[libc] Modular printf option (float only) (#147426)

This adds LIBC_CONF_PRINTF_MODULAR, which causes floating point support
(later, others) to be weakly linked into the implementation.
__printf_modular becomes the main entry point of the implementaiton, an
printf itself wraps __printf_modular. printf it also contains a
BFD_RELOC_NONE relocation to bring in the float aspect.

See issue #146159 for context.

clang/include/clang/Basic/AttrDocs.td

@@ -9854,9 +9854,9 @@ not all aspects of the implementation are needed for a given call, the compiler
 may redirect the call to the identifier given as the first argument to the
 attribute (the modular implementation function).
 
-The second argument is a implementation name, and the remaining arguments are
+The second argument is an implementation name, and the remaining arguments are
 aspects of the format string for the compiler to report. The implementation
-name is an unevaluated identifier be in the C namespace.
+name is an unevaluated identifier in the C namespace.
 
 The compiler reports that a call requires an aspect by issuing a relocation for
 the symbol ``<impl_name>_<aspect>`` at the point of the call. This arranges for

libc/config/config.json

@@ -59,6 +59,10 @@
     "LIBC_COPT_PRINTF_DISABLE_BITINT": {
       "value": false,
       "doc": "Disable bitint length modifiers to reduce code size. Specifically the wNUM and wfNUM modifiers."
+    },
+    "LIBC_CONF_PRINTF_MODULAR": {
+      "value": false,
+      "doc": "Split printf implementation into modules that can be lazily linked in."
     }
   },
   "scanf": {

libc/docs/dev/index.rst

@@ -22,3 +22,4 @@ Navigate to the links below for information on the respective topics:
    undefined_behavior
    printf_behavior
    syscall_wrapper_refactor
+   modular_format

libc/docs/dev/modular_format.rst

@@ -0,0 +1,68 @@
+.. _modular_format:
+
+======================
+Modular format strings
+======================
+
+Introduction
+============
+
+Several C standard library functions (most notably, ``printf`` and ``scanf``),
+present a large amount of related features to the caller configured via a
+format string. This benefits code size at the caller, since format strings are
+typically quite dense, and the equivalent of many individual calls can be
+performed with only one. Overall this is a benefit, since to a function calls
+typically outnumber the one definition of that function.
+
+However, the implementations of various libc features gated behind aspects of
+those format strings can be large enough that they completely swamp the
+programs that call them. Floating point and errno conversion in particular can
+involve large tables which may be wholly dead. However, due to the format
+string structure, this code is dead in a way previously invisible to the
+compiler.
+
+To address this, an clang attribute was introduced: ``modular_format(<impl_fn>,
+<impl_name>, <aspects>...)``. This adds to the semantics of the existing
+``format`` attribute (which must also be present, if implicitly.) The first
+argument is a symbol naming a modular version of the implementation; this
+version only weakly refers to "aspects" of the implementation that may not be
+necessary for certain format strings. The second argument is general
+"implementation name" string, and the remaining arguments are a list of handled
+aspects of the format string. When the compiler sees that a given call only
+needs a fixed set of aspects of the implementation, it may redirect the call to
+the implementation function and emit a series of relocations to symbols named
+``<impl_name>_<aspect>``. These in turn bring the needed aspects of the call
+into the link. The default entrypoints fall the modular ones, except they bring
+in every possible implementation aspect.
+
+Mechanism
+=========
+
+This functionality is currently gated behind ``LIBC_COPT_PRINTF_MODULAR``. When
+set, the ``printf``-family functions gain modular variants, and the regular
+variants are modified to call them and emit NONE relocations against all
+implementation aspects. 
+
+The implementation aspects are defined in headers using the
+``LIBC_PRINTF_MODULE((<decl>), { <body> })`` macro. If
+``LIBC_COPT_PRINTF_MODULAR`` is not defined, then this macro makes these
+``LIBC_INLINE`` definitions as per usual. Otherwise, for normal usage, these
+become weak declarations, which causes any references to the module to become
+weak. The implementations are moved to a dedicated impl file for groups of
+modules. These define the aspect symbol and the module impls by defining
+``LIBC_PRINTF_DEFINE_MODULES`` before including the header. This causes the to
+be brought into the link exactly when the aspect symbol is referenced.
+
+Template functions present a special complication: the implementation must
+instantiate them for any value that may be used. Since the purpose of the
+templates is to implement a fixed interface, the possible arguments should
+always be fixed and finite. Accordingly, libc contains def files to enumerate
+possible arguments and provide handling for each. Templates are instantiated in
+the headers whenever ``LIBC_PRINTF_DEFINE_MODULES`` is defined.
+
+libc and the compiler may understand different sets of aspect names, but their
+understanding of what an aspect name means must be identical. libc reports the
+set of aspect names that it needs a verdict on, and the compiler will only
+provide a verdict for those aspects. If libc asks for a verdict on an aspect
+unknown to the compiler, the aspect must be summarily considered to be
+required.

