[Bug regression/20973] [4.0/4.1 Regression] kdelibs (khtml) miscompiled by reload

["matz at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From matz at suse dot de  2005-04-12 19:05 -------
The problem is in reload.c:find_dummy_reload.  It tries to use the input reg 
as reload register for an in-out reload and has certain conditions when it 
can't do so: 
  /* Consider using IN if OUT was not acceptable 
     or if OUT dies in this insn (like the quotient in a divmod insn). 
     We can't use IN unless it is dies in this insn, 
     which means we must know accurately which hard regs are live. 
     Also, the result can't go in IN if IN is used within OUT, 
     or if OUT is an earlyclobber and IN appears elsewhere in the insn.  */ 
  if (hard_regs_live_known 
      && REG_P (in) 
      && REGNO (in) < FIRST_PSEUDO_REGISTER 
      && (value == 0 
          || find_reg_note (this_insn, REG_UNUSED, real_out)) 
      && find_reg_note (this_insn, REG_DEAD, real_in) 
      && !fixed_regs[REGNO (in)] 
      && HARD_REGNO_MODE_OK (REGNO (in), 
 
But this doesn't check if IN is used uninitialized.  In that case it also 
can't be used.  It's not immediately clear to me how to check for this, 
as nowhere is it noted that this or that pseudo is actually uninitialized 
and only therefore got a register by global.c.  One could look at the  
global_live_at_start of the first block, if it mentions the original pseudo 
number of the IN operand. 
 
The problem of course being that at this point the hardregs already are 
substituted into the REG expressions.  So one would have to trust the 
original regnos noted there.  And it's not clear that this is the only 
place in reload which is confused by hardregs corresponding to uninitialized 
pseudos. 

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20973