Fix reading objects created with -fembed-bitcode-marker.

[comex]

Summary

Currently, this fails with many tools, e.g.

$ clang -fembed-bitcode-marker -c -o test.o test.c
$ nm test.o
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/nm: test.o The file was not recognized as a valid object file

-fembed-bitcode-marker creates a LLVM,bitcode section consisting of a single byte. When reading the object file, IRObjectFile::findBitcodeInObject succeeds, causing SymbolicFile::createSymbolicFile to try to read the "bitcode" rather than using the outer Mach-O data - when then fails.

Fix this by making findBitcodeInObject return an error if the section size <= 1.

[steven_wu]

Thanks for fixing this. LGTM.

[steven_wu]

Additional comment, I didn't realize llvm-nm now prioritize to print bitcode symbol table, rather than the symbol table from the object file. There should be an option to choose which one to see and maybe also an option to see both.

@pcc, how is the symbol table designed for embedded bitcode for LTO? Does the native object side has a symbol table?

[jyknight]

In D44373#1036189, @steven_wu wrote:

Thanks for fixing this. LGTM.

Looks like this never actually got committed?

[steven_wu]

In D44373#1436308, @jyknight wrote:

In D44373#1036189, @steven_wu wrote:

Thanks for fixing this. LGTM.

Looks like this never actually got committed?

Thanks for catching this. Looks like @comex doesn't have commit right. I will do that.

llvm/trunk/lib/Object/IRObjectFile.cpp

 
 Expected<MemoryBufferRef>
 IRObjectFile::findBitcodeInObject(const ObjectFile &Obj) {
   for (const SectionRef &Sec : Obj.sections()) {
     if (Sec.isBitcode()) {
       StringRef SecContents;
       if (std::error_code EC = Sec.getContents(SecContents))
         return errorCodeToError(EC);
+      if (SecContents.size() <= 1)
+        return errorCodeToError(object_error::bitcode_section_not_found);
       return MemoryBufferRef(SecContents, Obj.getFileName());
     }
   }
 
   return errorCodeToError(object_error::bitcode_section_not_found);
 }
 
 Expected<MemoryBufferRef>

llvm/trunk/test/Object/nm-bitcode.test

+# Inputs generated with:
+# echo 'int hello() { return 5; }' > test.c
+# clang -O -fembed-bitcode -c -o macho-bitcode-x86_64.o test.c
+# clang -O -fembed-bitcode-marker -c -o macho-bitcode-marker-x86_64.o test.c
+
+RUN: llvm-nm -a %p/Inputs/macho-bitcode-x86_64.o \
+RUN:         | FileCheck %s
+
+RUN: llvm-nm -a %p/Inputs/macho-bitcode-marker-x86_64.o \
+RUN:         | FileCheck %s
+
+CHECK: T _hello