I’m trying to puzzle something out… I hope this question isn’t too goofy.
I’m coding a macro to logically compare a halfword in storage to a 2-byte
unsigned constant that is calculated by the macro assembler. I want to generate
baseless code using as few instructions as possible, and I’d like to use the
fewest number of registers possible (although I have no problem using the
high-halves of any registers). My preference is to use CLHHSI to do the
compare, because no literal is required for the constant.
However: CLHHSI requires that the constant be the second (immediate) operand.
There will be times when the macro parameters will indicate that that
comparands must be reversed (i.e., where the immediate operand must be the
first comparand), in which case I can’t use CLHHSI.
I know that I can always code the comparands in the correct order by loading
them into the high-halves of R0 and R1, then use CLHHR. Or, of course, I can
generate a CLC with a literal. But I figured I’d ask: if I use CLHHSI, then (if
the macro determines that it’s necessary) can I modify the resulting condition
code to appear as though the original comparands were swapped? (I.e.: CC=00
stays the same, CC=01 becomes 10, CC=10 becomes 01.)
So far, this is what I’ve come up with (just as an example of the macro’s
generated code):
CLHHSI HALF,X'A24B' I like CLHHSI… no literal needed
JE DONE if the comparands are equal, don’t modify the
CC
IPM R1
XILF R1,X'30000000' flip the bits in the CC
SPM R1 set the new CC, program mask remains unmodified
DONE DS 0H
Is that the best I can do? Or might there be some clever way to modify the CC
using fewer (or shorter) instructions? (Or is my entire concept ridiculous?)
Thanks so much,
David
