[LLVMdev] Bug in llvm/ADT/ArrayRef.h?

[Andreas Weber]

Hi,

I think I ran into a rather subtle bug inside llvm/ADT/ArrayRef.h which
only shows up when compiling the code with GCC-4.8 with switched-off
optimizations. (Both clang and GCC-4.7 don't trigger the bug.)

I already filed a bug against GCC-4.8 which was rejected by the GCC-folks
as being invalid, because the code (basically ArrayRef.h)  "is doing
something bad - it's retaining a pointer to a temporary object across a
function call." They also provided a detailed explanation for their
opinion, which I think is correct. See this link for the full story:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61015

The following program demonstrates the bug using LLVM's own ArrayRef:
#define __STDC_LIMIT_MACROS
#define __STDC_CONSTANT_MACROS

#include <cstdio>
#include <llvm/ADT/ArrayRef.h>

class Obj {};
class SpecialObj : public Obj {};

int main()
{
  SpecialObj* pSpecial = new SpecialObj();

  llvm::ArrayRef<Obj*> arrayRef( pSpecial ); //Breaks on GCC-4.8.

  /* Obj* pObj = pSpecial;
  llvm::ArrayRef<Obj*> arrayRef( pObj ); //Possible Workaround */

  int someStackArray[500];
  memset( someStackArray, 0xdd, sizeof(someStackArray) );

  if( arrayRef[0] != pSpecial )
    printf( "This shouldn't happen: %p\n", arrayRef[0] );
  else
    printf( "Expected behaviour.\n" );
  return 0;
}

Compiling (and then executing) this program with
  g++-4.8 -Wall -O0 -std=c++11 -I./LLVM_SVN/installed/include main.cpp
prints: "This shouldn't happen: 0xdddddddddddddddd"

Compiling with
  clang++ -Wall -O0 -std=c++11 -I./LLVM_SVN/installed/include main.cpp
prints: "Expected behaviour."

I think (as a quick fix) we should remove the const qualifier from
ArrayRef's CTor argument, so that the above code won't compile anymore and
thus avoiding a silent failure.
To be precise:
Change llvm/ADT/ArrayRef.h:57 from
 ArrayRef(const T &OneElt)
      : Data(&OneElt), Length(1) {}
to
 ArrayRef(T &OneElt)
      : Data(&OneElt), Length(1) {}

Best regards,
Andreas
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