libc/include/CMakeLists.txt

@@ -360,6 +360,7 @@ add_header_macro(
   ../libc/include/stdio.yaml
   stdio.h
   DEPENDS
+    .llvm-libc-macros._LIBC_MODULAR_FORMAT_PRINTF
     .llvm-libc-macros.file_seek_macros
     .llvm-libc-macros.null_macro
     .llvm-libc-macros.stdio_macros

libc/include/llvm-libc-macros/CMakeLists.txt

@@ -3,15 +3,19 @@ function(add_macro_header name)
   cmake_parse_arguments(
     "MACRO_HEADER"
     "" # Optional arguments
-    "HDR" # Single value arguments
+    "HDR;DEST_HDR" # Single value arguments
     "DEPENDS" # Multi-value arguments
     ${ARGN}
   )
+  if (MACRO_HEADER_DEST_HDR)
+    set(dest_header_arg DEST_HDR ${MACRO_HEADER_DEST_HDR})
+  endif()
   if(TARGET libc.include.llvm-libc-macros.${LIBC_TARGET_OS}.${name})
     add_header(
       ${name}
       HDR
         ${MACRO_HEADER_HDR}
+      ${dest_header_arg}
       DEPENDS
         .${LIBC_TARGET_OS}.${name}
         ${MACRO_HEADER_DEPENDS}
@@ -21,6 +25,7 @@ function(add_macro_header name)
       ${name}
       HDR
         ${MACRO_HEADER_HDR}
+      ${dest_header_arg}
       DEPENDS
         ${MACRO_HEADER_DEPENDS}
     )
@@ -395,3 +400,18 @@ add_macro_header(
     sysexits-macros.h
 )
 
+if (LIBC_CONF_MODULAR_FORMAT)
+  add_macro_header(
+    _LIBC_MODULAR_FORMAT_PRINTF
+    HDR
+      _LIBC_MODULAR_FORMAT_PRINTF.h
+  )
+else()
+  add_macro_header(
+    _LIBC_MODULAR_FORMAT_PRINTF
+    HDR
+      _LIBC_MODULAR_FORMAT_PRINTF-disable.h
+      DEST_HDR
+      _LIBC_MODULAR_FORMAT_PRINTF.h
+  )
+endif()

libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF-disable.h

@@ -0,0 +1,14 @@
+//===-- Definition to disable modular format macro for printf -------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
+#define LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
+
+#define _LIBC_MODULAR_FORMAT_PRINTF(MODULAR_IMPL_FN)
+
+#endif // LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H

libc/include/llvm-libc-macros/_LIBC_MODULAR_FORMAT_PRINTF.h

@@ -0,0 +1,15 @@
+//===-- Definition of modular format macro for printf ---------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
+#define LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H
+
+#define _LIBC_MODULAR_FORMAT_PRINTF(MODULAR_IMPL_FN)                           \
+  __attribute__((modular_format(MODULAR_IMPL_FN, "__printf", "float")))
+
+#endif // LLVM_LIBC_MACROS_LIBC_MODULAR_FORMAT_PRINTF_H

libc/include/stdio.yaml

@@ -31,6 +31,8 @@ functions:
       - type: char **__restrict
       - type: const char *__restrict
       - type: '...'
+    attributes:
+      - _LIBC_MODULAR_FORMAT_PRINTF(__asprintf_modular)
   - name: clearerr
     standards:
       - stdc
@@ -276,6 +278,8 @@ functions:
     arguments:
       - type: const char *__restrict
       - type: '...'
+    attributes:
+      - _LIBC_MODULAR_FORMAT_PRINTF(__printf_modular)
   - name: putc
     standards:
       - stdc
@@ -347,6 +351,8 @@ functions:
       - type: size_t
       - type: const char *__restrict
       - type: '...'
+    attributes:
+      - _LIBC_MODULAR_FORMAT_PRINTF(__snprintf_modular)
   - name: sprintf
     standards:
       - stdc
@@ -355,6 +361,8 @@ functions:
       - type: char *__restrict
       - type: const char *__restrict
       - type: '...'
+    attributes:
+      - _LIBC_MODULAR_FORMAT_PRINTF(__sprintf_modular)
   - name: sscanf
     standards:
       - stdc
@@ -378,6 +386,8 @@ functions:
       - type: char **__restrict
       - type: const char *__restrict
       - type: va_list
+    attributes:
+      - _LIBC_MODULAR_FORMAT_PRINTF(__vasprintf_modular)
   - name: vfprintf
     standards:
       - stdc
@@ -393,6 +403,8 @@ functions:
     arguments:
       - type: const char *__restrict
       - type: va_list
+    attributes:
+      - _LIBC_MODULAR_FORMAT_PRINTF(__vprintf_modular)
   - name: vsnprintf
     standards:
       - stdc
@@ -402,6 +414,8 @@ functions:
       - type: size_t
       - type: const char *__restrict
       - type: va_list
+    attributes:
+      - _LIBC_MODULAR_FORMAT_PRINTF(__vsnprintf_modular)
   - name: vsprintf
     standards:
       - stdc
@@ -410,6 +424,8 @@ functions:
       - type: char *__restrict
       - type: const char *__restrict
       - type: va_list
+    attributes:
+      - _LIBC_MODULAR_FORMAT_PRINTF(__vsprintf_modular)
   - name: vsscanf
     standards:
       - stdc

libc/src/stdio/CMakeLists.txt

@@ -116,10 +116,15 @@ add_entrypoint_object(
     libc.src.stdio.scanf_core.string_reader
 )
 
