[OpenMP] Fix the linkage for kernel environment global variable

[tianshilei1992]

Summary

This patch fixes the issue that multiple definition of kernel environment global
variables can occur because of wrong linkage.

Fixes #64284.

[jhuber6]

Assuming this is truly weak_odr, i.e. single definition that gets a symbol in the ELF.

[tianshilei1992]

In D156955#4555952, @jhuber6 wrote:

Assuming this is truly weak_odr, i.e. single definition that gets a symbol in the ELF.

Our previous _exec_mode is actually WeakAnyLinkage, which could potentially lead to mismatch.

[tianshilei1992]

rebase and fix test issue

llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp

Function *Fn = getOrCreateRuntimeFunctionPtr(
       omp::RuntimeFunction::OMPRTL___kmpc_target_init);
   const DataLayout &DL = Fn->getParent()->getDataLayout();
 
   Twine DynamicEnvironmentName = KernelName + "_dynamic_environment";
   Constant *DynamicEnvironmentInitializer =
       ConstantStruct::get(DynamicEnvironment, {DebugIndentionLevelVal});
   Constant *DynamicEnvironmentGV = new GlobalVariable(
       M, DynamicEnvironment, /* IsConstant */ false,
-      GlobalValue::InternalLinkage, DynamicEnvironmentInitializer,
+      GlobalValue::WeakODRLinkage, DynamicEnvironmentInitializer,
       DynamicEnvironmentName,
       /* InsertBefore */ nullptr, llvm::GlobalValue::NotThreadLocal,
       DL.getDefaultGlobalsAddressSpace());
   if (DynamicEnvironmentGV->getType() != DynamicEnvironmentPtr)
     DynamicEnvironmentGV = ConstantExpr::getAddrSpaceCast(
         DynamicEnvironmentGV, DynamicEnvironmentPtr);
 
   Constant *ConfigurationEnvironmentInitializer = ConstantStruct::get(
       ConfigurationEnvironment, {
                                     UseGenericStateMachineVal,
                                     MayUseNestedParallelismVal,
                                     IsSPMDVal,
                                 });
   Constant *KernelEnvironmentInitializer = ConstantStruct::get(
       KernelEnvironment, {
                              ConfigurationEnvironmentInitializer,
                              Ident,
                              DynamicEnvironmentGV,
                          });
   Twine KernelEnvironmentName = KernelName + "_kernel_environment";
   Constant *KernelEnvironmentGV = new GlobalVariable(
-      M, KernelEnvironment, /* IsConstant */ true, GlobalValue::ExternalLinkage,
+      M, KernelEnvironment, /* IsConstant */ true, GlobalValue::WeakODRLinkage,
       KernelEnvironmentInitializer, KernelEnvironmentName,
       /* InsertBefore */ nullptr, llvm::GlobalValue::NotThreadLocal,
       DL.getDefaultGlobalsAddressSpace());
   if (KernelEnvironmentGV->getType() != KernelEnvironmentPtr)
     KernelEnvironmentGV = ConstantExpr::getAddrSpaceCast(KernelEnvironmentGV,
                                                          KernelEnvironmentPtr);
 
   CallInst *ThreadKind = Builder.CreateCall(Fn, {KernelEnvironmentGV});

mlir/test/Target/LLVMIR/omptarget-region-device-llvm.mlir

omp.target   {
       omp.terminator
     }
     llvm.return
   }
 }
 
 // CHECK:      @[[SRC_LOC:.*]] = private unnamed_addr constant [23 x i8] c"{{[^"]*}}", align 1
 // CHECK:      @[[IDENT:.*]] = private unnamed_addr constant %struct.ident_t { i32 0, i32 2, i32 0, i32 22, ptr @[[SRC_LOC]] }, align 8
-// CHECK:      @[[DYNA_ENV:.*]] = internal global %struct.DynamicEnvironmentTy zeroinitializer
-// CHECK:      @[[KERNEL_ENV:.*]] = constant %struct.KernelEnvironmentTy { %struct.ConfigurationEnvironmentTy { i8 1, i8 1, i8 1 }, ptr @[[IDENT]], ptr @[[DYNA_ENV]] }
+// CHECK:      @[[DYNA_ENV:.*]] = weak_odr global %struct.DynamicEnvironmentTy zeroinitializer
+// CHECK:      @[[KERNEL_ENV:.*]] = weak_odr constant %struct.KernelEnvironmentTy { %struct.ConfigurationEnvironmentTy { i8 1, i8 1, i8 1 }, ptr @[[IDENT]], ptr @[[DYNA_ENV]] }
 // CHECK:      define weak_odr protected void @__omp_offloading_{{[^_]+}}_{{[^_]+}}_omp_target_region__l{{[0-9]+}}(ptr %[[ADDR_A:.*]], ptr %[[ADDR_B:.*]], ptr %[[ADDR_C:.*]])
 // CHECK:        %[[INIT:.*]] = call i32 @__kmpc_target_init(ptr @[[KERNEL_ENV]])
 // CHECK-NEXT:   %[[CMP:.*]] = icmp eq i32 %3, -1
 // CHECK-NEXT:   br i1 %[[CMP]], label %[[LABEL_ENTRY:.*]], label %[[LABEL_EXIT:.*]]
 // CHECK:        [[LABEL_ENTRY]]:
 // CHECK:        %[[TMP_A:.*]] = alloca ptr, align 8
 // CHECK:        store ptr %[[ADDR_A]], ptr %[[TMP_A]], align 8
 // CHECK:        %[[PTR_A:.*]] = load ptr, ptr %[[TMP_A]], align 8