[llvm-objcopy] Initial support for wasm in llvm-objcopy

Currently only supports simple copying, other operations to follow.

Reviewers: sbc100, alexshap, jhenderson

Differential Revision: https://reviews.llvm.org/D70930

This is a reland of a928d127a with a one-line fix to ensure that
the wasm version number is written as little-endian (it's the only
field in all of the binary format that's not a single byte or an
LEB, but we may have to watch out more when we start handling the
linking section).
diff --git a/llvm/tools/llvm-objcopy/wasm/WasmObjcopy.cpp b/llvm/tools/llvm-objcopy/wasm/WasmObjcopy.cpp
new file mode 100644
index 0000000..41816a0
--- /dev/null
+++ b/llvm/tools/llvm-objcopy/wasm/WasmObjcopy.cpp
@@ -0,0 +1,65 @@
+//===- WasmObjcopy.cpp ----------------------------------------------------===//
+//
+// 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 "WasmObjcopy.h"
+#include "Buffer.h"
+#include "CopyConfig.h"
+#include "Object.h"
+#include "Reader.h"
+#include "Writer.h"
+#include "llvm-objcopy.h"
+#include "llvm/Support/Errc.h"
+
+namespace llvm {
+namespace objcopy {
+namespace wasm {
+
+using namespace object;
+
+static Error handleArgs(const CopyConfig &Config, Object &Obj) {
+  if (!Config.AddGnuDebugLink.empty() || !Config.BuildIdLinkDir.empty() ||
+      Config.BuildIdLinkInput || Config.BuildIdLinkOutput ||
+      Config.ExtractPartition || !Config.SplitDWO.empty() ||
+      !Config.SymbolsPrefix.empty() || !Config.AllocSectionsPrefix.empty() ||
+      Config.DiscardMode != DiscardType::None || Config.NewSymbolVisibility ||
+      !Config.SymbolsToAdd.empty() || !Config.RPathToAdd.empty() ||
+      !Config.OnlySection.empty() || !Config.SymbolsToGlobalize.empty() ||
+      !Config.SymbolsToKeep.empty() || !Config.SymbolsToLocalize.empty() ||
+      !Config.SymbolsToRemove.empty() ||
+      !Config.UnneededSymbolsToRemove.empty() ||
+      !Config.SymbolsToWeaken.empty() || !Config.SymbolsToKeepGlobal.empty() ||
+      !Config.SectionsToRename.empty() || !Config.SetSectionAlignment.empty() ||
+      !Config.SetSectionFlags.empty() || !Config.SymbolsToRename.empty() ||
+      !Config.ToRemove.empty() || !Config.DumpSection.empty() ||
+      !Config.AddSection.empty()) {
+    return createStringError(
+        llvm::errc::invalid_argument,
+        "no flags are supported yet, only basic copying is allowed");
+  }
+  return Error::success();
+}
+
+Error executeObjcopyOnBinary(const CopyConfig &Config,
+                             object::WasmObjectFile &In, Buffer &Out) {
+  Reader TheReader(In);
+  Expected<std::unique_ptr<Object>> ObjOrErr = TheReader.create();
+  if (!ObjOrErr)
+    return createFileError(Config.InputFilename, ObjOrErr.takeError());
+  Object *Obj = ObjOrErr->get();
+  assert(Obj && "Unable to deserialize Wasm object");
+  if (Error E = handleArgs(Config, *Obj))
+    return createFileError(Config.InputFilename, std::move(E));
+  Writer TheWriter(*Obj, Out);
+  if (Error E = TheWriter.write())
+    return createFileError(Config.OutputFilename, std::move(E));
+  return Error::success();
+}
+
+} // end namespace wasm
+} // end namespace objcopy
+} // end namespace llvm