IL on GE 90-30 PLC

The short answer is no, you have to use JMPC statements. This is because GE's instruction list is just assembly language, and I doubt that processor has conditional execution of individual instructions.

However, there's no reason that a math function can't have an extra Enable parameter. In fact that's usually how an IEC-61131-3 compliant runtime implements math ladder logic instructions (i.e. every ladder instruction is a function where the first parameter is a boolean Enable input). So you can usually call these ladder logic functions from instruction list, and you can set the EN input to condition the execution.

I just don't know if you can access the ladder logic math functions from instruction list in a GE controller... sorry! If you could, that would solve your problem. Try using the standard GE function call syntax in your instruction list, and use the Ladder math function names.

Looking at this manual, according to page 5-12:

Functions are also unconditionally executed, regardless of the result of a previous instruction.

and

Function Blocks are also unconditionally executed, regardless of the result of a previous instruction.

However, I find it interesting that on page 5-13, it says:

The table also indicates whether the function or function block requires the accumulator to be initialized by placing an LD Bool instruction in front of the function block.

...since LD_BOOL is an instruction list instruction that loads a BOOL into the accumulator before calling the function block. Sounds to me like that's the Enable parameter I was talking about above.

If all else fails, it looks like you can write your own "blocks" a.k.a. subroutines, so you could wrap the math function you want into another subroutine that also takes an Enable input as a parameter.