[libc] Move the definitions of the standard IO streams to the platform

[jhuber6]

Summary

This patch moves the definitions of the standard IO streams to the
platform file definition. This is necessary because previously we had a
level of indirection where the stream's FILE * was initialized based
on the pointer to the internal __llvm_libc version. This cannot be
resolved ahead of time by the linker because the address will not be
known until runtime. This caused the previous implementation to emit a
global constructor to initialize the pointer to the actual FILE *. By
moving these definitions so that we can bind their address to the
original file type we can avoid this global constructor.

This file keeps the entrypoints, but makes them empty files only
containing an external reference. This is so they still appear as
entrypoints and get emitted as declarations in the generated headers.

[jhuber6]

I don't know if keeping the entrypoints is necessary but I don't know how to make it emit the extern FILE *stderr definitions without them.

[jhuber6]

Fix extern C

[sivachandra]

Thanks for catching the problem and the patch to fix it.

libc/src/__support/File/gpu/file.cpp

 
 static GPUFile StdOut(0UL, File::ModeFlags(File::OpenMode::APPEND));
 File *stdout = &StdOut;
 
 static GPUFile StdErr(0UL, File::ModeFlags(File::OpenMode::APPEND));
 File *stderr = &StdErr;
 
 } // namespace __llvm_libc
+
+// Provide the external defintitions of the standard IO streams.
+extern "C" {
+FILE *stdin = reinterpret_cast<FILE *>(&__llvm_libc::StdIn);
+FILE *stderr = reinterpret_cast<FILE *>(&__llvm_libc::StdErr);
+FILE *stdout = reinterpret_cast<FILE *>(&__llvm_libc::StdOut);
+}

libc/src/__support/File/linux/file.cpp

 File *stdout = &StdOut;
 
 constexpr size_t STDERR_BUFFER_SIZE = 0;
 static LinuxFile StdErr(2, nullptr, STDERR_BUFFER_SIZE, _IONBF, false,
                         File::ModeFlags(File::OpenMode::APPEND));
 File *stderr = &StdErr;
 
 } // namespace __llvm_libc
+
+// Provide the external defintitions of the standard IO streams.
+extern "C" {
+FILE *stdin = reinterpret_cast<FILE *>(&__llvm_libc::StdIn);
+FILE *stderr = reinterpret_cast<FILE *>(&__llvm_libc::StdErr);
+FILE *stdout = reinterpret_cast<FILE *>(&__llvm_libc::StdOut);
+}

libc/src/stdio/stderr.cpp

 //===-- Definition of the global stderr object ----------------------------===//
 //
 // 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/File/file.h"
 
 #include <stdio.h>
 
-extern "C" {
-FILE *stderr = reinterpret_cast<FILE *>(__llvm_libc::stderr);
-}
+extern "C" FILE *stderr;

libc/src/stdio/stdin.cpp

 //===-- Definition of the global stdin object -----------------------------===//
 //
 // 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/File/file.h"
 
 #include <stdio.h>
 
-extern "C" {
-FILE *stdin = reinterpret_cast<FILE *>(__llvm_libc::stdin);
-}
+extern "C" FILE *stdin;

libc/src/stdio/stdout.cpp

 //===-- Definition of the global stdout object ----------------------------===//
 //
 // 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/File/file.h"
 
 #include <stdio.h>
 
-extern "C" {
-FILE *stdout = reinterpret_cast<FILE *>(__llvm_libc::stdout);
-}
+extern "C" FILE *stdout;