[LTO] An option to disable automatic bitcode upgrade

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.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D109869

llvm/lib/Object/IRSymtab.cpp

@@ -14,6 +14,7 @@
 #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"
@@ -22,13 +23,13 @@
 #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"
@@ -41,6 +42,10 @@
 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"
@@ -402,20 +407,22 @@ Expected<FileContents> irsymtab::readBitcode(const BitcodeFileContents &BFC) {
     return make_error<StringError>("Bitcode file does not contain any modules",
                                    inconvertibleErrorCode());
 
-  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.
-  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);
+  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.
+    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()},