Re: Loop optimization bug with Ada front end on PPC (and probably Alpha)

[Corey Minyard <minyard at acm dot org>]

That patch looks a lot better than mine :-).  However, I'm getting a 
segv compiling unwind-dw2-fde.c.  I put a print in (I can't use gdb 5.0 
with the output of gcc 3.0 on Linux PowerPC), and it's dying in the line: 

  && INSN_IN_RANGE_P (JUMP_LABEL (insn), loop->top, loop->cont)

It looks like loop->top is alway NULL.  I'm working with a slightly old 
CVS, maybe I should update, or is this a real problem?

-Corey

Richard Henderson wrote:

>The problem is not limited to doloop -- the unroller ought
>to hit this problem as well.  I'm testing the following.
>
>
>
>r~
>
>
>
>Index: loop.c
>===================================================================
>RCS file: /cvs/gcc/gcc/gcc/loop.c,v
>retrieving revision 1.369
>diff -c -p -d -r1.369 loop.c
>*** loop.c	2001/11/15 23:44:56	1.369
>--- loop.c	2001/11/26 01:52:34
>*************** typedef struct loop_replace_args
>*** 282,293 ****
>    rtx insn;
>  } loop_replace_args;
>  
>- /* Nonzero iff INSN is between START and END, inclusive.  */
>- #define INSN_IN_RANGE_P(INSN, START, END)	\
>-   (INSN_UID (INSN) < max_uid_for_loop		\
>-    && INSN_LUID (INSN) >= INSN_LUID (START)	\
>-    && INSN_LUID (INSN) <= INSN_LUID (END))
>- 
>  /* Indirect_jump_in_function is computed once per function.  */
>  static int indirect_jump_in_function;
>  static int indirect_jump_in_function_p PARAMS ((rtx));
>--- 282,287 ----
>Index: loop.h
>===================================================================
>RCS file: /cvs/gcc/gcc/gcc/loop.h,v
>retrieving revision 1.56
>diff -c -p -d -r1.56 loop.h
>*** loop.h	2001/10/29 22:13:40	1.56
>--- loop.h	2001/11/26 01:52:34
>*************** Software Foundation, 59 Temple Place - S
>*** 50,55 ****
>--- 50,60 ----
>  #define REGNO_FIRST_LUID(REGNO) uid_luid[REGNO_FIRST_UID (REGNO)]
>  #define REGNO_LAST_LUID(REGNO) uid_luid[REGNO_LAST_UID (REGNO)]
>  
>+ /* Nonzero iff INSN is between START and END, inclusive.  */
>+ #define INSN_IN_RANGE_P(INSN, START, END)	\
>+   (INSN_UID (INSN) < max_uid_for_loop		\
>+    && INSN_LUID (INSN) >= INSN_LUID (START)	\
>+    && INSN_LUID (INSN) <= INSN_LUID (END))
>  
>  /* A "basic induction variable" or biv is a pseudo reg that is set
>     (within this loop) only by incrementing or decrementing it.  */
>Index: unroll.c
>===================================================================
>RCS file: /cvs/gcc/gcc/gcc/unroll.c,v
>retrieving revision 1.147
>diff -c -p -d -r1.147 unroll.c
>*** unroll.c	2001/11/21 00:50:56	1.147
>--- unroll.c	2001/11/26 01:52:34
>*************** loop_iterations (loop)
>*** 3480,3485 ****
>--- 3480,3486 ----
>    int unsigned_p, compare_dir, final_larger;
>    rtx last_loop_insn;
>    rtx reg_term;
>+   rtx insn;
>    struct iv_class *bl;
>  
>    loop_info->n_iterations = 0;
>*************** loop_iterations (loop)
>*** 3705,3710 ****
>--- 3706,3762 ----
>  
>    if (initial_value == 0)
>      return 0;
>+ 
>+   /* Some code transformations can result in code akin to
>+ 
>+ 	LOOP_BEG
>+ 	  goto start;
>+ 	top:
>+ 	  i++;
>+ 	start:
>+ 	  ...
>+ 	LOOP_CONT
>+ 	  if (i < n) goto top;
>+ 	LOOP_END
>+ 
>+      In this situation, we skip the increment the first time through
>+      the loop, which results in an incorrect estimate of the number
>+      of iterations.  As we did for GIVs above, adjust the initial value
>+      to compensate.  */
>+ 
>+   off_by_one = 0;
>+   for (insn = loop->start; insn != loop->top; insn = NEXT_INSN (insn))
>+     if (GET_CODE (insn) == JUMP_INSN)
>+       {
>+ 	if (any_uncondjump_p (insn)
>+ 	    && JUMP_LABEL (insn)
>+ 	    && INSN_IN_RANGE_P (JUMP_LABEL (insn), loop->top, loop->cont))
>+ 	  {
>+ 	    if (reg_set_between_p (bl->biv->src_reg, insn, JUMP_LABEL (insn)))
>+ 	      off_by_one = 1;
>+ 	    break;
>+ 	  }
>+ 	/* No idea what's going on.  */
>+ 	if (loop_dump_stream)
>+ 	  fprintf (loop_dump_stream,
>+ 		   "Loop iterations: Confused by jump before loop top.\n");
>+ 	return 0;
>+       }
>+ 
>+   if (off_by_one)
>+     {
>+       if (loop_dump_stream)
>+ 	fprintf (loop_dump_stream,
>+ 		 "Loop iterations: Basic induction var skips initial incr.\n");
>+       if (GET_CODE (increment) != CONST_INT)
>+ 	{
>+ 	  if (loop_dump_stream)
>+ 	    fprintf (loop_dump_stream,
>+ 		     "Loop iterations: Can't adjust with non-constant incr.\n");
>+ 	  return 0;
>+ 	}
>+       initial_value = plus_constant (initial_value, -INTVAL (increment));
>+     }
>  
>    unsigned_p = 0;
>    off_by_one = 0;
>