[OpenMP] Add implicit data sharing support when offloading to NVIDIA GPUs using OpenMP device offloading

Summary:
This patch is part of the development effort to add support in the current OpenMP GPU offloading implementation for implicitly sharing variables between a target region executed by the team master thread and the worker threads within that team.

This patch is the first of three required for successfully performing the implicit sharing of master thread variables with the worker threads within a team. The remaining two patches are:
- Patch D38978 to the LLVM NVPTX backend which ensures the lowering of shared variables to an device memory which allows the sharing of references;
- Patch (coming soon) is a patch to libomptarget runtime library which ensures that a list of references to shared variables is properly maintained.

A simple code snippet which illustrates an implicit data sharing situation is as follows:

```
#pragma omp target
{
   // master thread only
   int v;
   #pragma omp parallel
   {
      // worker threads
      // use v
   }
}
```

Variable v is implicitly shared from the team master thread which executes the code in between the target and parallel directives. The worker threads must operate on the latest version of v, including any updates performed by the master.

The code generated in this patch relies on the LLVM NVPTX patch (mentioned above) which prevents v from being lowered in the thread local memory of the master thread thus making the reference to this variable un-shareable with the workers. This ensures that the code generated by this patch is correct.
Since the parallel region is outlined the passing of arguments to the outlined regions must preserve the original order of arguments. The runtime therefore maintains a list of references to shared variables thus ensuring their passing in the correct order. The passing of arguments to the outlined parallel function is performed in a separate function which the data sharing infrastructure constructs in this patch. The function is inlined when optimizations are enabled.

Reviewers: hfinkel, carlo.bertolli, arpith-jacob, Hahnfeld, ABataev, caomhin

Reviewed By: ABataev

Subscribers: cfe-commits, jholewinski

Differential Revision: https://reviews.llvm.org/D38976

llvm-svn: 318773

2 files changed, 182 insertions, 23 deletions

diff --git a/clang/lib/CodeGen/CGOpenMPRuntimeNVPTX.cpp b/clang/lib/CodeGen/CGOpenMPRuntimeNVPTX.cpp
index 5a119003e5b..0b8d79e28b6 100644
--- a/clang/lib/CodeGen/CGOpenMPRuntimeNVPTX.cpp
+++ b/clang/lib/CodeGen/CGOpenMPRuntimeNVPTX.cpp
@@ -296,6 +296,7 @@ void CGOpenMPRuntimeNVPTX::emitGenericKernel(const OMPExecutableDirective &D,
   EntryFunctionState EST;
   WorkerFunctionState WST(CGM);
   Work.clear();
+  WrapperFunctionsMap.clear();
 
   // Emit target region as a standalone region.
   class NVPTXPrePostActionTy : public PrePostActionTy {
@@ -468,7 +469,7 @@ static void setPropertyExecutionMode(CodeGenModule &CGM, StringRef Name,
 }
 
 void CGOpenMPRuntimeNVPTX::emitWorkerFunction(WorkerFunctionState &WST) {
-  auto &Ctx = CGM.getContext();
+  ASTContext &Ctx = CGM.getContext();
 
   CodeGenFunction CGF(CGM, /*suppressNewContext=*/true);
   CGF.disableDebugInfo();
@@ -511,7 +512,10 @@ void CGOpenMPRuntimeNVPTX::emitWorkerLoop(CodeGenFunction &CGF,
   CGF.InitTempAlloca(ExecStatus, Bld.getInt8(/*C=*/0));
   CGF.InitTempAlloca(WorkFn, llvm::Constant::getNullValue(CGF.Int8PtrTy));
 
-  llvm::Value *Args[] = {WorkFn.getPointer()};
+  // Set up shared arguments
+  Address SharedArgs =
+      CGF.CreateDefaultAlignTempAlloca(CGF.Int8PtrPtrTy, "shared_args");
+  llvm::Value *Args[] = {WorkFn.getPointer(), SharedArgs.getPointer()};
   llvm::Value *Ret = CGF.EmitRuntimeCall(
       createNVPTXRuntimeFunction(OMPRTL_NVPTX__kmpc_kernel_parallel), Args);
   Bld.CreateStore(Bld.CreateZExt(Ret, CGF.Int8Ty), ExecStatus);
@@ -530,6 +534,9 @@ void CGOpenMPRuntimeNVPTX::emitWorkerLoop(CodeGenFunction &CGF,
   // Signal start of parallel region.
   CGF.EmitBlock(ExecuteBB);
 
+  // Current context
+  ASTContext &Ctx = CGF.getContext();
+
   // Process work items: outlined parallel functions.
   for (auto *W : Work) {
     // Try to match this outlined function.
@@ -545,14 +552,18 @@ void CGOpenMPRuntimeNVPTX::emitWorkerLoop(CodeGenFunction &CGF,
     // Execute this outlined function.
     CGF.EmitBlock(ExecuteFNBB);
 