+set(sprintf_srcs sprintf.cpp)
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND sprintf_srcs sprintf_modular.cpp)
+endif()
+
 add_entrypoint_object(
   sprintf
   SRCS
-    sprintf.cpp
+    ${sprintf_srcs}
   HDRS
     sprintf.h
   DEPENDS
@@ -131,10 +136,14 @@ add_entrypoint_object(
     libc.src.__support.CPP.limits
 )
 
+set(snprintf_srcs snprintf.cpp)
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND snprintf_srcs snprintf_modular.cpp)
+endif()
 add_entrypoint_object(
   snprintf
   SRCS
-    snprintf.cpp
+    ${snprintf_srcs}
   HDRS
     snprintf.h
   DEPENDS
@@ -146,10 +155,14 @@ add_entrypoint_object(
     libc.src.__support.CPP.limits
 )
 
+set(asprintf_srcs asprintf.cpp)
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND asprintf_srcs asprintf_modular.cpp)
+endif()
 add_entrypoint_object(
   asprintf
   SRCS
-    asprintf.cpp
+    ${asprintf_srcs}
   HDRS
     asprintf.h
   DEPENDS
@@ -160,10 +173,15 @@ add_entrypoint_object(
     libc.src.__support.CPP.limits
 )
 
+set(vsprintf_srcs vsprintf.cpp)
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND vsprintf_srcs vsprintf_modular.cpp)
+endif()
+
 add_entrypoint_object(
   vsprintf
   SRCS
-    vsprintf.cpp
+    ${vsprintf_srcs}
   HDRS
     vsprintf.h
   DEPENDS
@@ -175,10 +193,15 @@ add_entrypoint_object(
     libc.src.__support.CPP.limits
 )
 
+set(vsnprintf_srcs vsnprintf.cpp)
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND vsnprintf_srcs vsnprintf_modular.cpp)
+endif()
+
 add_entrypoint_object(
   vsnprintf
   SRCS
-    vsnprintf.cpp
+    ${vsnprintf_srcs}
   HDRS
     vsnprintf.h
   DEPENDS
@@ -190,10 +213,15 @@ add_entrypoint_object(
     libc.src.__support.CPP.limits
 )
 
+set(vasprintf_srcs vasprintf.cpp)
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND vasprintf_srcs vasprintf_modular.cpp)
+endif()
+
 add_entrypoint_object(
   vasprintf
   SRCS
-    vasprintf.cpp
+    ${vasprintf_srcs}
   HDRS
     vasprintf.h
   DEPENDS

libc/src/stdio/asprintf.cpp

@@ -26,7 +26,12 @@ LLVM_LIBC_FUNCTION(int, asprintf,
                                  // and pointer semantics, as well as handling
                                  // destruction automatically.
   va_end(vlist);
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  auto ret_val = printf_core::vasprintf_internal<true>(buffer, format, args);
+#else
   auto ret_val = printf_core::vasprintf_internal(buffer, format, args);
+#endif
   if (!ret_val.has_value()) {
     libc_errno = printf_core::internal_error_to_errno(ret_val.error());
     return -1;

libc/src/stdio/asprintf.h

@@ -14,6 +14,7 @@
 namespace LIBC_NAMESPACE_DECL {
 
 int asprintf(char **__restrict s, const char *__restrict format, ...);
+int __asprintf_modular(char **__restrict s, const char *__restrict format, ...);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/asprintf_modular.cpp

@@ -0,0 +1,43 @@
+//===-- Implementation of asprintf_modular ----------------------*- C++ -*-===//
+//
+// 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/__support/CPP/limits.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/asprintf.h"
+#include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/error_mapper.h"
+#include "src/stdio/printf_core/vasprintf_internal.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, __asprintf_modular,
+                   (char **__restrict buffer, 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);
+  auto ret_val = printf_core::vasprintf_internal<true>(buffer, format, args);
+  if (!ret_val.has_value()) {
+    libc_errno = printf_core::internal_error_to_errno(ret_val.error());
+    return -1;
+  }
+  if (ret_val.value() > static_cast<size_t>(cpp::numeric_limits<int>::max())) {
+    libc_errno =
+        printf_core::internal_error_to_errno(-printf_core::OVERFLOW_ERROR);
+    return -1;
+  }
+
+  return static_cast<int>(ret_val.value());
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/baremetal/CMakeLists.txt

@@ -211,10 +211,14 @@ add_entrypoint_object(
     libc.include.stdio
 )
 
+set(printf_srcs printf.cpp)
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND printf_srcs printf_modular.cpp)
+endif()
 add_entrypoint_object(
   printf
   SRCS
-    printf.cpp
+    ${printf_srcs}
   HDRS
     ../printf.h
   DEPENDS
@@ -345,10 +349,14 @@ add_entrypoint_object(
     libc.src.__support.arg_list
 )
 
