[clang] [CIR] Implement weak ref and alias attribute handling (PR #195972)

[Andy Kaylor via cfe-commits]

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@@ -3374,10 +3379,93 @@ void CIRGenModule::release() {
 
   emitLLVMUsed();
 
+  // Classic codegen calls `checkAliases` here to validate any alias
+  // definitions emitted during codegen.
+  assert(!cir::MissingFeatures::checkAliases());
+
   // There's a lot of code that is not implemented yet.
   assert(!cir::MissingFeatures::cgmRelease());
 }
 
+void CIRGenModule::emitAliasDefinition(GlobalDecl gd) {
+  const auto *d = cast<ValueDecl>(gd.getDecl());
+  const AliasAttr *aa = d->getAttr<AliasAttr>();
+  assert(aa && "Not an alias?");
+
+  StringRef mangledName = getMangledName(gd);
+
+  if (aa->getAliasee() == mangledName) {
+    diags.Report(aa->getLocation(), diag::err_cyclic_alias) << 0;
+    return;
+  }
+
+  // If there is a definition in the module, then it wins over the alias.
+  // This is dubious, but allow it to be safe. Just ignore the alias.
+  mlir::Operation *entry = getGlobalValue(mangledName);
+  if (entry) {
+    auto entryGV = mlir::dyn_cast<cir::CIRGlobalValueInterface>(entry);
+    if (entryGV && !entryGV.isDeclaration())
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andykaylor wrote:

Unfortuantely, `isDeclaration` comes from MLIR's `SymbolOpInterface` so it would be a big lift to change that. We don't have an `isDefinition` method, but I suppose it's easy enough to add one.

https://github.com/llvm/llvm-project/pull/195972