llvm dev - May 2019 - Why did "llvm.memcpy" changed when emitting an object file?

Hello Johannes and Tim,

Thank you for your answers! They really helped me.

Can I influence if intrinsics are lowered or not?
I set llvm::EngineBuilder::setOptLevel to
"llvm::CodeGenOpt::Aggressive"

And I couldn't find any flag in llvm::TargetOptions that might effect this
behavior.

Kind greetings
Björn

-----Original Message-----
From: Doerfert, Johannes <jdoerfert at anl.gov>
Sent: Donnerstag, 9. Mai 2019 08:55
To: Gaier, Bjoern <Bjoern.Gaier at horiba.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Why did "llvm.memcpy" changed when emitting an
object file?

Hi Björn,

Your observation is correct, LLVM can "recreate" actual function calls
for intrinsics calls.

Generally, there is no single way LLVM intrinsics are lowered. It depends on the
intrinsic and the target (afaik). Some, especially target specific ones, are
lowered to assembly sequences. Others, are simply dropped or replaced by one of
their arguments (some annotations).
Memory operations, among others, can be "lowered" to calls to library
calls, e.g., in IntrinsicLowering::LowerIntrinsicCall. Though, memcpy can also
be lowered to a sequence of load/stores in
SelectionDAG::getMemcpy() (via getMemcpyLoadsAndStores). I guess the fact that
you use the JIT interface might be a reason you see the former lowering but I
don't know for sure.

I hope this general answer helps, otherwise you can probably get a more detailed
response from a backend person.

Cheers,
  Johannes


On 05/09, Gaier, Bjoern via llvm-dev wrote:
> Dear LLVM-Mailing list,
>
> I have a beginners question again - please don't be mad at me!
>
> I was playing around with llvm::Module instance and ended in iterating over
all global values and functions. I then noticed the function
"llvm.memcpy". As I understood, this is an intrinsic coming from the
LLVM.
>
> So far so good! I then emitted an object file with the
llvm::ExecutionEngine and noticed an undefined references to "memcpy".
Now I'm surprised! To me, it looks like that the intrinsic turned into a
'normal' function. I checked the iteration of the functions and were not
able to locate any "memcpy" there, this is why I'm assuming the
intrinsic changed.
>
> I wonder if this is true - I always thought that intrinsics will be
replaced by specific predefined assembly instructions, even the MSVC compiler
provides a memcpy intrinsic, so I don't understand why this function would
appear, when it was an intrinsic before. Did I maybe configured the
EngineBuilder wrong? Or the ExecutionEngine itself?
>
> Kind greetings
> Björn
> Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816,
> USt.ID-Nr. DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr.
> Robert Plank, Markus Bode, Heiko Lampert, Takashi Nagano, Takeshi
> Fukushima. Junichi Tajika
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

--

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, USt.ID-Nr.
DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus
Bode, Heiko Lampert, Takashi Nagano, Takeshi Fukushima. Junichi Tajika

On Thu, 9 May 2019 at 09:04, Gaier, Bjoern <Bjoern.Gaier at horiba.com>
wrote:
> Can I influence if intrinsics are lowered or not?
You can't guarantee memcpy won't be called, if that's what you mean.
If nothing else, most targets simply don't have the code to build a
physical loop to do the copy. You have to find a way to provide it in
your JIT.

Cheers.

Tim.

Sorry, I was not precise enough with my question.

I wanted to ask if I can give the JIT a hint how it should treat those intrinsic
functions. Like "always create a function call" or "always use
the intrinsic if available". I know that the Clang compiler has such flags
but I don't know if the JIT interfaces has those too.

Kind greetings
Björn

-----Original Message-----
From: Tim Northover <t.p.northover at gmail.com>
Sent: Donnerstag, 9. Mai 2019 10:46
To: Gaier, Bjoern <Bjoern.Gaier at horiba.com>
Cc: Doerfert, Johannes <jdoerfert at anl.gov>; llvm-dev <llvm-dev at
lists.llvm.org>
Subject: Re: [llvm-dev] Why did "llvm.memcpy" changed when emitting an
object file?

On Thu, 9 May 2019 at 09:04, Gaier, Bjoern <Bjoern.Gaier at horiba.com>
wrote:
> Can I influence if intrinsics are lowered or not?
You can't guarantee memcpy won't be called, if that's what you mean.
If nothing else, most targets simply don't have the code to build a physical
loop to do the copy. You have to find a way to provide it in your JIT.

Cheers.

Tim.
Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, USt.ID-Nr.
DE 114 165 789 Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus
Bode, Heiko Lampert, Takashi Nagano, Takeshi Fukushima. Junichi Tajika