[libc] Modular printf option (float only)

clang/include/clang/Basic/Attr.td

@@ -5240,3 +5240,14 @@ 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

@@ -9605,3 +9605,37 @@ 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

@@ -6751,6 +6751,29 @@ 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
 //===----------------------------------------------------------------------===//
@@ -7681,6 +7704,10 @@ 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;
   }
 }
 

libc/config/config.json

@@ -51,6 +51,10 @@
     "LIBC_CONF_PRINTF_RUNTIME_DISPATCH": {
       "value": true,
       "doc": "Use dynamic dispatch for the output mechanism to reduce code size."
+    },
+    "LIBC_CONF_PRINTF_MODULAR": {
+      "value": true,
+      "doc": "Split printf implementation into modules that can be lazily linked in."
     }
   },
   "scanf": {

libc/docs/configure.rst

@@ -45,6 +45,7 @@ to learn about the defaults for your platform and target.
     - ``LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_DYADIC_FLOAT``: Use dyadic float for faster and smaller but less accurate printf doubles.
     - ``LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_FLOAT320``: Use an alternative printf float implementation based on 320-bit floats
     - ``LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE``: Use large table for better printf long double performance.
+    - ``LIBC_CONF_PRINTF_MODULAR``: Split printf implementation into modules that can be lazily linked in.
     - ``LIBC_CONF_PRINTF_RUNTIME_DISPATCH``: Use dynamic dispatch for the output mechanism to reduce code size.
 * **"pthread" options**
     - ``LIBC_CONF_RAW_MUTEX_DEFAULT_SPIN_COUNT``: Default number of spins before blocking if a mutex is in contention (default to 100).

libc/src/stdio/generic/CMakeLists.txt

@@ -412,10 +412,15 @@ if(LLVM_LIBC_FULL_BUILD)
   )
 endif()
 
+set(printf_srcs printf.cpp)
+if (LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND printf_srcs printf_modular.cpp)
+endif()
+
 add_generic_entrypoint_object(
   printf
   SRCS
-    printf.cpp
+    ${printf_srcs}
   HDRS
     ../printf.h
   DEPENDS

libc/src/stdio/generic/printf_modular.cpp

@@ -0,0 +1,40 @@
+//===-- Implementation of printf_modular-----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdio/printf.h"
+
+#include "src/__support/File/file.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/printf_core/vfprintf_internal.h"
+
+#include "hdr/types/FILE.h"
+#include <stdarg.h>
+
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
+#define PRINTF_STDOUT LIBC_NAMESPACE::stdout
+#else // LIBC_COPT_STDIO_USE_SYSTEM_FILE
+#define PRINTF_STDOUT ::stdout
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, __printf_modular,
+                   (const char *__restrict format, ...)) {
+  va_list vlist;
+  va_start(vlist, format);
+  internal::ArgList args(vlist); // This holder class allows for easier copying
+                                 // and pointer semantics, as well as handling
+                                 // destruction automatically.
+  va_end(vlist);
+  int ret_val = printf_core::vfprintf_internal_modular(
+      reinterpret_cast<::FILE *>(PRINTF_STDOUT), format, args);
+  return ret_val;
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/printf.h

@@ -15,6 +15,7 @@
 namespace LIBC_NAMESPACE_DECL {
 
 int printf(const char *__restrict format, ...);
+int __printf_modular(const char *__restrict format, ...);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/printf_core/CMakeLists.txt

@@ -28,6 +28,9 @@ endif()
 if(LIBC_CONF_PRINTF_RUNTIME_DISPATCH)
   list(APPEND printf_config_copts "-DLIBC_COPT_PRINTF_RUNTIME_DISPATCH")
 endif()
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND printf_config_copts "-DLIBC_COPT_PRINTF_MODULAR")
+endif()
 if(printf_config_copts)
   list(PREPEND printf_config_copts "COMPILE_OPTIONS")
 endif()
@@ -114,10 +117,12 @@ add_header_library(
     libc.src.__support.StringUtil.error_to_string
 )
 
-add_header_library(
+add_object_library(
   printf_main
   HDRS
     printf_main.h
+  SRCS
+    float_impl.cpp
   DEPENDS
     .parser
     .converter

libc/src/stdio/printf_core/float_dec_converter.h

@@ -1122,6 +1122,18 @@ LIBC_INLINE int convert_float_dec_auto_typed(Writer<write_mode> *writer,
   }
 }
 
+template <WriteMode write_mode>
+LIBC_PRINTF_MODULAR_DECL int
+convert_float_decimal(Writer<write_mode> *writer, const FormatSection &to_conv);
+template <WriteMode write_mode>
+LIBC_PRINTF_MODULAR_DECL int
+convert_float_dec_exp(Writer<write_mode> *writer, const FormatSection &to_conv);
+template <WriteMode write_mode>
+LIBC_PRINTF_MODULAR_DECL int
+convert_float_dec_auto(Writer<write_mode> *writer,
+                       const FormatSection &to_conv);
+
+#ifdef LIBC_PRINTF_DEFINE_MODULAR
 // TODO: unify the float converters to remove the duplicated checks for inf/nan.
 
