DWARF parsing and writing support using LLVM (#2520)

This imports LLVM code for DWARF handling. That code has the
Apache 2 license like us. It's also the same code used to
emit DWARF in the common toolchain, so it seems like a safe choice.

This adds two passes: --dwarfdump which runs the same code LLVM
runs for llvm-dwarfdump. This shows we can parse it ok, and will
be useful for debugging. And --dwarfupdate writes out the DWARF
sections (unchanged from what we read, so it just roundtrips - for
updating we need #2515).

This puts LLVM in thirdparty which is added here.

All the LLVM code is behind USE_LLVM_DWARF, which is on
by default, but off in JS for now, as it increases code size by 20%.

This current approach imports the LLVM files directly. This is not
how they are intended to be used, so it required a bunch of
local changes - more than I expected actually, for the platform-specific
stuff. For now this seems to work, so it may be good enough, but
in the long term we may want to switch to linking against libllvm.
A downside to doing that is that binaryen users would need to
have an LLVM build, and even in the waterfall builds we'd have a
problem - while we ship LLVM there anyhow, we constantly update
it, which means that binaryen would need to be on latest llvm all
the time too (which otherwise, given DWARF is quite stable, we
might not need to constantly update).

An even larger issue is that as I did this work I learned about how
DWARF works in LLVM, and while the reading code is easy to
reuse, the writing code is trickier. The main code path is heavily
integrated with the MC layer, which we don't have - we might want
to create a "fake MC layer" for that, but it sounds hard. Instead,
there is the YAML path which is used mostly for testing, and which
can convert DWARF to and from YAML and from binary. Using
the non-YAML parts there, we can convert binary DWARF to
the YAML layer's nice Info data, then convert that to binary. This
works, however, this is not the path LLVM uses normally, and it
supports only some basic DWARF sections - I had to add ranges
support, in fact. So if we need more complex things, we may end
up needing to use the MC layer approach, or consider some other
DWARF library. However, hopefully that should not affect the core
binaryen code which just calls a library for DWARF stuff.

Helps #2400

1 files changed, 88 insertions, 0 deletions

diff --git a/third_party/llvm-project/include/llvm/Support/FileOutputBuffer.h b/third_party/llvm-project/include/llvm/Support/FileOutputBuffer.h
new file mode 100644
index 000000000..bdc1425d4
--- /dev/null
+++ b/third_party/llvm-project/include/llvm/Support/FileOutputBuffer.h
@@ -0,0 +1,88 @@
+//=== FileOutputBuffer.h - File Output Buffer -------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Utility for creating a in-memory buffer that will be written to a file.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_FILEOUTPUTBUFFER_H
+#define LLVM_SUPPORT_FILEOUTPUTBUFFER_H
+
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/DataTypes.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/FileSystem.h"
+
+namespace llvm {
+/// FileOutputBuffer - This interface provides simple way to create an in-memory
+/// buffer which will be written to a file. During the lifetime of these
+/// objects, the content or existence of the specified file is undefined. That
+/// is, creating an OutputBuffer for a file may immediately remove the file.
+/// If the FileOutputBuffer is committed, the target file's content will become
+/// the buffer content at the time of the commit.  If the FileOutputBuffer is
+/// not committed, the file will be deleted in the FileOutputBuffer destructor.
+class FileOutputBuffer {
+public:
+  enum {
+    /// set the 'x' bit on the resulting file
+    F_executable = 1,
+
+    /// Don't use mmap and instead write an in-memory buffer to a file when this
+    /// buffer is closed.
+    F_no_mmap = 2,
+  };
+
+  /// Factory method to create an OutputBuffer object which manages a read/write
+  /// buffer of the specified size. When committed, the buffer will be written
+  /// to the file at the specified path.
+  ///
+  /// When F_modify is specified and \p FilePath refers to an existing on-disk
+  /// file \p Size may be set to -1, in which case the entire file is used.
+  /// Otherwise, the file shrinks or grows as necessary based on the value of
+  /// \p Size.  It is an error to specify F_modify and Size=-1 if \p FilePath
+  /// does not exist.
+  static Expected<std::unique_ptr<FileOutputBuffer>>
+  create(StringRef FilePath, size_t Size, unsigned Flags = 0);
+
+  /// Returns a pointer to the start of the buffer.
+  virtual uint8_t *getBufferStart() const = 0;
+
+  /// Returns a pointer to the end of the buffer.
+  virtual uint8_t *getBufferEnd() const = 0;
+
+  /// Returns size of the buffer.
+  virtual size_t getBufferSize() const = 0;
+
+  /// Returns path where file will show up if buffer is committed.
+  StringRef getPath() const { return FinalPath; }
+
+  /// Flushes the content of the buffer to its file and deallocates the
+  /// buffer.  If commit() is not called before this object's destructor
+  /// is called, the file is deleted in the destructor. The optional parameter
+  /// is used if it turns out you want the file size to be smaller than
+  /// initially requested.
+  virtual Error commit() = 0;
+
+  /// If this object was previously committed, the destructor just deletes
+  /// this object.  If this object was not committed, the destructor
+  /// deallocates the buffer and the target file is never written.
+  virtual ~FileOutputBuffer() {}
+
+  /// This removes the temporary file (unless it already was committed)
+  /// but keeps the memory mapping alive.
+  virtual void discard() {}
+
+protected:
+  FileOutputBuffer(StringRef Path) : FinalPath(Path) {}
+
+  std::string FinalPath;
+};
+} // end namespace llvm
+
+#endif