[LLVMbugs] [Bug 23007] New: With LTO, dead stripped ObjC protocols can result in linker error

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23007

            Bug ID: 23007
           Summary: With LTO, dead stripped ObjC protocols can result in
                    linker error
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: peter at pcc.me.uk
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

On Darwin:

$ cat foo.mm
#import <Foundation/Foundation.h>

@protocol CrAppProtocol
- (bool)isHandlingSendEvent;
@end

bool foo(NSObject *obj) {
  return [obj conformsToProtocol:@protocol(CrAppProtocol)];
}
$ cat main.c
int main() {}
$ export DYLD_LIBRARY_PATH=$HOME/src/llvm-build-rel/lib 
$ $HOME/src/llvm-build-rel/bin/clang -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk
-c -flto foo.mm main.c
$ ar cru libfoo.a foo.o
$ $HOME/src/llvm-build-rel/bin/clang -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk
-flto -Wl,-ObjC -Wl,-dead_strip main.o libfoo.a
ld: warning: '__Z3fooP8NSObject' is implemented in bitcode, but it was loaded
too late
Undefined symbols for architecture x86_64:
  "_objc_msgSend", referenced from:
      import-atom in libfoo.a(foo.o)
  "l_OBJC_PROTOCOL_REFERENCE_$_CrAppProtocol", referenced from:
      import-atom in libfoo.a(foo.o)
  "l_OBJC_LABEL_PROTOCOL_$_CrAppProtocol", referenced from:
      import-atom in libfoo.a(foo.o)
  "l_OBJC_PROTOCOL_$_CrAppProtocol", referenced from:
      l_OBJC_LABEL_PROTOCOL_$_CrAppProtocol in lto.o
      l_OBJC_PROTOCOL_REFERENCE_$_CrAppProtocol in lto.o
      import-atom in libfoo.a(foo.o)
      l_OBJC_LABEL_PROTOCOL_$_CrAppProtocol in libfoo.a(foo.o)
      l_OBJC_PROTOCOL_REFERENCE_$_CrAppProtocol in libfoo.a(foo.o)
ld: symbol(s) not found for architecture x86_64
clang-3.7: error: linker command failed with exit code 1 (use -v to see
invocation)

Reverting 209227 and 207979 in LLVM appears to fix this problem.

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