llvm dev - Sep 2019 - Question on llvm.mem* intrinsics

Hi there lowering experts,

Can the llvm.mem* intrinsics ever turn into a library call? Or do
they invariably turn into inline code?

This comes up because there was a patch to StackProtector to use
CaptureTracking instead of a home-grown analysis, which changes
the treatment of calls to intrinsics.  (The old code treated them
as normal calls, the new code decides intrinsics can never capture.)
But if llvm.mem* intrinsics can turn into real function calls, we
need to pay attention to that.

Thanks,
--paulr

On 9/10/19 2:09 PM, via llvm-dev wrote:
> Hi there lowering experts,
>
> Can the llvm.mem* intrinsics ever turn into a library call? Or do
> they invariably turn into inline code?

These can certainly turn into library calls. See, for example, the code in:

   SelectionDAG::getMemcpy (in SelectionDAG.cpp), which is called by the 
code in SelectionDAGBuilder::visitIntrinsicCall (in 
SelectionDAGBuilder.cpp) handling Intrinsic::memcpy.

  -Hal

>
> This comes up because there was a patch to StackProtector to use
> CaptureTracking instead of a home-grown analysis, which changes
> the treatment of calls to intrinsics.  (The old code treated them
> as normal calls, the new code decides intrinsics can never capture.)
> But if llvm.mem* intrinsics can turn into real function calls, we
> need to pay attention to that.
>
> Thanks,
> --paulr
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

> -----Original Message-----
> From: Finkel, Hal J. [mailto:hfinkel at anl.gov]
> Sent: Tuesday, September 10, 2019 3:35 PM
> To: Robinson, Paul; llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] Question on llvm.mem* intrinsics
> 
> 
> On 9/10/19 2:09 PM, via llvm-dev wrote:
> > Hi there lowering experts,
> >
> > Can the llvm.mem* intrinsics ever turn into a library call? Or do
> > they invariably turn into inline code?
> 
> 
> These can certainly turn into library calls. See, for example, the code
> in:
> 
>    SelectionDAG::getMemcpy (in SelectionDAG.cpp), which is called by the
> code in SelectionDAGBuilder::visitIntrinsicCall (in
> SelectionDAGBuilder.cpp) handling Intrinsic::memcpy.
> 
>   -Hal
Thanks Hal!

There are certainly places in LLVM that assume intrinsics are never
real calls.  `isLoweredToCall()` returns false for all intrinsics.
I was suspicious about the mem* calls though, and happy to get a
confirmation.

--paulr
> 
> 
> >
> > This comes up because there was a patch to StackProtector to use
> > CaptureTracking instead of a home-grown analysis, which changes
> > the treatment of calls to intrinsics.  (The old code treated them
> > as normal calls, the new code decides intrinsics can never capture.)
> > But if llvm.mem* intrinsics can turn into real function calls, we
> > need to pay attention to that.
> >
> > Thanks,
> > --paulr
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory