[libc] add noexcept to external function headers

[michaelrj]

Summary

To improve code generation for C++ code that directly includes our
headers, the external function definitions will now be marked noexcept.
This may not be necessary for the internal definitions since we build
with the -fno-exceptions flag.

[sivachandra]

libc/include/__llvm-libc-common.h

35

Nit: The other names like _Noreturn are actually C keywords. In this case, though the identifier names prefixed with _<Capital letter> are allowed for standard library use, we should probably obscure this a little more into __NOEXCEPT similar to __BEGIN_C_DECLS.

[michaelrj]

change the noexcept macro name to avoid collisions

[michaelrj]

rebase before land

libc/config/linux/api.td

let Defn = [{
 
     #ifdef NDEBUG
     #define assert(e) (void)0
     #else
 
     #ifdef __cplusplus
     extern "C"
     #endif
-    _Noreturn void __assert_fail(const char *, const char *, unsigned, const char *);
+    _Noreturn void __assert_fail(const char *, const char *, unsigned, const char *) __NOEXCEPT;
 
     #define assert(e)  \
       ((e) ? (void)0 : __assert_fail(#e, __FILE__, __LINE__, __PRETTY_FUNCTION__))
 
     #endif
   }];
 }
 

libc/include/__llvm-libc-common.h

 #define _Static_assert static_assert
 
 #undef _Alignof
 #define _Alignof alignof
 
 #undef _Thread_local
 #define _Thread_local thread_local
 
+#undef __NOEXCEPT
+#define __NOEXCEPT noexcept
+
 #else // not __cplusplus
 
 #undef __BEGIN_C_DECLS
 #define __BEGIN_C_DECLS
 
 #undef __END_C_DECLS
 #define __END_C_DECLS
 
 #undef __restrict
 #define __restrict restrict // C99 and above support the restrict keyword.
 
+#undef __NOEXCEPT
+#define __NOEXCEPT
+
 #endif // __cplusplus
 
 #endif // LLVM_LIBC___COMMON_H

libc/test/src/time/gmtime_test.cpp

 //===-- Unittests for gmtime ----------------------------------------------===//
 //
 // 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/time/gmtime.h"
 #include "src/time/time_utils.h"
 #include "test/ErrnoSetterMatcher.h"
 #include "test/src/time/TmMatcher.h"
 #include "utils/UnitTest/Test.h"
 
 #include <errno.h>
 #include <limits.h>
-#include <string.h>
 
 using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
 using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
 using __llvm_libc::time_utils::TimeConstants;
 
 TEST(LlvmLibcGmTime, OutOfRange) {
   time_t seconds =
       1 + INT_MAX * static_cast<int64_t>(

libc/test/src/time/mktime_test.cpp

 #include "src/time/time_utils.h"
 #include "test/ErrnoSetterMatcher.h"
 #include "test/src/time/TmHelper.h"
 #include "test/src/time/TmMatcher.h"
 #include "utils/UnitTest/Test.h"
 
 #include <errno.h>
 #include <limits.h>
-#include <string.h>
 
 using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
 using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
 using __llvm_libc::time_utils::TimeConstants;
 
 static inline time_t call_mktime(struct tm *tm_data, int year, int month,
                                  int mday, int hour, int min, int sec, int wday,
                                  int yday) {

libc/utils/HdrGen/PrototypeTestGen/PrototypeTestGen.cpp

for (const auto &entrypoint : EntrypointNamesOption) {
     OS << "  static_assert(__llvm_libc::cpp::is_same_v<" << returnType << '(';
     auto args = functionSpec->getValueAsListOfDefs("Args");
     for (size_t i = 0, size = args.size(); i < size; ++i) {
       llvm::Record *argType = args[i]->getValueAsDef("ArgType");
       OS << G.getTypeAsString(argType);
       if (i < size - 1)
         OS << ", ";
     }
-    OS << "), decltype(" << entrypoint << ")>, ";
+    OS << ") __NOEXCEPT, decltype(" << entrypoint << ")>, ";
     OS << '"' << entrypoint
        << " prototype in TableGen does not match public header" << '"';
     OS << ");\n";
   }
 
   OS << '\n';
   OS << "  return 0;\n";
   OS << "}\n\n";
 
   // We provide dummy malloc and free implementations to support the case
   // when LLVM libc does to include them.
-  OS << "void *malloc(size_t) { return nullptr; }\n";
-  OS << "void *realloc(void *, size_t) { return nullptr; }\n";
-  OS << "void free(void *) {}\n";
+  OS << "void *malloc(size_t) __NOEXCEPT { return nullptr; }\n";
+  OS << "void *realloc(void *, size_t) __NOEXCEPT { return nullptr; }\n";
+  OS << "void free(void *) __NOEXCEPT {}\n";
 
   return false;
 }
 
 int main(int argc, char *argv[]) {
   llvm::cl::ParseCommandLineOptions(argc, argv);
   return TableGenMain(argv[0], TestGeneratorMain);
 }

libc/utils/HdrGen/PublicAPICommand.cpp

for (auto &Name : EntrypointNameList) {
     auto ArgsList = FunctionSpec->getValueAsListOfDefs("Args");
     for (size_t i = 0; i < ArgsList.size(); ++i) {
       llvm::Record *ArgType = ArgsList[i]->getValueAsDef("ArgType");
       OS << G.getTypeAsString(ArgType);
       if (i < ArgsList.size() - 1)
         OS << ", ";
     }
 
-    OS << ");\n\n";
+    OS << ") __NOEXCEPT;\n\n";
   }
 
   // Make another pass over entrypoints to emit object declarations.
   for (const auto &Name : EntrypointNameList) {
     if (G.ObjectSpecMap.find(Name) == G.ObjectSpecMap.end())
       continue;
     llvm::Record *ObjectSpec = G.ObjectSpecMap[Name];
     auto Type = ObjectSpec->getValueAsString("Type");