[CIR] Defer definitions of global variables until they are used. (#142496)

This change adds support for deferring global variable definitions until
first use whenever it is possible to do so. Although deferring function
definitions uses much of the same implementation, function deferral will
be added in a follow-up change.

1 files changed, 215 insertions, 3 deletions

diff --git a/clang/lib/CIR/CodeGen/CIRGenModule.cpp b/clang/lib/CIR/CodeGen/CIRGenModule.cpp
index 0ec5dce..eb291de 100644
--- a/clang/lib/CIR/CodeGen/CIRGenModule.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenModule.cpp
@@ -202,6 +202,99 @@ mlir::Location CIRGenModule::getLoc(SourceRange cRange) {
   return mlir::FusedLoc::get({begin, end}, metadata, builder.getContext());
 }
 
+mlir::Operation *
+CIRGenModule::getAddrOfGlobal(GlobalDecl gd, ForDefinition_t isForDefinition) {
+  const Decl *d = gd.getDecl();
+
+  if (isa<CXXConstructorDecl>(d) || isa<CXXDestructorDecl>(d)) {
+    errorNYI(d->getSourceRange(),
+             "getAddrOfGlobal: C++ constructor/destructor");
+    return nullptr;
+  }
+
+  if (isa<CXXMethodDecl>(d)) {
+    errorNYI(d->getSourceRange(), "getAddrOfGlobal: C++ method decl");
+    return nullptr;
+  }
+
+  if (isa<FunctionDecl>(d)) {
+    errorNYI(d->getSourceRange(), "getAddrOfGlobal: function decl");
+    return nullptr;
+  }
+
+  return getAddrOfGlobalVar(cast<VarDecl>(d), /*ty=*/nullptr, isForDefinition)
+      .getDefiningOp();
+}
+
+void CIRGenModule::emitGlobalDecl(const clang::GlobalDecl &d) {
+  // We call getAddrOfGlobal with isForDefinition set to ForDefinition in
+  // order to get a Value with exactly the type we need, not something that
+  // might have been created for another decl with the same mangled name but
+  // different type.
+  mlir::Operation *op = getAddrOfGlobal(d, ForDefinition);
+
+  // In case of different address spaces, we may still get a cast, even with
+  // IsForDefinition equal to ForDefinition. Query mangled names table to get
+  // GlobalValue.
+  if (!op)
+    op = getGlobalValue(getMangledName(d));
+
+  assert(op && "expected a valid global op");
+
+  // Check to see if we've already emitted this. This is necessary for a
+  // couple of reasons: first, decls can end up in deferred-decls queue
+  // multiple times, and second, decls can end up with definitions in unusual
+  // ways (e.g. by an extern inline function acquiring a strong function
+  // redefinition). Just ignore those cases.
+  // TODO: Not sure what to map this to for MLIR
+  mlir::Operation *globalValueOp = op;
+  if (auto gv = dyn_cast<cir::GetGlobalOp>(op))
+    globalValueOp =
+        mlir::SymbolTable::lookupSymbolIn(getModule(), gv.getNameAttr());
+
+  if (auto cirGlobalValue =
+          dyn_cast<cir::CIRGlobalValueInterface>(globalValueOp))
+    if (!cirGlobalValue.isDeclaration())
+      return;
+
+  // If this is OpenMP, check if it is legal to emit this global normally.
+  assert(!cir::MissingFeatures::openMP());
+
+  // Otherwise, emit the definition and move on to the next one.
+  emitGlobalDefinition(d, op);
+}
+
+void CIRGenModule::emitDeferred() {
+  // Emit code for any potentially referenced deferred decls. Since a previously
+  // unused static decl may become used during the generation of code for a
+  // static function, iterate until no changes are made.
+
+  assert(!cir::MissingFeatures::openMP());
+  assert(!cir::MissingFeatures::deferredVtables());
+  assert(!cir::MissingFeatures::cudaSupport());
+
+  // Stop if we're out of both deferred vtables and deferred declarations.
+  if (deferredDeclsToEmit.empty())
+    return;
+
+  // Grab the list of decls to emit. If emitGlobalDefinition schedules more
+  // work, it will not interfere with this.
+  std::vector<GlobalDecl> curDeclsToEmit;
+  curDeclsToEmit.swap(deferredDeclsToEmit);
+
+  for (const GlobalDecl &d : curDeclsToEmit) {
+    emitGlobalDecl(d);
+
+    // If we found out that we need to emit more decls, do that recursively.
+    // This has the advantage that the decls are emitted in a DFS and related
+    // ones are close together, which is convenient for testing.
+    if (!deferredDeclsToEmit.empty()) {
+      emitDeferred();
+      assert(deferredDeclsToEmit.empty());
+    }
+  }
+}
+
 void CIRGenModule::emitGlobal(clang::GlobalDecl gd) {
   if (const auto *cd = dyn_cast<clang::OpenACCConstructDecl>(gd.getDecl())) {
     emitGlobalOpenACCDecl(cd);
@@ -240,8 +333,33 @@ void CIRGenModule::emitGlobal(clang::GlobalDecl gd) {
     }
   }
 
