[llvm-dev] [MemorySSA] A question about how to update the MemorySSA when we call RecursivelyDeleteTriviallyDeadInstructions

[Wei Mi via llvm-dev]

On Fri, Apr 28, 2017 at 12:00 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
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> On Fri, Apr 28, 2017 at 11:29 AM, Wei Mi <wmi at google.com> wrote:
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>> When we erase a memory access instruction, existing passes using
>> memory ssa like GVN, NewGVN, GVNHoist and EarlyCSE uses
>> MemorySSAUpdater to do the update before the instruction is erased.
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>> However, if we call llvm::RecursivelyDeleteTriviallyDeadInstructions
>> to find out dead instruction recursively and memory access
>> instructions may be erased inside the recursive process, we need a way
>> to update MemorySSA for those instructions. Note MemorySSA update
>> should be done before the instruction is erased.
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>> Is it acceptable to have an API with MemorySSAUpdater as a param,
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> I think so, that is what we do in other APIs.
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>> or
>> it is better for MemoryUseOrDef to have a callback value handler to do
>> the update transparently,
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> For removal, the order of removal matters
> Just like with regular IR, you must remove the uses (or ensure a single
> value for arguments) for MemoryPhi nodes before trying to remove them.
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> Thus it would likely be easy to cause subtle to trigger bugs with a callback
> handler.
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>> or there are better ways to be suggested.
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I see. Thanks for the explanation!

Wei.

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>> Thanks,
>> Wei.
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