[llvm-dev] LoopVectorize fails to vectorize loops with induction variables with PtrToInt/IntToPtr conversions

[Hal Finkel via llvm-dev]

On 06/20/2017 01:41 PM, Adrien Guinet wrote:
> On 06/20/2017 03:26 AM, Hal Finkel wrote:
>> Hi, Adrien,
> Hello Hal!
>
> Thanks for your answer!
>
>> Thanks for reporting this. I recommend that you file a bug report at
>> https://bugs.llvm.org/
> Will do!
>
>> Whenever I see reports of missed optimization opportunities in the face
>> of ptrtoint/inttoptr, my first question is: why are these instructions
>> present in the first place? At the IR level, use of inttoptr is highly
>> discouraged. Our aliasing analysis, for example, does not look through
>> them, and so you'll generally see a lot of missed optimizations when
>> they're around.
>> In this case, inttoptr seems to be introduced by SROA. SROA should not
>> be introducing inttoptr, but rather should be using GEPs on i8* (at
>> least), to avoid introducing pointers that our AA can't analyze.
> It looks so indeed!
>
>> Beyond that, if we need to handle inttoptr/ptrtoint in SCEV, then maybe
>> there's a way to make it smarter about the expressions with which it can
>> deal.
> One of the solution I was thinking of was making SCEV "understand"
> inttoptr(ptrtoint()), as this is clearly a no-op (and what is happening
> here), but it seems not that straightforward to do (hence this
> discussion to discuss what should be done :)).
>
>> I'm actually not sure to what "aliasing rules" the comment you
>> quote below is referring. I can certainly understand not being able to
>> place "inbounds" on some generated GEPs, but otherwise this seems
>> non-obvious to me (i.e. either the expander can identify a base pointer
>> from which to generate the GEP, or it can't, in which case it needs to
>> generate a inttoptr).
> The way I understand it is that, for a pointer to a fixed-size array P,
> by using I=ptrtoint(P)/arithmetic operations(I)/inttoptr(I) we can
> potentially end-up with a pointer which is not in the original values
> that could be computed from the original array (like an out-of-bound
> pointer), and thus not "reconstruct" a GEP from it. That's why SCEV
> wouldn't consider them.

This is exactly what I meant by not being able to create "inbounds" 
GEPs. However, aside from that, I don't recall our semantics restricting 
the formation of "out of bounds" pointers (although there certainly are 
restrictions on the uses of such pointers).

>   IIRC, I tried to add ptrtoint/inttoptr as
> "no-op" in SCEV, but then the cost-model seemed to be broken (a very
> high value was given at some point), and thus vectorization still didn't
> happen.
>
> I was also thinking: even if SROA is fixed, nothing prevents another
> pass from introducing such constructions in the future, so maybe the
> solution is to have a pass (like the one I wrote) that try and cleanup
> before any usage of SCEV? (or done by InstCombine for instance, as this
> basically removes the inttoptr(ptrtoint()) combination).
> Or having the "inttoptr(ptrtoint())" composition understood by SCEV as a
> no-op might also be a cleaner "long-term" solution...
>
> What do you think?

While it is true that nothing prevents it, technically, over time we've 
been removing code from LLVM that creates these in favor of code that 
uses GEPs. I think it would be counter productive to teach some parts of 
the optimizer about inttoptr/ptrtoint, and not BasicAA and all of the 
rest of it. There are certainly some language constructs that must be 
lowered using inttoptr/ptrtoint at the IR level, but usage should be 
limited to those contexts.

The right solution here is to fix SROA, and any other passes that are 
doing this, to use GEPs instead. Only if that's not possible, should we 
consider other solutions. The reason we've chosen this path is that the 
task of teaching the rest of the optimizer to handle inttoptr/ptrtoint 
as truly first-class pointer-manipulation constructs, along with GEPs, 
is highly non-trivial. If, on top of that, we need some canonicalization 
procedure which transforms uses of inttoptr/ptrtoint into GEP-based 
expressions, we can do that too.

  -Hal


-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory