Re: [PATCH] Fix PR35607, another revenge of invariant addresses (ivopts)

[Zdenek Dvorak <rakdver at kam dot mff dot cuni dot cz>]

Hi,

> > Indeed.  Copied the list now.  And Diego who had much to say about
> > gimple grammar in the threads referenced above (read between the
> > two arrows ;).
> 
> This problem keeps surfacing again and again.  I have a small Ada testcase 
> that triggers it locally (with local Gigi changes) and there is no loop.

IMHO there are two possible ways to solve these problems:

1) something like the patch below (up to details of
   address_invariant_p, this is just a quick hack).
2) making TREE_INVARIANT behave equivalently to this patch after
   gimplification (or introducing a new tree flag with this altered
   semantics)

2) seems complicated and possibly causing further problems, so I would
propose to go with 1), unless there are compile-time performance
problems due to this change.

I tried to understand Diego's proposal
(http://gcc.gnu.org/ml/gcc/2007-07/msg00097.html), however it seems to
propose to keep relying on TREE_INVARIANT with its current semantics,
which seems wrong -- I think middle-end should not use TREE_INVARIANT at
all, as it is more or less redundant with SSA form,

Zdenek

Index: tree-gimple.c
===================================================================
*** tree-gimple.c	(revision 133282)
--- tree-gimple.c	(working copy)
*************** is_gimple_addressable (tree t)
*** 167,172 ****
--- 167,211 ----
  	  || INDIRECT_REF_P (t));
  }
  
+ /* Returns true if T is an object with invariant address.  */
+ 
+ static bool
+ address_invariant_p (const_tree t)
+ {
+   for (; handled_component_p (t); t = TREE_OPERAND (t, 0))
+     {
+       switch (TREE_CODE (t))
+ 	{
+ 	case ARRAY_REF:
+ 	  if (!is_gimple_min_invariant (TREE_OPERAND (t, 1)))
+ 	    return false;
+ 
+ 	  if (TREE_OPERAND (t, 2)
+ 	      && !is_gimple_min_invariant (TREE_OPERAND (t, 2)))
+ 	    return false;
+ 
+ 	  if (TREE_OPERAND (t, 3)
+ 	      && !is_gimple_min_invariant (TREE_OPERAND (t, 3)))
+ 	    return false;
+ 	  break;
+ 
+ 	case COMPONENT_REF:
+ 	  if (TREE_OPERAND (t, 2)
+ 	      && !is_gimple_min_invariant (TREE_OPERAND (t, 2)))
+ 	    return false;
+ 	  break;
+ 
+ 	default:
+ 	  break;
+ 	}
+     }
+ 
+   if (INDIRECT_REF_P (t))
+     return is_gimple_min_invariant (TREE_OPERAND (t, 0));
+ 
+   return CONSTANT_CLASS_P (t) || DECL_P (t);
+ }
+ 
  /* Return true if T is a GIMPLE minimal invariant.  It's a restricted
     form of function invariant.  */
  
*************** is_gimple_min_invariant (const_tree t)
*** 176,182 ****
    switch (TREE_CODE (t))
      {
      case ADDR_EXPR:
!       return TREE_INVARIANT (t);
  
      case INTEGER_CST:
      case REAL_CST:
--- 215,223 ----
    switch (TREE_CODE (t))
      {
      case ADDR_EXPR:
!       if (!TREE_INVARIANT (t))
! 	return false;
!       return address_invariant_p (TREE_OPERAND (t, 0));
  
      case INTEGER_CST:
      case REAL_CST: