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Thu Jul 2 09:26:10 PDT 2020 · https://lists.llvm.org/pipermail/llvm-dev/2020-July/143027.html

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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[llvm-dev] Redundant ptrtoint/inttoptr instructions