Re: optimization/6713: Regression wrt 3.0.4: g++ -O2 leads to seg fault at run time

[Glen Nakamura <glen@imodulo.com>]

The following reply was made to PR optimization/6713; it has been noted by GNATS.

From: Glen Nakamura <glen@imodulo.com>
To: gcc-gnats@gcc.gnu.org, mark@ecriticalinc.com, pcarlini@unitus.it,
	gcc-bugs@gcc.gnu.org, nobody@gcc.gnu.org
Cc:  
Subject: Re: optimization/6713: Regression wrt 3.0.4: g++ -O2 leads to seg fault at run time
Date: Wed, 22 May 2002 16:31:39 -1000

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6713
 
 FYI: the problem also goes away when compiling w/ -O2 -fno-strength-reduce.
 I'm not sure how it's supposed to work, but strength_reduce() in loop.c
 eliminates reg 60 but leaves a REG_EQUAL note referring to it within the loop.
 I think this REG_EQUAL note is cause of the problem...  If I eliminate it,
 the problem goes away.  BTW, changing ++i to i++ in the for loop avoids the
 problem since the REG_EQUAL note is not emitted in that case.
 
 Any thoughts on how to fix this?  I'm not familiar enough with the loop
 optimizer to produce a proper patch.
 
 - Glen Nakamura
 
 
 [snip]
 giv at 482 reduced to (reg 135)
 giv at 480 reduced to (reg 135)
 biv 60 was eliminated.
 Reg 60: biv eliminated
 [snip]
 (note 351 591 492 NOTE_INSN_LOOP_BEG)
 [snip]
 (note 529 474 482 [bb 6] NOTE_INSN_BASIC_BLOCK)
 
 (insn 482 529 588 (set (reg/s/f:SI 111)
         (reg/f:SI 135)) -1 (nil)
     (expr_list:REG_EQUAL (plus:SI (reg/v:SI 60)   <<< PROBLEM NOTE???
             (const_int 1 [0x1]))
         (nil)))
 
 (insn 588 482 489 (set (reg/s/f:SI 111)
         (reg/f:SI 135)) -1 (nil)
     (nil))
 
 (note 489 588 490 0x40660980 NOTE_INSN_BLOCK_END)
 [snip]
 (note 497 408 499 NOTE_INSN_LOOP_END)
 [snip]