-    // Insert call to work function.
-    // FIXME: Pass arguments to outlined function from master thread.
-    auto *Fn = cast<llvm::Function>(W);
-    Address ZeroAddr =
-        CGF.CreateDefaultAlignTempAlloca(CGF.Int32Ty, /*Name=*/".zero.addr");
-    CGF.InitTempAlloca(ZeroAddr, CGF.Builder.getInt32(/*C=*/0));
-    llvm::Value *FnArgs[] = {ZeroAddr.getPointer(), ZeroAddr.getPointer()};
-    emitCall(CGF, Fn, FnArgs);
+    // Insert call to work function via shared wrapper. The shared
+    // wrapper takes exactly three arguments:
+    //   - the parallelism level;
+    //   - the master thread ID;
+    //   - the list of references to shared arguments.
+    //
+    // TODO: Assert that the function is a wrapper function.s
+    Address Capture = CGF.EmitLoadOfPointer(SharedArgs,
+       Ctx.getPointerType(
+          Ctx.getPointerType(Ctx.VoidPtrTy)).castAs<PointerType>());
+    emitCall(CGF, W, {Bld.getInt16(/*ParallelLevel=*/0),
+        getMasterThreadID(CGF), Capture.getPointer()});
 
     // Go to end of parallel region.
     CGF.EmitBranch(TerminateBB);
@@ -618,16 +629,18 @@ CGOpenMPRuntimeNVPTX::createNVPTXRuntimeFunction(unsigned Function) {
   }
   case OMPRTL_NVPTX__kmpc_kernel_prepare_parallel: {
     /// Build void __kmpc_kernel_prepare_parallel(
-    /// void *outlined_function);
-    llvm::Type *TypeParams[] = {CGM.Int8PtrTy};
+    /// void *outlined_function, void ***args, kmp_int32 nArgs);
+    llvm::Type *TypeParams[] = {CGM.Int8PtrTy,
+        CGM.Int8PtrPtrTy->getPointerTo(0), CGM.Int32Ty};
     llvm::FunctionType *FnTy =
         llvm::FunctionType::get(CGM.VoidTy, TypeParams, /*isVarArg*/ false);
     RTLFn = CGM.CreateRuntimeFunction(FnTy, "__kmpc_kernel_prepare_parallel");
     break;
   }
   case OMPRTL_NVPTX__kmpc_kernel_parallel: {
-    /// Build bool __kmpc_kernel_parallel(void **outlined_function);
-    llvm::Type *TypeParams[] = {CGM.Int8PtrPtrTy};
+    /// Build bool __kmpc_kernel_parallel(void **outlined_function, void ***args);
+    llvm::Type *TypeParams[] = {CGM.Int8PtrPtrTy,
+        CGM.Int8PtrPtrTy->getPointerTo(0)};
     llvm::Type *RetTy = CGM.getTypes().ConvertType(CGM.getContext().BoolTy);
     llvm::FunctionType *FnTy =
         llvm::FunctionType::get(RetTy, TypeParams, /*isVarArg*/ false);
@@ -846,8 +859,17 @@ void CGOpenMPRuntimeNVPTX::emitNumTeamsClause(CodeGenFunction &CGF,
 llvm::Value *CGOpenMPRuntimeNVPTX::emitParallelOutlinedFunction(
     const OMPExecutableDirective &D, const VarDecl *ThreadIDVar,
     OpenMPDirectiveKind InnermostKind, const RegionCodeGenTy &CodeGen) {
-  return CGOpenMPRuntime::emitParallelOutlinedFunction(D, ThreadIDVar,
-                                                       InnermostKind, CodeGen);
+
+  auto *OutlinedFun = cast<llvm::Function>(
+    CGOpenMPRuntime::emitParallelOutlinedFunction(
+          D, ThreadIDVar, InnermostKind, CodeGen));
+  if (!isInSpmdExecutionMode()) {
+    llvm::Function *WrapperFun =
+        createDataSharingWrapper(OutlinedFun, D);
+    WrapperFunctionsMap[OutlinedFun] = WrapperFun;
+  }
+
+  return OutlinedFun;
 }
 
 llvm::Value *CGOpenMPRuntimeNVPTX::emitTeamsOutlinedFunction(
@@ -899,15 +921,52 @@ void CGOpenMPRuntimeNVPTX::emitGenericParallelCall(
     CodeGenFunction &CGF, SourceLocation Loc, llvm::Value *OutlinedFn,
     ArrayRef<llvm::Value *> CapturedVars, const Expr *IfCond) {
   llvm::Function *Fn = cast<llvm::Function>(OutlinedFn);
+  llvm::Function *WFn = WrapperFunctionsMap[Fn];
+  assert(WFn && "Wrapper function does not exist!");
+
+  // Force inline this outlined function at its call site.
+  Fn->setLinkage(llvm::GlobalValue::InternalLinkage);
 
