[llvm-dev] Placement of static allocas

[Mahesha S via llvm-dev]

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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