[LTO] An option to disable automatic bitcode upgrade

[hoy]

Summary

Bitcode upgrade automatically takes in place when the current compiler version doesn't match the one building the input modules. The upgrade is very expansive for large applications and unnecessary when users are sure the two compilers do not have a version issue, even if their versions mismatch literally. An optional to disable the automatic bitcode upgrade will be handy in such case.

[wenlei]

llvm/lib/Object/IRSymtab.cpp

436

Upgrade for this case is probably always needed to recreate symbol table. I think that bypassing upgrade from benign version mismatch is all we need. Correspondingly, we could rename the switch as diisable-bitcode-version-upgrade. WDYT?

[hoy]

llvm/lib/Object/IRSymtab.cpp

436

Good point. Symbol table mismatch isn't what we are targeting.

[hoy]

Updating D109869: [LTO] An option to disable automatic bitcode upgrade

[wenlei]

lgtm, thanks.

[hoy]

Rebasing.

llvm/lib/Object/IRSymtab.cpp

 //===- IRSymtab.cpp - implementation of IR symbol tables ------------------===//
 //
 // 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 "llvm/Object/IRSymtab.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Triple.h"
+#include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/IR/Comdat.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/GlobalAlias.h"
 #include "llvm/IR/GlobalObject.h"
 #include "llvm/IR/Mangler.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
-#include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/MC/StringTableBuilder.h"
 #include "llvm/Object/IRObjectFile.h"
 #include "llvm/Object/ModuleSymbolTable.h"
 #include "llvm/Object/SymbolicFile.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/StringSaver.h"
 #include "llvm/Support/VCSRevision.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <string>
 #include <utility>
 #include <vector>
 
 using namespace llvm;
 using namespace irsymtab;
 
+cl::opt<bool> DisableBitcodeVersionUpgrade(
+    "disable-bitcode-version-upgrade", cl::init(false), cl::Hidden,
+    cl::desc("Disable automatic bitcode upgrade for version mismatch"));
+
 static const char *PreservedSymbols[] = {
 #define HANDLE_LIBCALL(code, name) name,
 #include "llvm/IR/RuntimeLibcalls.def"
 #undef HANDLE_LIBCALL
     // There are global variables, so put it here instead of in
     // RuntimeLibcalls.def.
     // TODO: Are there similar such variables?
     "__ssp_canary_word",
static Expected<FileContents> upgrade(ArrayRef<BitcodeModule> BMs) {
   return std::move(FC);
 }
 
 Expected<FileContents> irsymtab::readBitcode(const BitcodeFileContents &BFC) {
   if (BFC.Mods.empty())
     return make_error<StringError>("Bitcode file does not contain any modules",
                                    inconvertibleErrorCode());
 
+  if (!DisableBitcodeVersionUpgrade) {
     if (BFC.StrtabForSymtab.empty() ||
         BFC.Symtab.size() < sizeof(storage::Header))
       return upgrade(BFC.Mods);
 
-  // We cannot use the regular reader to read the version and producer, because
-  // it will expect the header to be in the current format. The only thing we
-  // can rely on is that the version and producer will be present as the first
-  // struct elements.
+    // We cannot use the regular reader to read the version and producer,
+    // because it will expect the header to be in the current format. The only
+    // thing we can rely on is that the version and producer will be present as
+    // the first struct elements.
     auto *Hdr = reinterpret_cast<const storage::Header *>(BFC.Symtab.data());
     unsigned Version = Hdr->Version;
     StringRef Producer = Hdr->Producer.get(BFC.StrtabForSymtab);
     if (Version != storage::Header::kCurrentVersion ||
         Producer != kExpectedProducerName)
       return upgrade(BFC.Mods);
+  }
 
   FileContents FC;
   FC.TheReader = {{BFC.Symtab.data(), BFC.Symtab.size()},
                   {BFC.StrtabForSymtab.data(), BFC.StrtabForSymtab.size()}};
 
   // Finally, make sure that the number of modules in the symbol table matches
   // the number of modules in the bitcode file. If they differ, it may mean that
   // the bitcode file was created by binary concatenation, so we need to create
   // a new symbol table from scratch.
   if (FC.TheReader.getNumModules() != BFC.Mods.size())
     return upgrade(std::move(BFC.Mods));
 
   return std::move(FC);
 }