llvm dev - Jan 2022 - Usage of splitting heuristic among LLVM register allocators

Hi All,

As per understanding, only the Greedy register allocator has the splitting
heuristic which is  implemented using splitkit.cpp. The splitting logic is
only present in splitkit.cpp file of llvm codebase.
Whereas one of the Basic register allocator's comments state that it also
performs splitting but I am not able to find any use of api from
splitkit.cpp for splitting.

Please clarify in which register allocator splitting heuristic is being
used.

Thanks,
Rohit Aggarwal

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Hi Rohit,

The basic allocator splits live-ranges, but it does that in a very boring way.
Essentially, when you spill, you have to split the live-ranges and that’s what
the inline spiller, used by the basic allocator, does.

E.g., let say you have:
a = …
...
= a
...
= a

And you want to spill a.

What the spiller will do is split a into its stored and reloaded components:
a0 = …
st a0
…
a1 = ld
= a1
…
a2 = ld
= a2

As you can see, the split created a0, a1, and a2 from a.

Now, if you’re interested in fancy live-range splitting, then yes, only the
greedy allocator does that.

Cheers,
-Quentin
> On Jan 13, 2022, at 1:23 AM, Rohit Aggarwal via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi All,
> 
> As per understanding, only the Greedy register allocator has the splitting
heuristic which is  implemented using splitkit.cpp. The splitting logic is only
present in splitkit.cpp file of llvm codebase.
> Whereas one of the Basic register allocator's comments state that it
also performs splitting but I am not able to find any use of api from
splitkit.cpp for splitting.
> 
> Please clarify in which register allocator splitting heuristic is being
used.
> 
> Thanks,
> Rohit Aggarwal
> 
> 
> 
> Disclaimer:- This footer text is to convey that this email is sent by one
of the users of IITH. So, do not mark it as SPAM.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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