-  // TODO(CIR): Defer emitting some global definitions until later
-  emitGlobalDefinition(gd);
+  // Defer code generation to first use when possible, e.g. if this is an inline
+  // function. If the global must always be emitted, do it eagerly if possible
+  // to benefit from cache locality. Deferring code generation is necessary to
+  // avoid adding initializers to external declarations.
+  if (mustBeEmitted(global) && mayBeEmittedEagerly(global)) {
+    // Emit the definition if it can't be deferred.
+    emitGlobalDefinition(gd);
+    return;
+  }
+
+  // If we're deferring emission of a C++ variable with an initializer, remember
+  // the order in which it appeared on the file.
+  assert(!cir::MissingFeatures::deferredCXXGlobalInit());
+
+  llvm::StringRef mangledName = getMangledName(gd);
+  if (getGlobalValue(mangledName) != nullptr) {
+    // The value has already been used and should therefore be emitted.
+    addDeferredDeclToEmit(gd);
+  } else if (mustBeEmitted(global)) {
+    // The value must be emitted, but cannot be emitted eagerly.
+    assert(!mayBeEmittedEagerly(global));
+    addDeferredDeclToEmit(gd);
+  } else {
+    // Otherwise, remember that we saw a deferred decl with this name. The first
+    // use of the mangled name will cause it to move into deferredDeclsToEmit.
+    deferredDecls[mangledName] = gd;
+  }
 }
 
 void CIRGenModule::emitGlobalFunctionDefinition(clang::GlobalDecl gd,
@@ -402,6 +520,17 @@ CIRGenModule::getOrCreateCIRGlobal(StringRef mangledName, mlir::Type ty,
       CIRGenModule::createGlobalOp(*this, loc, mangledName, ty,
                                    /*insertPoint=*/entry.getOperation());
 
+  // This is the first use or definition of a mangled name.  If there is a
+  // deferred decl with this name, remember that we need to emit it at the end
+  // of the file.
+  auto ddi = deferredDecls.find(mangledName);
+  if (ddi != deferredDecls.end()) {
+    // Move the potentially referenced deferred decl to the DeferredDeclsToEmit
+    // list, and remove it from DeferredDecls (since we don't need it anymore).
+    addDeferredDeclToEmit(ddi->second);
+    deferredDecls.erase(ddi);
+  }
+
   // Handle things which are present even on external declarations.
   if (d) {
     if (langOpts.OpenMP && !langOpts.OpenMPSimd)
@@ -1121,12 +1250,88 @@ void CIRGenModule::emitTentativeDefinition(const VarDecl *d) {
   if (gv && !mlir::cast<cir::GlobalOp>(gv).isDeclaration())
     return;
 
-  assert(!cir::MissingFeatures::deferredDecls());
+  // If we have not seen a reference to this variable yet, place it into the
+  // deferred declarations table to be emitted if needed later.
+  if (!mustBeEmitted(d) && !gv) {
+    deferredDecls[mangledName] = d;
+    return;
+  }
 
