[clang] "modular_format" attribute for functions using format strings

clang/include/clang/Basic/Attr.td

@@ -5223,3 +5223,14 @@ def NonString : InheritableAttr {
   let Subjects = SubjectList<[Var, Field]>;
   let Documentation = [NonStringDocs];
 }
+
+def ModularFormat : InheritableAttr {
+  let Spellings = [Clang<"modular_format">];
+  let Args = [
+    IdentifierArgument<"ModularImplFn">,
+    StringArgument<"ImplName">,
+    VariadicStringArgument<"Aspects">
+  ];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [ModularFormatDocs];
+}

clang/include/clang/Basic/AttrDocs.td

@@ -9427,3 +9427,37 @@ silence diagnostics with code like:
   __attribute__((nonstring)) char NotAStr[3] = "foo"; // Not diagnosed
   }];
 }
+
+def ModularFormatDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``modular_format`` attribute can be applied to a function that bears the
+``format`` attribute (or standard library functions) to indicate that the
+implementation is modular on the format string argument. When the format string
+for a given call is constant, the compiler may redirect the call to the symbol
+given as the first argument to the attribute (the modular implementation
+function).
+
+The second argument is a implementation name, and the remaining arguments are
+aspects of the format string for the compiler to report. If the compiler does
+not understand an aspect, it must summarily report that the format string has
+that aspect.
+
+The compiler reports an aspect by issuing a relocation for the symbol
+``<impl_name>_<aspect>``. This arranges for code and data needed to support the
+aspect of the implementation to be brought into the link to satisfy weak
+references in the modular implemenation function.
+
+For example, say ``printf`` is annotated with
+``modular_format(__modular_printf, __printf, float)``. Then, a call to
+``printf(var, 42)`` would be untouched. A call to ``printf("%d", 42)`` would
+become a call to ``__modular_printf`` with the same arguments, as would
+``printf("%f", 42.0)``. The latter would be accompanied with a strong
+relocation against the symbol ``__printf_float``, which would bring floating
+point support for ``printf`` into the link.
+
+The following aspects are currently supported:
+
+- ``float``: The call has a floating point argument
+  }];
+}

clang/lib/CodeGen/CGCall.cpp

@@ -2560,6 +2560,20 @@ void CodeGenModule::ConstructAttributeList(StringRef Name,
 
     if (TargetDecl->hasAttr<ArmLocallyStreamingAttr>())
       FuncAttrs.addAttribute("aarch64_pstate_sm_body");
+
+    if (auto *ModularFormat = TargetDecl->getAttr<ModularFormatAttr>()) {
+      // TODO: Error checking
+      FormatAttr *Format = TargetDecl->getAttr<FormatAttr>();
+      StringRef Type = Format->getType()->getName();
+      std::string FormatIdx = std::to_string(Format->getFormatIdx());
+      std::string FirstArg = std::to_string(Format->getFirstArg());
+      SmallVector<StringRef> Args = {
+          Type, FormatIdx, FirstArg,
+          ModularFormat->getModularImplFn()->getName(),
+          ModularFormat->getImplName()};
+      llvm::append_range(Args, ModularFormat->aspects());
+      FuncAttrs.addAttribute("modular-format", llvm::join(Args, ","));
+    }
   }
 
   // Attach "no-builtins" attributes to:

clang/lib/Sema/SemaDeclAttr.cpp

@@ -6745,6 +6745,29 @@ static void handleVTablePointerAuthentication(Sema &S, Decl *D,
       CustomDiscriminationValue));
 }
 
+static void handleModularFormat(Sema &S, Decl *D, const ParsedAttr &AL) {
+  StringRef ImplName;
+  if (!S.checkStringLiteralArgumentAttr(AL, 1, ImplName))
+    return;
+  SmallVector<StringRef> Aspects;
+  for (unsigned I = 2, E = AL.getNumArgs(); I != E; ++I) {
+    StringRef Aspect;
+    if (!S.checkStringLiteralArgumentAttr(AL, I, Aspect))
+      return;
+    Aspects.push_back(Aspect);
+  }
+
+  // Store aspects sorted and without duplicates.
+  llvm::sort(Aspects);
+  Aspects.erase(llvm::unique(Aspects), Aspects.end());
+
+  // TODO: Type checking on identifier
+  // TODO: Merge attributes
+  D->addAttr(::new (S.Context) ModularFormatAttr(
+      S.Context, AL, AL.getArgAsIdent(0)->getIdentifierInfo(), ImplName,
+      Aspects.data(), Aspects.size()));
+}
+
 //===----------------------------------------------------------------------===//
 // Top Level Sema Entry Points
 //===----------------------------------------------------------------------===//
@@ -7669,6 +7692,10 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_VTablePointerAuthentication:
     handleVTablePointerAuthentication(S, D, AL);
     break;
+
+  case ParsedAttr::AT_ModularFormat:
+    handleModularFormat(S, D, AL);
+    break;
   }
 }