn.org> wrote: Subject: Re: EQU * considered harmful To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Date: Wednesday, August 1, 2018, 1:05 PM Well, one could argue that "DS" implies a variable, not instructions, and is therefore inappropriate as something on which to hang an instruction label. I like the idea of some kind of "instruction" attribute for EQU, with an error if you branched to a non-instruction symbol. I think I might argue for an EQU operand rather than a new opcode, but that is a quibble. J NEXTL DC CL(oddnumber)' ' NEXTL INSTRUCT You know that data mixed with instructions is just a performance KILLER on modern CPUs? They have separate i- and data caches, and mingling the two makes a mess that must be straightened out, at a cost of CPU cycles. Charles
