[OpenMP] Remove `register_requires` global constructor (#80460)

Summary:
Currently, OpenMP handles the `omp requires` clause by emitting a global
constructor into the runtime for every translation unit that requires
it. However, this is not a great solution because it prevents us from
having a defined order in which the runtime is accessed and used.

This patch changes the approach to no longer use global constructors,
but to instead group the flag with the other offloading entires that we
already handle. This has the effect of still registering each flag per
requires TU, but now we have a single constructor that handles
everything.

This function removes support for the old `__tgt_register_requires` and
replaces it with a warning message. We just had a recent release, and
the OpenMP policy for the past four releases since we switched to LLVM
is that we do not provide strict backwards compatibility between major
LLVM releases now that the library is versioned. This means that a user
will need to recompile if they have an old binary that relied on
`register_requires` having the old behavior. It is important that we
actively deprecate this, as otherwise it would not solve the problem of
having no defined init and shutdown order for `libomptarget`. The
problem of `libomptarget` not having a define init and shutdown order
cascades into a lot of other issues so I have a strong incentive to be
rid of it.

It is worth noting that the current `__tgt_offload_entry` only has space
for a 32-bit integer here. I am planning to overhaul these at some point
as well.

3 files changed, 0 insertions, 46 deletions

diff --git a/clang/lib/CodeGen/CGOpenMPRuntime.cpp b/clang/lib/CodeGen/CGOpenMPRuntime.cpp
index 4855e74..a7b72df 100644
--- a/clang/lib/CodeGen/CGOpenMPRuntime.cpp
+++ b/clang/lib/CodeGen/CGOpenMPRuntime.cpp
@@ -10100,44 +10100,6 @@ bool CGOpenMPRuntime::markAsGlobalTarget(GlobalDecl GD) {
   return !AlreadyEmittedTargetDecls.insert(D).second;
 }
 
-llvm::Function *CGOpenMPRuntime::emitRequiresDirectiveRegFun() {
-  // If we don't have entries or if we are emitting code for the device, we
-  // don't need to do anything.
-  if (CGM.getLangOpts().OMPTargetTriples.empty() ||
-      CGM.getLangOpts().OpenMPSimd || CGM.getLangOpts().OpenMPIsTargetDevice ||
-      (OMPBuilder.OffloadInfoManager.empty() &&
-       !HasEmittedDeclareTargetRegion && !HasEmittedTargetRegion))
-    return nullptr;
-
-  // Create and register the function that handles the requires directives.
-  ASTContext &C = CGM.getContext();
-
-  llvm::Function *RequiresRegFn;
-  {
-    CodeGenFunction CGF(CGM);
-    const auto &FI = CGM.getTypes().arrangeNullaryFunction();
-    llvm::FunctionType *FTy = CGM.getTypes().GetFunctionType(FI);
-    std::string ReqName = getName({"omp_offloading", "requires_reg"});
-    RequiresRegFn = CGM.CreateGlobalInitOrCleanUpFunction(FTy, ReqName, FI);
-    CGF.StartFunction(GlobalDecl(), C.VoidTy, RequiresRegFn, FI, {});
-    // TODO: check for other requires clauses.
-    // The requires directive takes effect only when a target region is
-    // present in the compilation unit. Otherwise it is ignored and not
-    // passed to the runtime. This avoids the runtime from throwing an error
-    // for mismatching requires clauses across compilation units that don't
-    // contain at least 1 target region.
-    assert((HasEmittedTargetRegion || HasEmittedDeclareTargetRegion ||
-            !OMPBuilder.OffloadInfoManager.empty()) &&
-           "Target or declare target region expected.");
-    CGF.EmitRuntimeCall(OMPBuilder.getOrCreateRuntimeFunction(
-                            CGM.getModule(), OMPRTL___tgt_register_requires),
-                        llvm::ConstantInt::get(
-                            CGM.Int64Ty, OMPBuilder.Config.getRequiresFlags()));
-    CGF.FinishFunction();
-  }
-  return RequiresRegFn;
-}
-
 void CGOpenMPRuntime::emitTeamsCall(CodeGenFunction &CGF,
                                     const OMPExecutableDirective &D,
                                     SourceLocation Loc,
diff --git a/clang/lib/CodeGen/CGOpenMPRuntime.h b/clang/lib/CodeGen/CGOpenMPRuntime.h
index b01b39a..c320642 100644
--- a/clang/lib/CodeGen/CGOpenMPRuntime.h
+++ b/clang/lib/CodeGen/CGOpenMPRuntime.h
@@ -1407,10 +1407,6 @@ public:
   /// \param GD Global to scan.
   virtual bool emitTargetGlobal(GlobalDecl GD);
 
-  /// Creates and returns a registration function for when at least one
-  /// requires directives was used in the current module.
-  llvm::Function *emitRequiresDirectiveRegFun();
-
   /// Creates all the offload entries in the current compilation unit
   /// along with the associated metadata.
   void createOffloadEntriesAndInfoMetadata();
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 836cd34..77fb3a6 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -838,10 +838,6 @@ void CodeGenModule::Release() {
       AddGlobalCtor(CudaCtorFunction);
   }
   if (OpenMPRuntime) {
-    if (llvm::Function *OpenMPRequiresDirectiveRegFun =
-            OpenMPRuntime->emitRequiresDirectiveRegFun()) {
-      AddGlobalCtor(OpenMPRequiresDirectiveRegFun, 0);
-    }
     OpenMPRuntime->createOffloadEntriesAndInfoMetadata();
     OpenMPRuntime->clear();
   }