Is it possible to make decisions in assembly without using `jump` and `goto` at all?

Question

In this question, some answers showed how it is possible to make decisions without using "if" statements, however I suspect this is possible because "if" is not the only statement that generates jump instructions.

Given a fixed compiled language (in example C++), can the generated assembly do some kind of decisions without using jump and goto instructions?

Please give an example alternative to a simple if/else statement that does not use such instructions in case of affirmative answer.

Answer 1

The ARM architecture has an interesting conditional execution trait. Running in full ARM mode, nearly every instruction can have a condition attached to it. These are the same conditions that are used on the Branch instructions. An instruction such as add r0, r0, #15 will always execute, but an instruction such as addeq r0, r0, #15 will only execute if the zero flag is set. An equivalent using branches would be:

    beq afteradd       ; skip add if equal
    add r0, r0, #15    ; add 15 to R0
afteradd:

When running on a core that uses Thumb-2, the conditional execution is more limited, but still possible by using the IT instruction. This instruction will create an "if-then" construct without using branches. The IT construct is actually part of ARM's Unified Assembly Language, and should be used whether you're writing for ARM or Thumb-2. Here's the UAL version of the conditional add from above.

it eq                 ; if equal
addeq r0, r0, #15     ; then add 15 to r0

The IT construct can include more than just a single instruction. Using a series of T and E in the instruction, you can add more conditional instructions. In the below example, we will add 15 to R0 if the zero flag is set, otherwise we will subtract 15 from R0. The ITE means, fairly literally, if-then-else. The first following instruction should have a condition that matches your ITE condition, and then the second instruction will become the "else" and should have a condition that is the opposite of the ITE condition.

ite eq                ; if equal
addeq r0, r0, #15     ; then add 15 to r0
subne r0, r0, #15     ; else subtract 15 from r0

I guess this kind of does use if, which may go against what you were asking. In terms of the execution, this implements the if without using a jump, though.