-  auto &&L0ParallelGen = [this, Fn](CodeGenFunction &CGF, PrePostActionTy &) {
+  auto &&L0ParallelGen = [this, WFn, &CapturedVars](CodeGenFunction &CGF,
+                                                    PrePostActionTy &) {
     CGBuilderTy &Bld = CGF.Builder;
 
-    // Prepare for parallel region. Indicate the outlined function.
-    llvm::Value *Args[] = {Bld.CreateBitOrPointerCast(Fn, CGM.Int8PtrTy)};
-    CGF.EmitRuntimeCall(
-        createNVPTXRuntimeFunction(OMPRTL_NVPTX__kmpc_kernel_prepare_parallel),
-        Args);
+    llvm::Value *ID = Bld.CreateBitOrPointerCast(WFn, CGM.Int8PtrTy);
+
+    if (!CapturedVars.empty()) {
+      // Prepare for parallel region. Indicate the outlined function.
+      Address SharedArgs =
+          CGF.CreateDefaultAlignTempAlloca(CGF.VoidPtrPtrTy,
+              "shared_args");
+      llvm::Value *SharedArgsPtr = SharedArgs.getPointer();
+      llvm::Value *Args[] = {ID, SharedArgsPtr,
+                             Bld.getInt32(CapturedVars.size())};
+
+      CGF.EmitRuntimeCall(
+          createNVPTXRuntimeFunction(OMPRTL_NVPTX__kmpc_kernel_prepare_parallel),
+          Args);
+
+      unsigned Idx = 0;
+      ASTContext &Ctx = CGF.getContext();
+      for (llvm::Value *V : CapturedVars) {
+        Address Dst = Bld.CreateConstInBoundsGEP(
+            CGF.EmitLoadOfPointer(SharedArgs,
+            Ctx.getPointerType(
+                Ctx.getPointerType(Ctx.VoidPtrTy)).castAs<PointerType>()),
+            Idx, CGF.getPointerSize());
+        llvm::Value *PtrV = Bld.CreateBitCast(V, CGF.VoidPtrTy);
+        CGF.EmitStoreOfScalar(PtrV, Dst, /*Volatile=*/false,
+            Ctx.getPointerType(Ctx.VoidPtrTy));
+        Idx++;
+      }
+    } else {
+      llvm::Value *Args[] = {ID,
+          llvm::ConstantPointerNull::get(CGF.VoidPtrPtrTy->getPointerTo(0)),
+          /*nArgs=*/Bld.getInt32(0)};
+      CGF.EmitRuntimeCall(
+          createNVPTXRuntimeFunction(OMPRTL_NVPTX__kmpc_kernel_prepare_parallel),
+          Args);
+    }
 
     // Activate workers. This barrier is used by the master to signal
     // work for the workers.
@@ -922,7 +981,7 @@ void CGOpenMPRuntimeNVPTX::emitGenericParallelCall(
     syncCTAThreads(CGF);
 
     // Remember for post-processing in worker loop.
-    Work.push_back(Fn);
+    Work.emplace_back(WFn);
   };
 
