How can I represent this if statement as an ARM instruction?

Question

if (R1 > R2 || R3 > R4)
    R1 = R1 + 1

I'm having trouble figuring out what to do with multiple conditions. Would the instructions look something like this?

CMP     R1, R2
CMPNE   R3, R4
ADDEQ   R1, R1, #1

You have the right approach, except your code does `if (R1 == R2 || R3 == R4)`, as you use `NE`/`EQ` instead of `LE`/`GT` — Chris Dodd, Aug 10 '15 at 19:02
possible duplicate of [Double Conditions in ARM assembly](http://stackoverflow.com/questions/19781523/double-conditions-in-arm-assembly) — artless noise, Aug 11 '15 at 18:31

Variable Length Coder · Answer 1 · 2015-08-10T23:59:08.803

1

if (R1 > R2 || R3 > R4)
    R1 = R1 + 1

The OR condition makes this tricky since it requires a short-circuit evaluation. What you can do is make a fall through comparison that gets evaluated only if r1 is NOT greater than r2, and then have the add instruction look at the results of the flag from the last comparison done.

cmp   r1, r2     ; compare r1 and r2
cmpls r3, r4     ; if lower or same, compare r3 and r4
addhi r1, r1, #1 ; if higher, add 1

edited Aug 10 '15 at 23:59

answered Aug 10 '15 at 19:02

Variable Length Coder

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you want `hi` rather than `pl` for unsigned greater than -- `pl` tests signed(R3-R4) >= 0 – Chris Dodd Aug 10 '15 at 19:07

How can I represent this if statement as an ARM instruction?

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