   // The tentative definition is the only definition.
   emitGlobalVarDefinition(d);
 }
 
+bool CIRGenModule::mustBeEmitted(const ValueDecl *global) {
+  // Never defer when EmitAllDecls is specified.
+  if (langOpts.EmitAllDecls)
+    return true;
+
+  const auto *vd = dyn_cast<VarDecl>(global);
+  if (vd &&
+      ((codeGenOpts.KeepPersistentStorageVariables &&
+        (vd->getStorageDuration() == SD_Static ||
+         vd->getStorageDuration() == SD_Thread)) ||
+       (codeGenOpts.KeepStaticConsts && vd->getStorageDuration() == SD_Static &&
+        vd->getType().isConstQualified())))
+    return true;
+
+  // TODO(cir): We do want to defer function decls, but it's not implemented.
+  assert(!cir::MissingFeatures::deferredFuncDecls());
+  if (isa<FunctionDecl>(global))
+    return true;
+
+  return getASTContext().DeclMustBeEmitted(global);
+}
+
+bool CIRGenModule::mayBeEmittedEagerly(const ValueDecl *global) {
+  // In OpenMP 5.0 variables and function may be marked as
+  // device_type(host/nohost) and we should not emit them eagerly unless we sure
+  // that they must be emitted on the host/device. To be sure we need to have
+  // seen a declare target with an explicit mentioning of the function, we know
+  // we have if the level of the declare target attribute is -1. Note that we
+  // check somewhere else if we should emit this at all.
+  if (langOpts.OpenMP >= 50 && !langOpts.OpenMPSimd) {
+    std::optional<OMPDeclareTargetDeclAttr *> activeAttr =
+        OMPDeclareTargetDeclAttr::getActiveAttr(global);
+    if (!activeAttr || (*activeAttr)->getLevel() != (unsigned)-1)
+      return false;
+  }
+
+  const auto *fd = dyn_cast<FunctionDecl>(global);
+  if (fd) {
+    // Implicit template instantiations may change linkage if they are later
+    // explicitly instantiated, so they should not be emitted eagerly.
+    if (fd->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
+      return false;
+    // Defer until all versions have been semantically checked.
+    if (fd->hasAttr<TargetVersionAttr>() && !fd->isMultiVersion())
+      return false;
+    if (langOpts.SYCLIsDevice) {
+      errorNYI(fd->getSourceRange(), "mayBeEmittedEagerly: SYCL");
+      return false;
+    }
+  }
+  const auto *vd = dyn_cast<VarDecl>(global);
+  if (vd)
+    if (astContext.getInlineVariableDefinitionKind(vd) ==
+        ASTContext::InlineVariableDefinitionKind::WeakUnknown)
+      // A definition of an inline constexpr static data member may change
+      // linkage later if it's redeclared outside the class.
+      return false;
+
+  // If OpenMP is enabled and threadprivates must be generated like TLS, delay
+  // codegen for global variables, because they may be marked as threadprivate.
+  if (langOpts.OpenMP && langOpts.OpenMPUseTLS &&
+      astContext.getTargetInfo().isTLSSupported() && isa<VarDecl>(global) &&
+      !global->getType().isConstantStorage(astContext, false, false) &&
+      !OMPDeclareTargetDeclAttr::isDeclareTargetDeclaration(global))
+    return false;
+
+  assert((fd || vd) &&
+         "Only FunctionDecl and VarDecl should hit this path so far.");
+  return true;
+}
+
 static bool shouldAssumeDSOLocal(const CIRGenModule &cgm,
                                  cir::CIRGlobalValueInterface gv) {
   if (gv.hasLocalLinkage())
@@ -1394,6 +1599,13 @@ CIRGenModule::getGlobalVisibilityAttrFromDecl(const Decl *decl) {
   return cirVisibility;
 }
 
+void CIRGenModule::release() {
+  emitDeferred();
+
+  // There's a lot of code that is not implemented yet.
+  assert(!cir::MissingFeatures::cgmRelease());
+}
+
 mlir::Type CIRGenModule::convertType(QualType type) {
   return genTypes.convertType(type);
 }