+set(vprintf_srcs vprintf.cpp)
+if(LIBC_CONF_PRINTF_MODULAR)
+  list(APPEND vprintf_srcs vprintf_modular.cpp)
+endif()
 add_entrypoint_object(
   vprintf
   SRCS
-    vprintf.cpp
+    ${vprintf_srcs}
   HDRS
     ../vprintf.h
   DEPENDS

libc/src/stdio/baremetal/printf.cpp

@@ -26,7 +26,12 @@ LLVM_LIBC_FUNCTION(int, printf, (const char *__restrict format, ...)) {
                                  // destruction automatically.
   va_end(vlist);
 
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  return vfprintf_internal<true>(stdout, format, args);
+#else
   return vfprintf_internal(stdout, format, args);
+#endif
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/baremetal/printf_modular.cpp

@@ -0,0 +1,33 @@
+//===-- Implementation of printf_modular for baremetal ----------*- C++ -*-===//
+//
+// 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 "hdr/stdio_macros.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/baremetal/vfprintf_internal.h"
+
+#include <stdarg.h>
+
+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);
+
+  return vfprintf_internal<true>(stdout, format, args);
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/baremetal/vfprintf.cpp

@@ -25,7 +25,12 @@ LLVM_LIBC_FUNCTION(int, vfprintf,
                                  // and pointer semantics, as well as handling
                                  // destruction automatically.
 
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  return vfprintf_internal<true>(stream, format, args);
+#else
   return vfprintf_internal(stream, format, args);
+#endif
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/baremetal/vfprintf_internal.h

@@ -38,6 +38,7 @@ LIBC_INLINE int write_hook(cpp::string_view str_view, void *cookie) {
 
 } // namespace internal
 
+template <bool use_modular = false>
 LIBC_INLINE int vfprintf_internal(::FILE *__restrict stream,
                                   const char *__restrict format,
                                   internal::ArgList &args) {
@@ -48,7 +49,12 @@ LIBC_INLINE int vfprintf_internal(::FILE *__restrict stream,
                                  stream);
   printf_core::Writer writer(wb);
 
-  auto retval = printf_core::printf_main(&writer, format, args);
+  auto retval = [&] {
+    if constexpr (use_modular)
+      return printf_core::printf_main_modular(&writer, format, args);
+    else
+      return printf_core::printf_main(&writer, format, args);
+  }();
   if (!retval.has_value()) {
     libc_errno = printf_core::internal_error_to_errno(retval.error());
     return -1;

libc/src/stdio/baremetal/vprintf.cpp

@@ -24,7 +24,12 @@ LLVM_LIBC_FUNCTION(int, vprintf,
                                  // and pointer semantics, as well as handling
                                  // destruction automatically.
 
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  return vfprintf_internal<true>(stdout, format, args);
+#else
   return vfprintf_internal(stdout, format, args);
+#endif
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/baremetal/vprintf_modular.cpp

@@ -0,0 +1,30 @@
+//===-- Implementation of vprintf_modular -----------------------*- C++ -*-===//
+//
+// 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/vprintf.h"
+
+#include "hdr/stdio_macros.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/baremetal/vfprintf_internal.h"
+
+#include <stdarg.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, __vprintf_modular,
+                   (const char *__restrict format, va_list vlist)) {
+  internal::ArgList args(vlist); // This holder class allows for easier copying
+                                 // and pointer semantics, as well as handling
+                                 // destruction automatically.
+
+  return vfprintf_internal<true>(stdout, format, args);
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/generic/CMakeLists.txt

@@ -416,6 +416,10 @@ if(LLVM_LIBC_FULL_BUILD)
   )
 endif()
 
+if (LIBC_CONF_PRINTF_MODULAR AND NOT LIBC_TARGET_OS_IS_BAREMETAL)
+  message(FATAL_ERROR "modular printf is only supported in baremetal stdio")
+endif()
+
 add_generic_entrypoint_object(
   printf
   SRCS

libc/src/stdio/gpu/CMakeLists.txt

@@ -1,3 +1,7 @@
+if (LIBC_CONF_PRINTF_MODULAR)
+  message(FATAL_ERROR "modular printf is only supported in baremetal stdio")
+endif()
+
 add_entrypoint_object(
   stdin
   SRCS

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()
@@ -150,10 +153,12 @@ add_header_library(
     ${wchar_deps}
 )
 
-add_header_library(
+add_object_library(
   printf_main
   HDRS
     printf_main.h
+  SRCS
+    float_impl.cpp
   DEPENDS
     .parser
     .converter

libc/src/stdio/printf_core/converter.h

@@ -28,6 +28,37 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace printf_core {
 
