[llvm-dev] Lowering For Loops to use architecture "loop" instruction

[Dilan Manatunga via llvm-dev]

I guess my area of confusion is how do I know that the br_cc instruction is
for a loop and not just an if statement. I am still getting familiar with
the backend process, so sorry for any dumb questions.

-Dilan

On Wed, Jun 1, 2016 at 8:43 PM Tim Northover <t.p.northover at gmail.com>
wrote:

> On 1 June 2016 at 20:18, Dilan Manatunga via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > I'm working on project which involves writing a backend for a
> hypothetical
> > architecture. I am currently trying to figure out the best way to
> translate
> > for loops to use a specialized "loop" instruction the architecture
> supports.
> > The instruction is similar X86's loop instruction, where a register is
> > automatically decremented and the condition is automatically checked to
> see
> > if loop execution should continue.
>
> This sounds like it's not something the TableGen patterns could
> handle, but ought to be fairly easy during ISelDAGToDAG. You'd be
> looking for a something like "(br_cc SETGT, (sub $iter, 1), 0,
> $GoAgain)". The C++ code is needed because your final LOOP instruction
> also has an output (induction variable for the next iteration) that
> needs to be propagated from the "(sub $iter, 1)".
>
> Some very vague and untested pseudo-code would be:
>
>     // To produce a (LOOP Chain, InductionVariable, Dest)
>     SDNode *SelectLOOPFromBR_CC(SDNode *N) {
>       // TODO: check whatever conditions (sub 1, SETGT, ..).
>       SDValue IV = N->getOperand(1);
>       SDNode *LOOP = CurDAG->getMachineNode(XYZ::LOOP, DL,
>               CurDAG->getVTList(MVT::Other, IV.getType()),
> IV.getOperand(0), N->getOperand(3));
>       CurDAG->ReplaceAllUsesWith(IV, SDValue(Loop, 1));
>       return LOOP;
>     }
>
> where the key point is that you have to manually call
> ReplaceAllUsesWith to move your induction variable onto the LOOP
> instruction. The rest is just bog-standard "match a BR_CC" code.
>
> The LOOP instruction would output a GPR (new induction variable) and
> take the old one as an input (tied via "let Constraints = ..." if
> they're really the same register), as well as a destination basic
> block.
>
> Hope this is a little helpful.
>
> Tim.
>
> P.S. Beware the position of the Chain changes between
> pre-isel/post-isel and I can never remember which way round it should
> be. There's at least a 50% chance I've got it wrong here. Probably
> closer to 75%.
>
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