[LLVMbugs] [Bug 3968] New: MachineSinking:: ProcessBlock crashes by moving iterator off the begining of ilist

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=3968

           Summary: MachineSinking::ProcessBlock crashes by moving iterator
                    off the begining of ilist
           Product: libraries
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Common Code Generator Code
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: stoklund at 2pi.dk
                CC: llvmbugs at cs.uiuc.edu


The attached bugpoint-generated file causes a crash in a debug build of llc:

$ bin/llvm-as < ~/machine-sinking-crash.ll | bin/llc
Stack dump:
0.      Program arguments: bin/llc 
1.      Running pass 'Machine code sinking' on function '@QRiterate'
Bus error

The crash happens in:

  ilist_iterator &operator--() {      // predecrement - Back up
    NodePtr = Traits::getPrev(NodePtr);
    assert(Traits::getNext(NodePtr) && "--'d off the beginning of an ilist!");
    return *this;
  }

MachineSinking::ProcessBlock calls operator-- on an iterator with a null prev
pointer. The assert then calls getNext with a null pointer, causing the bus
error.

There are two separate issues here:

1. The assertion should be: assert(NodePtr && "--'d off...
2. MachineSinking::ProcessBlock should not call operator-- on an invalid
iterator


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