+#ifndef LIBC_COPT_PRINTF_DISABLE_FLOAT
+LIBC_PRINTF_MODULE((template <WriteMode write_mode>
+                    int convert_float(Writer<write_mode> *writer,
+                                      const FormatSection &to_conv)),
+                   {
+                     switch (to_conv.conv_name) {
+                     case 'f':
+                     case 'F':
+                       return convert_float_decimal(writer, to_conv);
+                     case 'e':
+                     case 'E':
+                       return convert_float_dec_exp(writer, to_conv);
+                     case 'a':
+                     case 'A':
+                       return convert_float_hex_exp(writer, to_conv);
+                     case 'g':
+                     case 'G':
+                       return convert_float_dec_auto(writer, to_conv);
+                     }
+                     __builtin_unreachable();
+                   })
+#endif // not LIBC_COPT_PRINTF_DISABLE_FLOAT
+
+#ifdef LIBC_PRINTF_DEFINE_MODULES
+#define HANDLE_WRITE_MODE(MODE)                                                \
+  template int convert_float<WriteMode::MODE>(                                 \
+      Writer<WriteMode::MODE> * writer, const FormatSection &to_conv);
+#include "src/stdio/printf_core/write_modes.def"
+#undef HANDLE_WRITE_MODE
+#endif // LIBC_PRINTF_DEFINE_MODULES
+
 // convert will call a conversion function to convert the FormatSection into
 // its string representation, and then that will write the result to the
 // writer.
@@ -72,16 +103,13 @@ int convert(Writer<write_mode> *writer, const FormatSection &to_conv) {
 #ifndef LIBC_COPT_PRINTF_DISABLE_FLOAT
   case 'f':
   case 'F':
-    return convert_float_decimal(writer, to_conv);
   case 'e':
   case 'E':
-    return convert_float_dec_exp(writer, to_conv);
   case 'a':
   case 'A':
-    return convert_float_hex_exp(writer, to_conv);
   case 'g':
   case 'G':
-    return convert_float_dec_auto(writer, to_conv);
+    return convert_float(writer, to_conv);
 #endif // LIBC_COPT_PRINTF_DISABLE_FLOAT
 #ifdef LIBC_INTERNAL_PRINTF_HAS_FIXED_POINT
   case 'r':

libc/src/stdio/printf_core/float_impl.cpp

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+#define LIBC_PRINTF_DEFINE_MODULES
+#include "src/stdio/printf_core/converter.h"
+
+// Bring this file into the link if __printf_float is referenced.
+extern "C" void __printf_float() {}
+
+#endif // LIBC_COPT_PRINTF_MODULAR

libc/src/stdio/printf_core/printf_config.h

@@ -48,4 +48,24 @@
 
 // LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
 
+#ifdef LIBC_COPT_PRINTF_MODULAR
+#define LIBC_PRINTF_MODULE_DECL __attribute__((weak))
+#else
+#define LIBC_PRINTF_MODULE_DECL LIBC_INLINE
+#endif
+
+// LIBC_PRINTF_MODULE: Defines/declares a printf module.
+//
+// Usage: LIBC_PRINTF_MODULE((<signature>), { <body> })
+//
+// Note that the signature is parenthesized, but the body is not.
+
+#define LIBC_PRINTF_MODULE_UNWRAP(...) __VA_ARGS__
+#if !defined(LIBC_COPT_PRINTF_MODULAR) || defined(LIBC_PRINTF_DEFINE_MODULES)
+#define LIBC_PRINTF_MODULE(SIG, ...) LIBC_PRINTF_MODULE_UNWRAP SIG __VA_ARGS__
+#else
+#define LIBC_PRINTF_MODULE(SIG, ...)                                           \
+  LIBC_PRINTF_MODULE_UNWRAP SIG LIBC_PRINTF_MODULE_DECL;
+#endif
+
 #endif // LLVM_LIBC_SRC_STDIO_PRINTF_CORE_PRINTF_CONFIG_H

libc/src/stdio/printf_core/printf_main.h

@@ -23,9 +23,9 @@ namespace LIBC_NAMESPACE_DECL {
 namespace printf_core {
 
 template <WriteMode write_mode>
-ErrorOr<size_t> printf_main(Writer<write_mode> *writer,
-                            const char *__restrict str,
-                            internal::ArgList &args) {
+ErrorOr<size_t> printf_main_modular(Writer<write_mode> *writer,
+                                    const char *__restrict str,
+                                    internal::ArgList &args) {
   Parser<internal::ArgList> parser(str, args);
   int result = 0;
   for (FormatSection cur_section = parser.get_next_section();
@@ -42,6 +42,16 @@ ErrorOr<size_t> printf_main(Writer<write_mode> *writer,
   return writer->get_chars_written();
 }
 
+template <WriteMode write_mode>
+ErrorOr<size_t> printf_main(Writer<write_mode> *writer,
+                            const char *__restrict str,
+                            internal::ArgList &args) {
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+#endif
+  return printf_main_modular(writer, str, args);
+}
+
 } // namespace printf_core
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/printf_core/vasprintf_internal.h

@@ -41,6 +41,7 @@ LIBC_INLINE int resize_overflow_hook(cpp::string_view new_str,
 
 constexpr size_t DEFAULT_BUFFER_SIZE = 200;
 
+template <bool use_modular = false>
 LIBC_INLINE ErrorOr<size_t> vasprintf_internal(char **ret,
                                                const char *__restrict format,
                                                internal::ArgList args) {
@@ -49,7 +50,12 @@ LIBC_INLINE ErrorOr<size_t> vasprintf_internal(char **ret,
                                  resize_overflow_hook);
   printf_core::Writer writer(wb);
 
