[llvm-dev] Redundant ptrtoint/inttoptr instructions

[Johannes Doerfert via llvm-dev]

Optimizing int2ptr + ptr2int (or vice versa) is tricky business.

I would prefer it to be outside SROA, I think.


We should be *very* careful and restrictive about it.

My initial thought is that the pattern below can be optimized

but I haven't spend too much time on it.


~ Johannes


On 7/2/20 11:26 AM, Alexey Zhikhartsev via llvm-dev wrote:
> Hi all,
>
> We noticed a lot of unnecessary ptrtoint instructions that stand in way of
> some of our optimizations; the code pattern looks like this:
>
> bb1:
>    %int1 = ptrtoint %struct.s* %ptr1 to i64
>
> bb2:
>    %int2 = ptrtoint %struct.s* %ptr2 to i64
>
> %bb3:
>    %phi.node = phi i64 [ %int1, %bb1 ], [%int2, %bb2 ]
>    %ptr = inttoptr i64 %phi.node to %struct.s*
>
> In short, the pattern above arises due to:
> 1. InstCombine introducing a bitcast while performing a canonicalization in
> combineLoadToOperationType() [1]
> 2. GVN performing "load coercion" and replacing a load with a ptrtoint
> (ptrtoint is due to the bitcast)
> 3. SROA replacing store- and load-instructions with phi nodes.
>
> The question is: is it a good idea to clean ptrtoint/inttoptr instructions
> inside SROA or should it be done higher in the pass pipeline? More details
> below.
>
> The canonicalization in instruction combining is the root cause of the
> problem, more information about the actual transformation and motivation
> behind it can be found in this RFC [2]. Based on our experiments, getting
> rid of the canonicalization reduces the number of ptrtoint instructions by
> 16% and inttoptr instructions by 25%. However, it seems that overall, LLVM
> benefits from the canonicalization and, at least back in 2015, many people
> supported adding it to LLVM. So, I would like to find a way to keep it
> while removing the ptrtoint-s that are unnecessary, and doing clean-up in
> SROA is a straightforward way to achieve that.
>
> What does everybody think?
>
> Thanks,
> Alexey
>
> [1] combineLoadToOperationType()
> https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp#L556
> [2] [LLVMdev] RFC: Missing canonicalization in LLVM
> http://lists.llvm.org/pipermail/llvm-dev/2015-January/080956.html
>
>
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