[OpenMPIRBuilder] Don't drop debug info for target region.

When an outlined function is generated for omp target region,
a corresponding DISubprogram was not being generated. This resulted in
all the debug information for the target region being dropped.

This commit adds DISubprogram for the outlined function if there is one
available for the parent function. It also updates the current debug
location so that the right scope is used for the entries in the outlined
function.

With this change in place, I can set source line breakpoint in target
region and run to them in debugger.

Author: abidh

flang/test/Lower/OpenMP/target-debug.f90

@@ -0,0 +1,18 @@
+!RUN: %flang_fc1 -triple amdgcn-amd-amdhsa %s -debug-info-kind=line-tables-only -fopenmp -fopenmp-is-target-device -emit-llvm -o - | FileCheck %s
+program test
+implicit none
+
+  integer(kind = 4) :: a, b, c, d
+
+  !$omp target map(tofrom: a, b, c, d)
+  a = a + 1
+  ! CHECK: !DILocation(line: [[@LINE-1]]
+  b = a + 2
+  ! CHECK: !DILocation(line: [[@LINE-1]]
+  c = a + 3
+  ! CHECK: !DILocation(line: [[@LINE-1]]
+  d = a + 4
+  ! CHECK: !DILocation(line: [[@LINE-1]]
+  !$omp end target
+
+end program test

llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp

@@ -31,6 +31,7 @@
 #include "llvm/IR/CallingConv.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
@@ -6498,10 +6499,41 @@ static Function *createOutlinedFunction(
       ParameterTypes.push_back(Arg->getType());
   }
 
+  auto BB = Builder.GetInsertBlock();
+  auto M = BB->getModule();
   auto FuncType = FunctionType::get(Builder.getVoidTy(), ParameterTypes,
                                     /*isVarArg*/ false);
-  auto Func = Function::Create(FuncType, GlobalValue::InternalLinkage, FuncName,
-                               Builder.GetInsertBlock()->getModule());
+  auto Func =
+      Function::Create(FuncType, GlobalValue::InternalLinkage, FuncName, M);
+
+  // If there's a DISubprogram associated with current function, then
+  // generate one for the outlined function.
+  if (Function *parentFunc = BB->getParent()) {
+    if (DISubprogram *SP = parentFunc->getSubprogram()) {
+      DICompileUnit *CU = SP->getUnit();
+      DIBuilder DB(*M, true, CU);
+      DebugLoc DL = Builder.getCurrentDebugLocation();
+      // TODO: We are using nullopt for arguments at the moment. This will need
+      // to be updated when debug data is being generated for variables.
+      DISubroutineType *Ty =
+          DB.createSubroutineType(DB.getOrCreateTypeArray(std::nullopt));
+      DISubprogram::DISPFlags SPFlags = DISubprogram::SPFlagDefinition |
+                                        DISubprogram::SPFlagOptimized |
+                                        DISubprogram::SPFlagLocalToUnit;
+
+      DISubprogram *OutlinedSP = DB.createFunction(
+          CU, FuncName, FuncName, SP->getFile(), DL.getLine(), Ty, DL.getLine(),
+          DINode::DIFlags::FlagArtificial, SPFlags);
+
+      // Attach subprogram to the function.
+      Func->setSubprogram(OutlinedSP);
+      // Update the CurrentDebugLocation in the builder so that right scope
+      // is used for things inside outlined function.
+      Builder.SetCurrentDebugLocation(
+          DILocation::get(Func->getContext(), DL.getLine(), DL.getCol(),
+                          OutlinedSP, DL.getInlinedAt()));
+    }
+  }
 
   // Save insert point.
   auto OldInsertPoint = Builder.saveIP();

llvm/unittests/Frontend/OpenMPIRBuilderTest.cpp

@@ -6006,6 +6006,10 @@ TEST_F(OpenMPIRBuilderTest, TargetRegion) {
   BasicBlock *FallbackBlock = Branch->getSuccessor(0);
   Iter = FallbackBlock->rbegin();
   CallInst *FCall = dyn_cast<CallInst>(&*(++Iter));
+  // 'F' has a dummy DISubprogram which causes OutlinedFunc to also
+  // have a DISubprogram. In this case, the call to OutlinedFunc needs
+  // to have a debug loc, otherwise verifier will complain.
+  FCall->setDebugLoc(DL);
   EXPECT_NE(FCall, nullptr);
 
   // Check that the correct aguments are passed in