[Bug optimization/14279] New: delete_null_pointer_checks removes live insn

["amodra at bigpond dot net dot au" <gcc-bugzilla at gcc dot gnu dot org>]

This testcase, compiled with -O2 -S, results in assembly that tests CR4 but
nowhere sets CR4.  Note that I'm reporting the bug against 3.4, but 3.4 needs to
have delete_null_pointer_checks enabled..  See
http://gcc.gnu.org/ml/gcc-patches/2004-02/msg02199.html

void assi (void);

struct C
{
  C *pMemPool;
  bool isEmpty();
  void decrementNumBlocks (C *const pCSG)
  {
    if (this == 0)
      assi ();
    if (pCSG)
      assi ();
    if (pCSG)
      assi ();
  }
};

void sqlofmblkEx (C *pMemSeg)
{
  C *pMemPool = __null;
  pMemPool = pMemSeg->pMemPool;
  pMemPool->decrementNumBlocks (pMemSeg);
  pMemSeg->isEmpty ();
}


The reason is that after cse but before delete_null_pointer_checks, we have rtl
like the following:

(insn 40 75 41 2 0x401ae7c0 (set (reg:CC 123)
        (compare:CC (reg/v/f:DI 118 [ pMemSeg ])
            (const_int 0 [0x0]))) 376 {*cmpdi_internal1} (nil)
    (nil))

(jump_insn 41 40 76 2 0x401ae7c0 (set (pc)
        (if_then_else (eq (reg:CC 123)
                (const_int 0 [0x0]))
            (label_ref 48)
            (pc))) 434 {*rs6000.md:13489} (nil)
    (nil))

(note 76 41 47 3 [bb 3] NOTE_INSN_BASIC_BLOCK)

(call_insn 47 76 48 3 0x401ae7c0 (parallel [
            (call (mem:SI (symbol_ref:DI ("_Z4assiv")) [0 S4 A8])
                (const_int 64 [0x40]))
            (use (const_int 0 [0x0]))
            (clobber (scratch:SI))
        ]) -1 (nil)
    (nil)
    (nil))

(code_label 48 47 77 4 3 "" [1 uses])

(note 77 48 53 4 [bb 4] NOTE_INSN_BASIC_BLOCK)

(jump_insn 53 77 78 4 0x401ae7c0 (set (pc)
        (if_then_else (eq (reg:CC 123)
                (const_int 0 [0x0]))
            (label_ref 62)
            (pc))) 434 {*rs6000.md:13489} (nil)
    (nil))


Note how both jumps use reg:CC 123.  delete_null_pointer_checks incorrectly
deletes insn 40 when finding that insn 41 is redundant, and is not clever enough
to also delete insn 53.

-- 
           Summary: delete_null_pointer_checks removes live insn
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: optimization
        AssignedTo: amodra at bigpond dot net dot au
        ReportedBy: amodra at bigpond dot net dot au
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: powerpc64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14279