-  auto ret_val = printf_core::printf_main(&writer, format, args);
+  auto ret_val = [&] {
+    if constexpr (use_modular)
+      return printf_core::printf_main_modular(&writer, format, args);
+    else
+      return printf_core::printf_main(&writer, format, args);
+  }();
   if (!ret_val.has_value()) {
     *ret = nullptr;
     return ret_val;

libc/src/stdio/printf_core/write_modes.def

@@ -0,0 +1,12 @@
+//===-- printf write mode definitions ----------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+HANDLE_WRITE_MODE(FILL_BUFF_AND_DROP_OVERFLOW)
+HANDLE_WRITE_MODE(FLUSH_TO_STREAM)
+HANDLE_WRITE_MODE(RESIZE_AND_FILL_BUFF)
+HANDLE_WRITE_MODE(RUNTIME_DISPATCH)

libc/src/stdio/printf_core/writer.h

@@ -21,12 +21,11 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace printf_core {
 
+#define HANDLE_WRITE_MODE(MODE) MODE,
 enum class WriteMode {
-  FILL_BUFF_AND_DROP_OVERFLOW,
-  FLUSH_TO_STREAM,
-  RESIZE_AND_FILL_BUFF,
-  RUNTIME_DISPATCH,
+#include "src/stdio/printf_core/write_modes.def"
 };
+#undef HANDLE_WRITE_MODE
 
 // Helper to omit the template argument if we are using runtime dispatch and
 // avoid multiple copies of the converter functions.

libc/src/stdio/snprintf.cpp

@@ -34,7 +34,12 @@ LLVM_LIBC_FUNCTION(int, snprintf,
   printf_core::DropOverflowBuffer wb(buffer, (buffsz > 0 ? buffsz - 1 : 0));
   printf_core::Writer writer(wb);
 
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  auto ret_val = printf_core::printf_main_modular(&writer, format, args);
+#else
   auto ret_val = printf_core::printf_main(&writer, format, args);
+#endif
   if (!ret_val.has_value()) {
     libc_errno = printf_core::internal_error_to_errno(ret_val.error());
     return -1;

libc/src/stdio/snprintf.h

@@ -16,6 +16,8 @@ namespace LIBC_NAMESPACE_DECL {
 
 int snprintf(char *__restrict buffer, size_t buffsz,
              const char *__restrict format, ...);
+int __snprintf_modular(char *__restrict buffer, size_t buffsz,
+                       const char *__restrict format, ...);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/snprintf_modular.cpp

@@ -0,0 +1,54 @@
+//===-- Implementation of snprintf_modular ----------------------*- C++ -*-===//
+//
+// 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/snprintf.h"
+
+#include "src/__support/CPP/limits.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/error_mapper.h"
+#include "src/stdio/printf_core/printf_main.h"
+#include "src/stdio/printf_core/writer.h"
+
+#include <stdarg.h>
+#include <stddef.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, __snprintf_modular,
+                   (char *__restrict buffer, size_t buffsz,
+                    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);
+  printf_core::DropOverflowBuffer wb(buffer, (buffsz > 0 ? buffsz - 1 : 0));
+  printf_core::Writer writer(wb);
+
+  auto ret_val = printf_core::printf_main_modular(&writer, format, args);
+  if (!ret_val.has_value()) {
+    libc_errno = printf_core::internal_error_to_errno(ret_val.error());
+    return -1;
+  }
+  if (buffsz > 0) // if the buffsz is 0 the buffer may be a null pointer.
+    wb.buff[wb.buff_cur] = '\0';
+
+  if (ret_val.value() > static_cast<size_t>(cpp::numeric_limits<int>::max())) {
+    libc_errno =
+        printf_core::internal_error_to_errno(-printf_core::OVERFLOW_ERROR);
+    return -1;
+  }
+
+  return static_cast<int>(ret_val.value());
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/sprintf.cpp

@@ -35,7 +35,12 @@ LLVM_LIBC_FUNCTION(int, sprintf,
                                      cpp::numeric_limits<size_t>::max());
   printf_core::Writer writer(wb);
 
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  auto ret_val = printf_core::printf_main_modular(&writer, format, args);
+#else
   auto ret_val = printf_core::printf_main(&writer, format, args);
+#endif
   if (!ret_val.has_value()) {
     libc_errno = printf_core::internal_error_to_errno(ret_val.error());
     return -1;

libc/src/stdio/sprintf.h

@@ -14,6 +14,8 @@
 namespace LIBC_NAMESPACE_DECL {
 
 int sprintf(char *__restrict buffer, const char *__restrict format, ...);
+int __sprintf_modular(char *__restrict buffer, const char *__restrict format,
+                      ...);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/sprintf_modular.cpp

@@ -0,0 +1,54 @@
+//===-- Implementation of sprintf_modular -----------------------*- C++ -*-===//
+//
+// 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/sprintf.h"
+
+#include "src/__support/CPP/limits.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/error_mapper.h"
+#include "src/stdio/printf_core/printf_main.h"
+#include "src/stdio/printf_core/writer.h"
+
+#include <stdarg.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, __sprintf_modular,
+                   (char *__restrict buffer, 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);
+
+  printf_core::DropOverflowBuffer wb(buffer,
+                                     cpp::numeric_limits<size_t>::max());
+  printf_core::Writer writer(wb);
+
+  auto ret_val = printf_core::printf_main_modular(&writer, format, args);
+  if (!ret_val.has_value()) {
+    libc_errno = printf_core::internal_error_to_errno(ret_val.error());
+    return -1;
+  }
+  wb.buff[wb.buff_cur] = '\0';
+
+  if (ret_val.value() > static_cast<size_t>(cpp::numeric_limits<int>::max())) {
+    libc_errno =
+        printf_core::internal_error_to_errno(-printf_core::OVERFLOW_ERROR);
+    return -1;
+  }
+
+  return static_cast<int>(ret_val.value());
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/vasprintf.cpp

