[llvm-dev] RFC: Memcpy inlining in IR

David Chisnall via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 21 05:16:38 PDT 2019

For CHERI, we have to be quite careful with memcpy because any pointer 
copy must be done with pointer load / store operations for all pointer 
sized-and-aligned places.  I believe I've now found four (maybe five?) 
different places in LLVM where memcpy is expanded.  Most of those are in 
the IR, not SelectionDAG.

If you're expanding to loads and stores, it's much better to do this at 
the IR level with SelectionDAG because you can insert flow control 
structures (so can emit loops), which I don't believe is a problem for 
GlobalISel.  Some targets do not expand to loads and stores, for example 
on some x86 variants memcpy is expanded to a single REP MOVSB 
instruction.  This is probably much easier to implement as a DAG pattern.


On 20/06/2019 23:48, Amara Emerson via llvm-dev wrote:
> I agree that this should be a very late pass. Doing it in the IR would 
> simplify the implementation in GlobalISel, but it would also allow us to 
> perhaps have one shared expansion/optimization pass between both SDISel 
> and GISel.
> Volkan may look at upstreaming a partial implementation he has downstream.
> Cheers,
> Amara
>> On Jun 20, 2019, at 3:22 AM, Sjoerd Meijer <Sjoerd.Meijer at arm.com 
>> <mailto:Sjoerd.Meijer at arm.com>> wrote:
>> Looks like there are a lot of opinions where memcpy expansion/inlining 
>> needs to happen: (late) IR, or if it is a backend problem, see also 
>> for example https://reviews.llvm.org/D35035. Complicating factor here 
>> is that efficient memcpy lowering is crucial for performance and 
>> code-size (and they occur a lot).
>> Either way, I agree that the TLI hooks are not SelectionDAG specific, 
>> they can be used in an IR lowering pass.
>> Cheers,
>> Sjoerd.
>> ------------------------------------------------------------------------
>> *From:*llvm-dev <llvm-dev-bounces at lists.llvm.org 
>> <mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of Roman Lebedev 
>> via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
>> *Sent:*20 June 2019 08:04
>> *To:*Amara Emerson
>> *Cc:*llvm-dev
>> *Subject:*Re: [llvm-dev] RFC: Memcpy inlining in IR
>> On Thu, Jun 20, 2019 at 6:42 AM Amara Emerson via llvm-dev
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> >
>> > Hi all,
>> >
>> > For GlobalISel, we’re exploring options for implementing inlining optimizations for memcpy and friends. However, looking around the existing implementation, I don’t see anything that would particularly be problematic for us to do it at the IR level.
>> >
>> > The existing TLI hooks to specify how certain memcpy calls should be lowered doesn’t have anything too SelectionDAG specific, and an IR lowering pass could be shared in future between SDAG and GISel. Does anyone see issues with this?
>> Sounds similar tohttps://reviews.llvm.org/D60318
>> It should be done *really* late in the middle-end pipeline though.
>> > Thanks,
>> > Amara
>> Roman.
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