 template <WriteMode write_mode>
@@ -1189,6 +1201,7 @@ LIBC_INLINE int convert_float_dec_auto(Writer<write_mode> *writer,
 
   return convert_inf_nan(writer, to_conv);
 }
+#endif
 
 } // namespace printf_core
 } // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/printf_core/float_dec_converter_limited.h

@@ -675,6 +675,17 @@ LIBC_INLINE int convert_float_dec_auto_typed(Writer<write_mode> *writer,
   return convert_float_typed<T>(writer, to_conv, float_bits, ConversionType::G);
 }
 
+template <WriteMode write_mode>
+LIBC_PRINTF_MODULAR_DECL int convert_float_decimal(Writer<write_mode> *writer,
+                                                 const FormatSection &to_conv);
+template <WriteMode write_mode>
+LIBC_PRINTF_MODULAR_DECL int convert_float_dec_exp(Writer<write_mode> *writer,
+                                                 const FormatSection &to_conv);
+template <WriteMode write_mode>
+LIBC_PRINTF_MODULAR_DECL int convert_float_dec_auto(Writer<write_mode> *writer,
+                                                  const FormatSection &to_conv);
+
+#ifdef LIBC_PRINTF_DEFINE_MODULAR
 template <WriteMode write_mode>
 LIBC_INLINE int convert_float_decimal(Writer<write_mode> *writer,
                                       const FormatSection &to_conv) {
@@ -692,6 +703,7 @@ LIBC_INLINE int convert_float_dec_auto(Writer<write_mode> *writer,
                                        const FormatSection &to_conv) {
   return convert_float_outer(writer, to_conv, ConversionType::G);
 }
+#endif
 
 } // namespace printf_core
 } // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/printf_core/float_hex_converter.h

@@ -25,6 +25,11 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace printf_core {
 
+template <WriteMode write_mode>
+LIBC_PRINTF_MODULAR_DECL int convert_float_hex_exp(Writer<write_mode> *writer,
+                                                 const FormatSection &to_conv);
+
+#ifdef LIBC_PRINTF_DEFINE_MODULAR
 template <WriteMode write_mode>
 LIBC_INLINE int convert_float_hex_exp(Writer<write_mode> *writer,
                                       const FormatSection &to_conv) {
@@ -254,6 +259,7 @@ LIBC_INLINE int convert_float_hex_exp(Writer<write_mode> *writer,
   }
   return WRITE_OK;
 }
+#endif
 
 } // namespace printf_core
 } // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/printf_core/float_impl.cpp

@@ -0,0 +1,56 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file instantiates the functionality needed for supporting floating
+/// point arguments in modular printf builds. Non-modular printf builds
+/// implicitly instantiate these functions.
+///
+//===----------------------------------------------------------------------===//
+
+#ifdef LIBC_COPT_PRINTF_MODULAR
+#include "src/__support/arg_list.h"
+
+#define LIBC_PRINTF_DEFINE_MODULAR
+#include "src/stdio/printf_core/float_dec_converter.h"
+#include "src/stdio/printf_core/float_hex_converter.h"
+#include "src/stdio/printf_core/parser.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace printf_core {
+template class Parser<internal::ArgList>;
+template class Parser<internal::DummyArgList<false>>;
+template class Parser<internal::DummyArgList<true>>;
+template class Parser<internal::StructArgList<false>>;
+template class Parser<internal::StructArgList<true>>;
+
+#define INSTANTIATE_CONVERT_FN(NAME)                                           \
+  template int NAME<WriteMode::FILL_BUFF_AND_DROP_OVERFLOW>(                   \
+      Writer<WriteMode::FILL_BUFF_AND_DROP_OVERFLOW> * writer,                 \
+      const FormatSection &to_conv);                                           \
+  template int NAME<WriteMode::FLUSH_TO_STREAM>(                               \
+      Writer<WriteMode::FLUSH_TO_STREAM> * writer,                             \
+      const FormatSection &to_conv);                                           \
+  template int NAME<WriteMode::RESIZE_AND_FILL_BUFF>(                          \
+      Writer<WriteMode::RESIZE_AND_FILL_BUFF> * writer,                        \
+      const FormatSection &to_conv);                                           \
+  template int NAME<WriteMode::RUNTIME_DISPATCH>(                              \
+      Writer<WriteMode::RUNTIME_DISPATCH> * writer,                            \
+      const FormatSection &to_conv)
+
+INSTANTIATE_CONVERT_FN(convert_float_decimal);
+INSTANTIATE_CONVERT_FN(convert_float_dec_exp);
+INSTANTIATE_CONVERT_FN(convert_float_dec_auto);
+INSTANTIATE_CONVERT_FN(convert_float_hex_exp);
+
+} // namespace printf_core
+} // namespace LIBC_NAMESPACE_DECL
+
+// Bring this file into the link if __printf_float is referenced.
+extern "C" void __printf_float() {}
+#endif