[PATCH] Improve write-only heuristics for SFT generation

[Richard Guenther]

This patch improves the heuristics for not generating SFTs for structures
that are written to only.  Originally these heuristics had been introduced
to fix a gfortran compile performance regression.  While retaining this
fix, the following patch improves the heuristic, so that for C++ with
inlining (like tramp3d), where uses of the original variables are scarce
and accesses are through chains of tmp = &foo; *tmp = ... temporaries.

The simple solution is to treat tmp = &foo; as a use of foo that makes
it (possibly) read/write.  I verified that this re-gains runtime 
performance for tramp3d due to SFTing of iterators with arrays and that
the compile-times for Polyhedron are unchanged.

Bootstrapped and regtested on x86_64-unknown-linux-gnu.

Ok for mainline?

Thanks,
Richard.


2006-03-02  Richard Guenther  <rguenther@suse.de>

	* tree-ssa-alias.c (find_used_portions): Consider taking
	the address as making the variable not write-only.

Index: tree-ssa-alias.c
===================================================================
*** tree-ssa-alias.c	(revision 111631)
--- tree-ssa-alias.c	(working copy)
*************** find_used_portions (tree *tp, int *walk_
*** 3071,3076 ****
--- 3071,3078 ----
  	    up->minused = 0;
  	    up->maxused = TREE_INT_CST_LOW (DECL_SIZE (var));
  	    up->implicit_uses = true;
+ 	    if (!lhs_p)
+ 	      up->write_only = false;
  
  	    up_insert (uid, up);
  	    *walk_subtrees = 0;