[llvm-dev] Problem with "[SimplifyCFG] Handle tail-sinking of more than 2 incoming branches"

Marcello Maggioni via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 2 16:54:46 PDT 2016

It’s not about weird restrictions we are adding, it’s about the fact that for that specific address space we cannot dynamically index into the memory.

The address needs to be a constant of some sort in the selection (it’s an hardware limitation, there’s nothing we can do about it) and the fact that it is transformed into a PHI makes it not constant anymore.

We will need to undo this into Codegen prepare by either reverse that putting the load back into the predecessors and creating a PHI of the result of the loads.

Having a way to opt-out or control what you want to sink I still think would be a added value to the optimization.

> On 2 Sep 2016, at 15:05, James Molloy <James.Molloy at arm.com> wrote:
> Hi,
>> On 2 Sep 2016, at 22:40, Marcello Maggioni <mmaggioni at apple.com <mailto:mmaggioni at apple.com>> wrote:
>>> What the optimization is doing is extracting the “getelementptr” , making an instruction out of it, sinking the load and using a PHI to select between the address.
>>> This breaks our selection of this types of loads.
> Are you referring to a downstream intrinsic, an upstream intrinsic or are you somehow adding strange restrictions yourself on what a load instruction can and can’t do?
> There’s a function canReplaceOperandWithVariable() that this optimisation uses to decide if it’s possible to do this. We use this to avoid making things that must be constant (like your example) variable. Perhaps this is doing something wrong? If you’re adding arbitrary restrictions on what loads can do though that’s never going to go well - I’m sorry but we just don’t support that use case. That’s what intrinsics are for.
> > Just a question. Why implementing it in SimplifyCFG and not as a separate pass like JumpThreading or something like that?
> Because sinking instructions into successors already exists in SimplifyCFG and has done for years. This is a small modification to make it a bit more clever. SimplifyCFG does lots of this kind of stuff already (switch->lookup table for example).
> Cheers,
> James
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