[llvm-dev] llvm.set.loop.iterations

[Björn Pettersson A via llvm-dev]

Hi Markus.

Can't you just custom lower the intrinsic at ISel, to some target specific pseudo/instruction. And then you introduce an ISD::SUB at some early point during ISel (hopefully making it possible for DAGCombine to fold away the ISD::ADD/ISD::SUB pair). Or are you afraid that the addition has been hoisted to a different basic block compared to the intrinsic (since DAGCombiner is local to a BB that would reduce the possibilities for folding away the ISD::SUB)?

Or what says that you need to wait until hwloop finalization (assuming that there is no target-generic MIR counterpart for llvm.set.loop.iterations, or is it)?

/Björn

________________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Markus Lavin via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Thursday, July 11, 2019 15:40
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] llvm.set.loop.iterations

After playing a bit with the newly introduced hardware loop framework I realize that the llvm.set.loop.iterations intrinsic takes as argument the number of iterations the loop will execute. In fact it goes all the way to, on IR, insert an addition of constant 1 to the number of taken backedges returned by SCEV.

If the machine instruction realizing the loop is interested in the number of branches-to-make / backedges-taken then this is slightly awkward as we need to subtract the constant 1. Of course if the iteration count was constant this is trivial but if it is passed in register then it is not so nice to have to insert these subtract instructions from a MIR pass (where the hwloop finalization is being done).

I wonder what would be the best way to deal with this.

One way would be to add a TTI hook gating the original addition but then the intrinsic will have two meanings depending on what this hook returns which is not good.

Another way would be to introduce a second intrinsic, say llvm.set.loop.backedges that corresponds to that value, but then we have yet another intrinsic.

A third option could be to have the original intrinsic take both values as arguments.

Any thoughts on this?

regards
Markus