[llvm-dev] [LLVM][RFC] Representing the target device information in the LLVM IR

Fri Apr 27 09:18:37 PDT 2018

On 04/26/2018 07:03 PM, Narayanaswamy, Ravi wrote:
Hi Hal,
   We are not trying to address issues where the object mapped are of different sizes between host and target with different ABI.

Why are you not trying to address that issue?

Good question.  It is a harder problem and no vendor except Intel (offloading from windows to Linux) required this and so were not pushing for this.

Also bring up a whole lot of problems with shared memory, how do use shared memory if the objects are different, are copies made, when is the original item synced up.

Ravi

The issue is when the objects are of same size like double which is 8bytes on both 32bit and 64bit platform.  If a double is used in a first_private on a target clause,  the 64 bit side will pass it as value whereas on the 32bit side since the value does not fit in the argument it will be passed as pointer to a double. There will be a mis-match at the call site and entry site on this value.
  The main reason for this change is that when we do backend outlining for target pragmas the targets information needs to be communicated to the backend to generate the tables with the right names.  Generate LLVM IR for passing this information is one mechanism and other is passing the command option to the backend.  For the later each pass which needs this info will have to change.
Thanks
Ravi

From: Hal Finkel [mailto:hfinkel at anl.gov]
Sent: Wednesday, April 25, 2018 5:50 PM
To: Lin, Jin <jin.lin at intel.com<mailto:jin.lin at intel.com>>; Friedman, Eli <efriedma at codeaurora.org<mailto:efriedma at codeaurora.org>>; 'llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>' <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] [LLVM][RFC] Representing the target device information in the LLVM IR

Hi, Jin,

Can you please back up a bit and talk about the programming environment in which this problem manifests?

If I have a host and a target with different ABIs, then it seems we have lots of problems. For one thing, the layouts of structures are different, the sizes of some integer types are different, the sizes of pointers are different, and so on. It seems like a solution in this space should address, somehow, this general translation problem. Fixing this particular problem with the dispatch function's parameters feels like only the tip of the iceberg. What if I'm passing a pointer to some structure, or a pointer to other pointers, etc.?

I understand that OpenMP v5 is expected to have some custom "mappers" to handle deep copying and translation. Is this related to the design space here?

Thanks again,

Hal

On 04/25/2018 07:22 PM, Lin, Jin via llvm-dev wrote:
For the firstprivate clause, the compiler generates code to pass it  by value or by reference to the outlined function. The reason the first private scalars is generally passed by value is for the performance reason.
For this particular case, the compiler cannot generate code to pass the double @gg by value under i386-pc-linux-gnu since the value is 64 bit while the architecture is 32bit.
For the host compilation, the compiler generates the code to pass the data as well as the outlined function name to the OMP runtime.
For the target compilation, the compiler generates the outlined function so that it can be called by the OMP runtime.
So, the compiler is required to generate a single call on the host to support all the targets. All the target versions must have the same interface. So the common interface of the outline function should be used. For this particular example, the variable @gcc should be passed by reference under x86_64-mic.
Please let me know if you have more questions.
Jin

From: Friedman, Eli [mailto:efriedma at codeaurora.org]
Sent: Wednesday, April 25, 2018 4:14 PM
To: Lin, Jin <jin.lin at intel.com><mailto:jin.lin at intel.com>; 'llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>' <llvm-dev at lists.llvm.org><mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [LLVM][RFC] Representing the target device information in the LLVM IR

On 4/25/2018 3:48 PM, Lin, Jin wrote:
Given a global variable @gg, the compiler has to generate code on the host to specify whether it is passed by value or passed by reference. In the following example, if the compiler generates the code for passing by value, the outlined function on the target i386-pc-linux-gnu cannot get the correct value since it assumes the variable @gg is passed by reference.

Here is the corresponding IR on the host side.
  %0 = load double, double* @gg, align 8, !tbaa !3
  %1 = bitcast double %0 to i64
   ...
  %12 = getelementptr inbounds [4 x i8*], [4 x i8*]* %.offload_baseptrs, i32 0, i32 2
  %13 = bitcast i8** %12 to i64*
  store i64 %1, i64* %13, align 8

Could you describe the overall process of calling an offloaded function in a bit more detail?  How do you describe the ABI of the called function to the OpenMP runtime?

I suspect you shouldn't be trying to store things which aren't pointers into offload_baseptrs.

-Eli

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Hal Finkel

Lead, Compiler Technology and Programming Languages

Leadership Computing Facility

Argonne National Laboratory

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Hal Finkel

Lead, Compiler Technology and Programming Languages

Leadership Computing Facility

Argonne National Laboratory
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