@@ -22,7 +22,12 @@ LLVM_LIBC_FUNCTION(int, vasprintf,
   internal::ArgList args(vlist); // This holder class allows for easier copying
                                  // and pointer semantics, as well as handling
                                  // destruction automatically.
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  auto ret_val = printf_core::vasprintf_internal<true>(ret, format, args);
+#else
   auto ret_val = printf_core::vasprintf_internal(ret, format, args);
+#endif
   if (!ret_val.has_value()) {
     libc_errno = printf_core::internal_error_to_errno(ret_val.error());
     return -1;

libc/src/stdio/vasprintf.h

@@ -16,6 +16,8 @@ namespace LIBC_NAMESPACE_DECL {
 
 int vasprintf(char **__restrict s, const char *__restrict format,
               va_list vlist);
+int __vasprintf_modular(char **__restrict s, const char *__restrict format,
+                        va_list vlist);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/vasprintf_modular.cpp

@@ -0,0 +1,38 @@
+//===-- Implementation of vasprintf_modular ---------------------*- C++ -*-===//
+//
+// 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/__support/CPP/limits.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/libc_errno.h"
+#include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/error_mapper.h"
+#include "src/stdio/printf_core/vasprintf_internal.h"
+#include "src/stdio/vasprintf.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, __vasprintf_modular,
+                   (char **__restrict ret, const char *__restrict format,
+                    va_list vlist)) {
+  internal::ArgList args(vlist); // This holder class allows for easier copying
+                                 // and pointer semantics, as well as handling
+                                 // destruction automatically.
+  auto ret_val = printf_core::vasprintf_internal<true>(ret, format, args);
+  if (!ret_val.has_value()) {
+    libc_errno = printf_core::internal_error_to_errno(ret_val.error());
+    return -1;
+  }
+  if (ret_val.value() > static_cast<size_t>(cpp::numeric_limits<int>::max())) {
+    libc_errno =
+        printf_core::internal_error_to_errno(-printf_core::OVERFLOW_ERROR);
+    return -1;
+  }
+  return static_cast<int>(ret_val.value());
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/vprintf.h

@@ -16,6 +16,7 @@
 namespace LIBC_NAMESPACE_DECL {
 
 int vprintf(const char *__restrict format, va_list vlist);
+int __vprintf_modular(const char *__restrict format, va_list vlist);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/vsnprintf.cpp

@@ -31,7 +31,12 @@ LLVM_LIBC_FUNCTION(int, vsnprintf,
   printf_core::DropOverflowBuffer wb(buffer, (buffsz > 0 ? buffsz - 1 : 0));
   printf_core::Writer writer(wb);
 
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  auto ret_val = printf_core::printf_main_modular(&writer, format, args);
+#else
   auto ret_val = printf_core::printf_main(&writer, format, args);
+#endif
   if (!ret_val.has_value()) {
     libc_errno = printf_core::internal_error_to_errno(ret_val.error());
     return -1;

libc/src/stdio/vsnprintf.h

@@ -17,6 +17,8 @@ namespace LIBC_NAMESPACE_DECL {
 
 int vsnprintf(char *__restrict buffer, size_t buffsz,
               const char *__restrict format, va_list vlist);
+int __vsnprintf_modular(char *__restrict buffer, size_t buffsz,
+                        const char *__restrict format, va_list vlist);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/vsnprintf_modular.cpp

@@ -0,0 +1,51 @@
+//===-- Implementation of vsnprintf_modular ---------------------*- C++ -*-===//
+//
+// 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/vsnprintf.h"
+
+#include "src/__support/CPP/limits.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/error_mapper.h"
+#include "src/stdio/printf_core/printf_main.h"
+#include "src/stdio/printf_core/writer.h"
+
+#include <stdarg.h>
+#include <stddef.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, __vsnprintf_modular,
+                   (char *__restrict buffer, size_t buffsz,
+                    const char *__restrict format, va_list vlist)) {
+  internal::ArgList args(vlist); // This holder class allows for easier copying
+                                 // and pointer semantics, as well as handling
+                                 // destruction automatically.
+  printf_core::DropOverflowBuffer wb(buffer, (buffsz > 0 ? buffsz - 1 : 0));
+  printf_core::Writer writer(wb);
+
+  auto ret_val = printf_core::printf_main_modular(&writer, format, args);
+  if (!ret_val.has_value()) {
+    libc_errno = printf_core::internal_error_to_errno(ret_val.error());
+    return -1;
+  }
+  if (buffsz > 0) // if the buffsz is 0 the buffer may be a null pointer.
+    wb.buff[wb.buff_cur] = '\0';
+
+  if (ret_val.value() > static_cast<size_t>(cpp::numeric_limits<int>::max())) {
+    libc_errno =
+        printf_core::internal_error_to_errno(-printf_core::OVERFLOW_ERROR);
+    return -1;
+  }
+
+  return static_cast<int>(ret_val.value());
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/vsprintf.cpp

