Re: c/7871: ICE on legal code, global register variables problems

[Jim Wilson <wilson@tuliptree.org>]

The following reply was made to PR c/7871; it has been noted by GNATS.

From: Jim Wilson <wilson@tuliptree.org>
To: gcc-gnats@gcc.gnu.org,  gcc-bugs@gcc.gnu.org,  gcc-prs@gcc.gnu.org, 
 rz@linux-m68k.org,  wilson@gcc.gnu.org,  rth@redhat.com
Cc:  
Subject: Re: c/7871: ICE on legal code, global register variables problems
Date: Thu, 27 Feb 2003 22:23:53 -0500

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 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7871
 
 PR 7871 was broken by this patch from Richard Henderson
 	http://gcc.gnu.org/ml/gcc-patches/2001-07/msg01050.html
 
 This PR uses a global register, and a store to the global register 
 before a call gets deleted as a dead store.
 
 The problem with this patch is that it doesn't distinguish between 
 regsters which are definitely set by calls, and registers which might be 
 set by calls.  In both cases, we can't do optimizations like cse across 
 the call.  However, only in the former case can we delete stores before 
 the call as dead.  In the latter case, a store before the call is not 
 dead, because the register might not be set in the call.
 
 I think every part of the patch is right except for the flow.c change. 
 The flow.c change marks global regs as killed by calls which is not 
 correct.  They might be killed, but we can't assume that, because then 
 we might delete a store that isn't dead.
 
 I propose the following patch to fix the bug.  I will check it in after 
 running the tests, and add the testcase to the testsuite if there isn't 
 already an equivalent one.
 
 Jim
 
 
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 2003-02-27  James E Wilson  <wilson@tuliptree.org>
 
 	* flow.c (propagate_one_insn): Don't assume that calls set global
 	registers.
 
 Index: flow.c
 ===================================================================
 RCS file: /cvs/gcc/gcc/gcc/flow.c,v
 retrieving revision 1.548
 diff -p -r1.548 flow.c
 *** flow.c	31 Jan 2003 06:52:48 -0000	1.548
 --- flow.c	28 Feb 2003 03:14:51 -0000
 *************** propagate_one_insn (pbi, insn)
 *** 1798,1806 ****
   	      mark_set_1 (pbi, CLOBBER, XEXP (XEXP (note, 0), 0),
   			  cond, insn, pbi->flags);
   
 ! 	  /* Calls change all call-used and global registers.  */
   	  for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
 ! 	    if (TEST_HARD_REG_BIT (regs_invalidated_by_call, i))
   	      {
   		/* We do not want REG_UNUSED notes for these registers.  */
   		mark_set_1 (pbi, CLOBBER, regno_reg_rtx[i], cond, insn,
 --- 1798,1810 ----
   	      mark_set_1 (pbi, CLOBBER, XEXP (XEXP (note, 0), 0),
   			  cond, insn, pbi->flags);
   
 ! 	  /* Calls change all call-used registers.  Calls may or may not
 ! 	     change global registers.  Since this will cause previous stores
 ! 	     to be deleted as dead, we must assume that global registers are
 ! 	     not set in the call.  */
   	  for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
 ! 	    if (TEST_HARD_REG_BIT (regs_invalidated_by_call, i)
 ! 		&& ! global_regs[i])
   	      {
   		/* We do not want REG_UNUSED notes for these registers.  */
   		mark_set_1 (pbi, CLOBBER, regno_reg_rtx[i], cond, insn,
 
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