   auto *RTLoc = emitUpdateLocation(CGF, Loc);
@@ -2318,3 +2377,92 @@ void CGOpenMPRuntimeNVPTX::emitOutlinedFunctionCall(
   }
   CGOpenMPRuntime::emitOutlinedFunctionCall(CGF, Loc, OutlinedFn, TargetArgs);
 }
+
+/// Emit function which wraps the outline parallel region
+/// and controls the arguments which are passed to this function.
+/// The wrapper ensures that the outlined function is called
+/// with the correct arguments when data is shared.
+llvm::Function *CGOpenMPRuntimeNVPTX::createDataSharingWrapper(
+    llvm::Function *OutlinedParallelFn, const OMPExecutableDirective &D) {
+  ASTContext &Ctx = CGM.getContext();
+  const auto &CS = *cast<CapturedStmt>(D.getAssociatedStmt());
+
+  // Create a function that takes as argument the source thread.
+  FunctionArgList WrapperArgs;
+  QualType Int16QTy =
+      Ctx.getIntTypeForBitwidth(/*DestWidth=*/16, /*Signed=*/false);
+  QualType Int32QTy =
+      Ctx.getIntTypeForBitwidth(/*DestWidth=*/32, /*Signed=*/false);
+  QualType Int32PtrQTy = Ctx.getPointerType(Int32QTy);
+  QualType VoidPtrPtrQTy = Ctx.getPointerType(Ctx.VoidPtrTy);
+  ImplicitParamDecl ParallelLevelArg(Ctx, Int16QTy, ImplicitParamDecl::Other);
+  ImplicitParamDecl WrapperArg(Ctx, Int32QTy, ImplicitParamDecl::Other);
+  ImplicitParamDecl SharedArgsList(Ctx, VoidPtrPtrQTy,
+      ImplicitParamDecl::Other);
+  WrapperArgs.emplace_back(&ParallelLevelArg);
+  WrapperArgs.emplace_back(&WrapperArg);
+  WrapperArgs.emplace_back(&SharedArgsList);
+
+  auto &CGFI =
+      CGM.getTypes().arrangeBuiltinFunctionDeclaration(Ctx.VoidTy, WrapperArgs);
+
+  auto *Fn = llvm::Function::Create(
+      CGM.getTypes().GetFunctionType(CGFI), llvm::GlobalValue::InternalLinkage,
+      OutlinedParallelFn->getName() + "_wrapper", &CGM.getModule());
+  CGM.SetInternalFunctionAttributes(/*D=*/nullptr, Fn, CGFI);
+  Fn->setLinkage(llvm::GlobalValue::InternalLinkage);
+
+  CodeGenFunction CGF(CGM, /*suppressNewContext=*/true);
+  CGF.StartFunction(GlobalDecl(), Ctx.VoidTy, Fn, CGFI, WrapperArgs);
+
+  const auto *RD = CS.getCapturedRecordDecl();
+  auto CurField = RD->field_begin();
+
+  // Get the array of arguments.
+  SmallVector<llvm::Value *, 8> Args;
+
+  // TODO: suppport SIMD and pass actual values
+  Args.emplace_back(llvm::ConstantPointerNull::get(
+      CGM.Int32Ty->getPointerTo()));
+  Args.emplace_back(llvm::ConstantPointerNull::get(
+      CGM.Int32Ty->getPointerTo()));
+
+  CGBuilderTy &Bld = CGF.Builder;
+  auto CI = CS.capture_begin();
+
+  // Load the start of the array
+  auto SharedArgs =
+      CGF.EmitLoadOfPointer(CGF.GetAddrOfLocalVar(&SharedArgsList),
+          VoidPtrPtrQTy->castAs<PointerType>());
+
+  // For each captured variable
+  for (unsigned I = 0; I < CS.capture_size(); ++I, ++CI, ++CurField) {
+    // Name of captured variable
+    StringRef Name;
+    if (CI->capturesThis())
+      Name = "this";
+    else
+      Name = CI->getCapturedVar()->getName();
+
+    // We retrieve the CLANG type of the argument. We use it to create
+    // an alloca which will give us the LLVM type.
+    QualType ElemTy = CurField->getType();
+    // If this is a capture by copy the element type has to be the pointer to
+    // the data.
+    if (CI->capturesVariableByCopy())
+      ElemTy = Ctx.getPointerType(ElemTy);
+
+    // Get shared address of the captured variable.
+    Address ArgAddress = Bld.CreateConstInBoundsGEP(
+        SharedArgs, I, CGF.getPointerSize());
+    Address TypedArgAddress = Bld.CreateBitCast(
+        ArgAddress, CGF.ConvertTypeForMem(Ctx.getPointerType(ElemTy)));
+    llvm::Value *Arg = CGF.EmitLoadOfScalar(TypedArgAddress,
+        /*Volatile=*/false, Int32PtrQTy, SourceLocation());
+    Args.emplace_back(Arg);
+  }
+
+  emitCall(CGF, OutlinedParallelFn, Args);
+  CGF.FinishFunction();
+  return Fn;
+}
diff --git a/clang/lib/CodeGen/CGOpenMPRuntimeNVPTX.h b/clang/lib/CodeGen/CGOpenMPRuntimeNVPTX.h
index a9d6386e6b9..5d13408318a 100644
--- a/clang/lib/CodeGen/CGOpenMPRuntimeNVPTX.h
+++ b/clang/lib/CodeGen/CGOpenMPRuntimeNVPTX.h
@@ -305,6 +305,17 @@ private:
   // target region and used by containing directives such as 'parallel'
   // to emit optimized code.
   ExecutionMode CurrentExecutionMode;
+
+  /// Map between an outlined function and its wrapper.
+  llvm::DenseMap<llvm::Function *, llvm::Function *> WrapperFunctionsMap;
+
+  /// Emit function which wraps the outline parallel region
+  /// and controls the parameters which are passed to this function.
+  /// The wrapper ensures that the outlined function is called
+  /// with the correct arguments when data is shared.
+  llvm::Function *
+  createDataSharingWrapper(llvm::Function *OutlinedParallelFn,
+      const OMPExecutableDirective &D);
 };
 
 } // CodeGen namespace.