229 RISC-V Examples - If/Elif/Else Conditional

This page covers an if/else if/else conditional statement in RISC-V.

This control-flow construct is used to select one among multiple mutually exclusive (only one is executed) conditions:

if (condition1) { ..do something.. }
else if (condition2) { ..do a different thing.. }
else { ..do something else.. }
..do things afterwards..

The flow execution, also called the Control-Flow Graph, for this portion of program is shown on the right.

The idea to create an implementation is that the assembly code gathers all conditions in one place (the top of the conditional block) and then branches to a block of code within the body of that conditional. Rather than skipping code given a case (as was done before) the code branches when it hits that case. This assembly code can have any number of conditionals and thus can be used for long if/elif/.../else blocks. This style of assembly code can also be used to implement a C/C++ switch-case construct.

Consider this example:

if (count == 1) { ..do something.. }
else if (count <= 10) { ..do a different something.. }
else { ..do something else.. }
..do things afterwards..

Then, to run through the cases we have:

    # Before conditional

    # Case 1
    li    t1, 1               # t1 <- 1
    beq   t0, t1, _f3Body1    # if count == 1 goto _f3Body1

    # Case 2
    li    t1, 10              # t1 <- 10
    ble   t0, t1, _f3Body2    # if count <= 10 goto _f3Body2

    # Case 3
    j     _f3Else

_f3Body1:
    # .. do something..
    j     _f3join

_f3Body3:
    # .. do a different something..
    j     _f3join

_f3Else:
    # .. do something else..

_f3join:
    # ..do things afterwards..

At the end of the code that starts with the label _f3Else there is no need for the jump j _f3join instruction because the flow of execution falls through to that code. This is often the case when going from the last of several alternative paths to the join point where all the paths join the execution.

Link to a file containing the above code

CMPUT 229 - Computer Organization and Architecture I

If/Elif/Else Conditional