[llvm-dev] Alloca, Inlining and Entry Basic Block

[Nicolai Hähnle via llvm-dev]

On Tue, Sep 14, 2021 at 1:49 PM Mahesha S via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi-
>
> Theoretically, alloca instructions can appear anywhere within the function
> body, but practically, it is better to keep them as a one cluster at the
> beginning of the entry basic block for better optimization opportunities.
>
> And, I think the front-end code gen phase mostly succeeds in clustering
> all alloca instructions at the beginning of the entry basic block.
> Nevertheless, it is not guaranteed always. One more case is inlining -
> which may leave alloca instructions other than in entry block.
>

The inliner explicitly scans for allocas in the callee entry block and
moves them to the caller's entry block.

In other words: if you're seeing surprising alloca's in the middle of a
function (i.e., _not_ VLAs like Roman mentioned), please first do a
root-cause into why those allocas end up where they do. If you're
unfamiliar with LLVM, such root-causing can be done with tools like the
-print-before-all command-line flags, which give you the IR between every
pass. You can go through that log to figure out where allocas first appear
in surprising places.

Cheers,
Nicolai


>
> So, I am thinking of adding a new late IR pass (after inlining)
> which deliberately moves all allocas which appear elsewhere to the
> beginning of the entry block.
>
> Now my questions are:
>
> (1) Can we neatly implement this pass? For example, what would happen if
> an alloca which appears within a loop is moved to the entry block?
> (2) Is there a possibility where some other later pass(es) other than
> inlining also can result in inserting alloca elsewhere, so the new proposed
> pass needs to be invoked multiple times?
>
> The main reason for the above ask is - I have my own doubt that if moving
> alloca to the entry block is such an easy task why not anyone from the
> community did not attempt it till date considering the importance of
> keeping the allocas at the beginning of the entry block for better
> optimization opportunities. If there are genuine reason for that, it is
> better to know it instead of blindly attempting to implement the pass.
>
> Thanks,
> Mahesha
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