[llvm-dev] Placement of static allocas

[Reid Kleckner via llvm-dev]

Frontends should place allocas in the entry block, and importantly, they
should appear before any instruction that can later expand into control
flow, such as an inlinable function call. Passes should not pessimize IR by
inserting control flow before static allocas. The doc you linked to seems
to cover that.

As far as rules go, this is not something that the verifier can enforce,
because static allocas don't carry a special "static" marker. The static
property is determined simply by the placement of the instruction. There is
the `inalloca` marker, but that's not relevant here.

On Tue, Sep 21, 2021 at 9:55 AM Mahesha S via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> As I understand it, the verifier does not complain about the
> (mis-)placement of alloca, probably because it is not something related to
> correctness (in theory), but it is related to optimization opportunities
> (in practice).
>
> I could see this documentation -
> https://llvm.org/docs/Frontend/PerformanceTips.html#use-of-allocas after
> this discussion -
> https://lists.llvm.org/pipermail/llvm-dev/2015-September/090191.html.
> But, nothing more on it.
>
> So not sure about - what is the general rule being set on placement of
> static allocas?, or, if there is any such rule in the first place? or,
> front-end and opt passes are free about the placement of static allocas?
>
> Thanks,
> Mahesha
>
>
> On Tue, Sep 21, 2021 at 9:56 PM Min-Yih Hsu <minyihh at uci.edu> wrote:
>
>> IIRC you can interleave debug intrinsics (e.g. llvm.dbg.declare) with
>> alloca instructions (at least the verifier doesn't complain). Not sure if
>> there are other intrinsics that fall into this category as well.
>>
>> -Min
>>
>> On Tue, Sep 21, 2021 at 7:45 AM Mahesha S via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi-
>>>
>>> Here is my understanding and assumption about the placement of static
>>> allocas:
>>>
>>> "All static allocas should appear in the entry basic block before any
>>> function call for better optimization opportunities. If there are
>>> interleaved static allocas with function call in-between, such an ir is
>>> considered broken, even though the ir is valid from correctness
>>> perspective. And if any pass is not adhering to the requirement that all
>>> static allocas should be placed in the entry block before any function
>>> call, then such a pass is considered broken since it may lead to surprising
>>> results in general."
>>>
>>> Let me know if my above understanding is correct or not.
>>>
>>> Thanks,
>>> Mahesha
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>>
>> --
>> Min-Yih Hsu
>> Ph.D Student in ICS Department, University of California, Irvine (UCI).
>>
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