[llvm-dev] Redundant ptrtoint/inttoptr instructions

[Michael Kruse via llvm-dev]

Unfortunately, inttoptr cannot be optimized away since it has an
effect on pointer aliasing semantis. See for instance the discussion
at https://bugs.llvm.org/show_bug.cgi?id=34548

Michael

Am Do., 2. Juli 2020 um 11:26 Uhr schrieb Alexey Zhikhartsev via
llvm-dev <llvm-dev at lists.llvm.org>:
>
> 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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