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

[Jim Wilson]

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

From: Jim Wilson <wilson@tuliptree.org>
To: Richard Henderson <rth@redhat.com>
Cc: gcc-gnats@gcc.gnu.org, gcc-bugs@gcc.gnu.org, rz@linux-m68k.org,
   wilson@gcc.gnu.org
Subject: Re: c/7871: ICE on legal code, global register variables problems
Date: 10 Mar 2003 00:16:23 -0500

 Let me recap a bit.  The current code calls mark_set_1 (...CLOBBER...)
 and mark_used_regs.  This causes CALL_INSNs to get a REG_UNDEAD note for
 a global_reg, which confuses combine into deleting the insn that
 initializes the global_reg.
 
 If I change this to mark_set_1 (...SET...), then there are no
 REG_DEAD/REG_UNUSED notes which is correct.  However, there are
 LOG_LINKS from the insn that sets the global_reg to the CALL_INSN, and
 from the CALL_INSN to the insn that uses the global_reg.  This causes
 combine to merge the 3 instructions together, and the result is that the
 first insn that sets the global_reg gets simplified away.  This could
 perhaps be fixed by modifying combine to know that calls and global_regs
 are special.
 
 If I avoid calling mark_set_1, then there are no REG_DEAD/REG_UNUSED
 notes, but there is still a LOG_LINK from the first insn that sets the
 global_reg to the CALL_INSN.  When combine merges an unrelated insn into
 the CALL_INSN, it sees a LOG_LINK that doesn't appear to belong on the
 CALL_INSN, so it moves it to the next insn which uses the global_reg. 
 Combine then merges two instructions that set/use the global_reg, and
 the first global_reg set before the call disappears again.
 
 Thus it seems that in order to get the right behavior, we need to avoid
 adding any REG_NOTES or LOG_LINKS for global_regs.
 
 I had to go back to gcc-2.95.1 to find a compiler that worked for my two
 testcases, and it has this behavior.  The change that broke it is:
 
 2000-04-07  Richard Henderson  <rth@cygnus.com>
 
 	* flow.c (loop_depth): Remove.
 	(reg_next_use, cc0_live, mem_set_list): Replace with ...
 	...
 	(mark_set_reg): Break out of mark_set_1.
 	...
 I didn't manage to find the mail message for this patch.  This command
 	cvs diff -p -r 1.247 -r 1.248 flow.c
 will show you the patch if you want to look at it.
 
 This is a large change that was mostly reorganizing code, but another
 thing it does is delete one line of code in mark_set_1 that handles
 global_regs like the frame pointer and arg pointer register.  If I re-add
 this one line of code, then both testcases I have start working again.
 
 I am off on a trip, so it will be a while before I am able to continue
 working on this.
 
 
 2003-03-10  James E Wilson  <wilson@tuliptree.org>
 
 	* flow.c (mark_set_1): Handle global_regs like the frame pointer.
 
 Index: flow.c
 ===================================================================
 RCS file: /cvs/gcc/gcc/gcc/flow.c,v
 retrieving revision 1.549
 diff -p -r1.549 flow.c
 *** flow.c	28 Feb 2003 10:11:47 -0000	1.549
 --- flow.c	10 Mar 2003 05:05:08 -0000
 *************** mark_set_1 (pbi, code, reg, cond, insn, 
 *** 2709,2714 ****
 --- 2709,2715 ----
   #if FRAME_POINTER_REGNUM != ARG_POINTER_REGNUM
         && ! (regno_first == ARG_POINTER_REGNUM && fixed_regs[regno_first])
   #endif
 +       && ! (regno_first < FIRST_PSEUDO_REGISTER && global_regs[regno_first])
         )
       {
         int some_was_live = 0, some_was_dead = 0;