@@ -32,7 +32,12 @@ LLVM_LIBC_FUNCTION(int, vsprintf,
                                      cpp::numeric_limits<size_t>::max());
   printf_core::Writer writer(wb);
 
+#ifdef LIBC_COPT_PRINTF_MODULAR
+  LIBC_INLINE_ASM(".reloc ., BFD_RELOC_NONE, __printf_float");
+  auto ret_val = printf_core::printf_main_modular(&writer, format, args);
+#else
   auto ret_val = printf_core::printf_main(&writer, format, args);
+#endif
   if (!ret_val.has_value()) {
     libc_errno = printf_core::internal_error_to_errno(ret_val.error());
     return -1;

libc/src/stdio/vsprintf.h

@@ -16,6 +16,8 @@ namespace LIBC_NAMESPACE_DECL {
 
 int vsprintf(char *__restrict buffer, const char *__restrict format,
              va_list vlist);
+int __vsprintf_modular(char *__restrict buffer, const char *__restrict format,
+                       va_list vlist);
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/stdio/vsprintf_modular.cpp

@@ -0,0 +1,50 @@
+//===-- Implementation of vsprintf_modular ----------------------*- C++ -*-===//
+//
+// 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/vsprintf.h"
+
+#include "src/__support/CPP/limits.h"
+#include "src/__support/arg_list.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/error_mapper.h"
+#include "src/stdio/printf_core/printf_main.h"
+#include "src/stdio/printf_core/writer.h"
+
+#include <stdarg.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, __vsprintf_modular,
+                   (char *__restrict buffer, const char *__restrict format,
+                    va_list vlist)) {
+  internal::ArgList args(vlist); // This holder class allows for easier copying
+                                 // and pointer semantics, as well as handling
+                                 // destruction automatically.
+
+  printf_core::DropOverflowBuffer wb(buffer,
+                                     cpp::numeric_limits<size_t>::max());
+  printf_core::Writer writer(wb);
+
+  auto ret_val = printf_core::printf_main_modular(&writer, format, args);
+  if (!ret_val.has_value()) {
+    libc_errno = printf_core::internal_error_to_errno(ret_val.error());
+    return -1;
+  }
+  wb.buff[wb.buff_cur] = '\0';
+
+  if (ret_val.value() > static_cast<size_t>(cpp::numeric_limits<int>::max())) {
+    libc_errno =
+        printf_core::internal_error_to_errno(-printf_core::OVERFLOW_ERROR);
+    return -1;
+  }
+  return static_cast<int>(ret_val.value());
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/utils/hdrgen/hdrgen/header.py

@@ -177,7 +177,7 @@ class HeaderFile:
                 for typ in self.all_types()
             }
             | {
-                PurePosixPath("llvm-libc-macros") / f"{attr}.h"
+                PurePosixPath("llvm-libc-macros") / f"{attr.split('(')[0]}.h"
                 for attr in self.all_attributes() - COMMON_ATTRIBUTES
             }
         )

libc/utils/hdrgen/tests/expected_output/test_header.h

@@ -13,6 +13,7 @@
 #include "llvm-libc-macros/float16-macros.h"
 
 #include "llvm-libc-macros/CONST_FUNC_A.h"
+#include "llvm-libc-macros/MACRO_ATTR.h"
 #include "llvm-libc-macros/test_more-macros.h"
 #include "llvm-libc-macros/test_small-macros.h"
 #include "llvm-libc-types/float128.h"
@@ -32,7 +33,7 @@ enum {
 
 __BEGIN_C_DECLS
 
-CONST_FUNC_A void func_a(void) __NOEXCEPT;
+CONST_FUNC_A MACRO_ATTR(A) void func_a(void) __NOEXCEPT;
 
 #ifdef LIBC_TYPES_HAS_FLOAT128
 float128 func_b(void) __NOEXCEPT;

libc/utils/hdrgen/tests/expected_output/test_small.json

@@ -5,6 +5,7 @@
     "includes": [
       "__llvm-libc-common.h",
       "llvm-libc-macros/CONST_FUNC_A.h",
+      "llvm-libc-macros/MACRO_ATTR.h",
       "llvm-libc-macros/test_more-macros.h",
       "llvm-libc-macros/test_small-macros.h",
       "llvm-libc-types/float128.h",

libc/utils/hdrgen/tests/expected_output/test_small_proxy.h

@@ -12,6 +12,7 @@
 #ifdef LIBC_FULL_BUILD
 
 #include "llvm-libc-macros/CONST_FUNC_A.h"
+#include "llvm-libc-macros/MACRO_ATTR.h"
 #include "llvm-libc-macros/test_more-macros.h"
 #include "llvm-libc-macros/test_small-macros.h"
 #include "llvm-libc-types/float128.h"

libc/utils/hdrgen/tests/input/merge1.yaml

@@ -17,3 +17,4 @@ functions:
       - stdc
     attributes:
       - CONST_FUNC_A
+      - MACRO_ATTR(A)