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, 80 insertions, 0 deletions

diff --git a/third_party/llvm-project/include/llvm/DebugInfo/DWARF/DWARFDebugPubTable.h b/third_party/llvm-project/include/llvm/DebugInfo/DWARF/DWARFDebugPubTable.h
new file mode 100644
index 000000000..ae57306b9
--- /dev/null
+++ b/third_party/llvm-project/include/llvm/DebugInfo/DWARF/DWARFDebugPubTable.h
@@ -0,0 +1,80 @@
+//===- DWARFDebugPubTable.h -------------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_DEBUGINFO_DWARF_DWARFDEBUGPUBTABLE_H
+#define LLVM_DEBUGINFO_DWARF_DWARFDEBUGPUBTABLE_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/BinaryFormat/Dwarf.h"
+#include "llvm/DebugInfo/DWARF/DWARFObject.h"
+#include <cstdint>
+#include <vector>
+
+namespace llvm {
+
+class raw_ostream;
+
+/// Represents structure for holding and parsing .debug_pub* tables.
+class DWARFDebugPubTable {
+public:
+  struct Entry {
+    /// Section offset from the beginning of the compilation unit.
+    uint64_t SecOffset;
+
+    /// An entry of the various gnu_pub* debug sections.
+    dwarf::PubIndexEntryDescriptor Descriptor;
+
+    /// The name of the object as given by the DW_AT_name attribute of the
+    /// referenced DIE.
+    StringRef Name;
+  };
+
+  /// Each table consists of sets of variable length entries. Each set describes
+  /// the names of global objects and functions, or global types, respectively,
+  /// whose definitions are represented by debugging information entries owned
+  /// by a single compilation unit.
+  struct Set {
+    /// The total length of the entries for that set, not including the length
+    /// field itself.
+    uint32_t Length;
+
+    /// This number is specific to the name lookup table and is independent of
+    /// the DWARF version number.
+    uint16_t Version;
+
+    /// The offset from the beginning of the .debug_info section of the
+    /// compilation unit header referenced by the set.
+    uint64_t Offset;
+
+    /// The size in bytes of the contents of the .debug_info section generated
+    /// to represent that compilation unit.
+    uint32_t Size;
+
+    std::vector<Entry> Entries;
+  };
+
+private:
+  std::vector<Set> Sets;
+
+  /// gnu styled tables contains additional information.
+  /// This flag determines whether or not section we parse is debug_gnu* table.
+  bool GnuStyle;
+
+public:
+  DWARFDebugPubTable(const DWARFObject &Obj, const DWARFSection &Sec,
+                     bool LittleEndian, bool GnuStyle);
+
+  void dump(raw_ostream &OS) const;
+
+  ArrayRef<Set> getData() { return Sets; }
+};
+
+} // end namespace llvm
+
+#endif // LLVM_DEBUGINFO_DWARF_DWARFDEBUGPUBTABLE_H