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

[Lin, Jin via llvm-dev]

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>; 'llvm-dev at lists.llvm.org' <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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