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

James Molloy via llvm-dev llvm-dev at lists.llvm.org
Sat Sep 3 03:13:37 PDT 2016


Yes, indeed, if you changed these loads into intrinsic loads this problem would go away, because we don’t transform intrinsics for this *exact reason*. There are other intrinsics that must take a constant operand - I’ve been working on a way to determine accurately if this is the case or not but it’s not in mainline yet.

I think having this restriction on the IR is outside LLVM’s model, and it’s likely break elsewhere too. I understand the hardware restriction, I just feel that it could be modelled more accurately in the IR :)


> On 3 Sep 2016, at 01:06, Krzysztof Parzyszek via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> On 9/2/2016 6:54 PM, Marcello Maggioni via llvm-dev wrote:
>> 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.
> Could you use an intrinsic for the load?
> An optimization could reasonably be expected not to replace a constant input to an intrinsic with a variable, so even if it fails at the moment, it could be addressed.
> -Krzysztof
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