[LLVMdev] distinguishing between real arguments and variable arguments

[Eli Friedman]

On Mon, Sep 26, 2011 at 1:40 PM, reed kotler <rkotler at mips.com> wrote:
> I'm doing the delay slot optimization for MIPS and I've noticed that two
> other ports that are doing this have to work around a problem in the
> instruction abstraction.
>
> Would be nice to fix this and not propagate the hack.
>
> The basic problem is that on a call, you want to get the set of explicit
> and implicit operands but NOT the variable operands.
>
> There is no way do this without hardcoding this information away from
> the TD file which is error prone.
> Architectures evolve and you can get new call instructions and then have
> a subtle delay slot bug.
>
> Any suggestions on how to fix LLVM so that we can get the non variable
> operands?

You can get the number of non-variable operands for a
MachineInstruction with getDesc()->getNumOperands().

That said, if I'm following this right, that isn't quite correct; you
really want to check whether a given operand's value is read before or
after the delay slot executes.  The difference might not matter in
practice, but you should probably at least leave a comment mentioning
that, though.

-Eli