[PATCH 3/4 v2] Enhance SCEV to follow copies of SSA_NAMEs.

[Alan Lawrence <alan dot lawrence at arm dot com>]

(Previous message: https://gcc.gnu.org/ml/gcc-patches/2015-12/msg02159.html)
On Sat, Dec 26, 2015 at 18:58 PM, Richard Biener <richard.guenther@gmail.com> wrote:

>> I'm not sure whether adding a pass_copy_prop is the right thing here, but since
>> loop-header-copying can create such opportunities, it seems good to take them!
>
> Aww.  I'd rather have general infrastructure (like SCEV) deal with
> those non-propagated copies.
>
> There are likely other missed propagation / folding opportunities
> caused from partial peeling.
>
> Richard.

I take your second point, but I am concerned that this leads to duplication of
copy-propagation code throughout the compiler?

However, this patch does that. Making analyze_initial_condition (alone) follow
copies, is enough to solve my case of missed vectorization of pr65947-2.c;
but I also used the routine in analyze_scalar_evolution_1, as it found copies
to follow in both bootstrap and a significant number of testcases:

c-c++-common/gomp/pr58472.c
gcc.c-torture/execute/20000422-1.c
gcc.c-torture/execute/20041213-2.c
gcc.c-torture/execute/20100430-1.c
gcc.c-torture/execute/pr49712.c
gcc.c-torture/execute/pr58640.c
gcc.dg/Warray-bounds-13.c
gcc.dg/graphite/block-{1,5,6}.c
gcc.dg/graphite/interchange-12.c
gcc.dg/graphite/interchange-mvt.c
gcc.dg/graphite/pr19910.c
gcc.dg/graphite/uns-block-1.c
gcc.dg/loop-unswitch-{2,4}.c
gcc.dg/pr59670.c
gcc.dg/torture/matrix-{1,2,3,4,5,6}.c
gcc.dg/torture/pr24750-1.c
gcc.dg/torture/transpose-{1,2,3,4,5,6}.c

...some of which were resolved to constants.

(Of course, this approach is not as thorough as adding a pass_copy_prop. E.g. it
does *not* enable vectorization of the inlined copy in pr65947-9.c.)

Bootstrapped + check-gcc + check-g++ on ARM, AArch64, x86_64.

gcc/ChangeLog:

	* tree-scalar-evolution.c (follow_copies): New.
	(analyze_initial_condition, analyze_scalar_evolution_1): Call previous.
---
 gcc/tree-scalar-evolution.c | 53 ++++++++++++++++++++++++++++++---------------
 1 file changed, 36 insertions(+), 17 deletions(-)

diff --git a/gcc/tree-scalar-evolution.c b/gcc/tree-scalar-evolution.c
index 9b33693..357eb8f 100644
--- a/gcc/tree-scalar-evolution.c
+++ b/gcc/tree-scalar-evolution.c
@@ -1522,6 +1522,37 @@ analyze_evolution_in_loop (gphi *loop_phi_node,
   return evolution_function;
 }
 
+/* While VAR is an SSA_NAME whose definition is a straight copy of another name,
+   or (perhaps via a degenerate phi) a constant, follows that definition.
+   Returns the constant, or the earliest SSA_NAME whose definition is neither
+   constant nor copy.  */
+
+static tree
+follow_copies (tree var)
+{
+  while (TREE_CODE (var) == SSA_NAME)
+    {
+      gimple *def = SSA_NAME_DEF_STMT (var);
+      if (gphi *phi = dyn_cast <gphi *> (def))
+	{
+	  tree rhs = degenerate_phi_result (phi);
+	  /* Don't follow degenerate phi's of SSA_NAMES, that can break
+	     loop-closed SSA form.  */
+	  if (rhs && is_gimple_min_invariant (rhs))
+	    var = rhs;
+	  break;
+	}
+      else if (gimple_assign_single_p (def)
+	       && !gimple_vuse (def)
+	       && (gimple_assign_rhs_code (def) == SSA_NAME
+		   || is_gimple_min_invariant (gimple_assign_rhs1 (def))))
+	var = gimple_assign_rhs1 (def);
+      else
+	break;
+    }
+  return var;
+}
+
 /* Given a loop-phi-node, return the initial conditions of the
    variable on entry of the loop.  When the CCP has propagated
    constants into the loop-phi-node, the initial condition is
@@ -1574,21 +1605,9 @@ analyze_initial_condition (gphi *loop_phi_node)
   if (init_cond == chrec_not_analyzed_yet)
     init_cond = chrec_dont_know;
 
-  /* During early loop unrolling we do not have fully constant propagated IL.
-     Handle degenerate PHIs here to not miss important unrollings.  */
-  if (TREE_CODE (init_cond) == SSA_NAME)
-    {
-      gimple *def = SSA_NAME_DEF_STMT (init_cond);
-      if (gphi *phi = dyn_cast <gphi *> (def))
-	{
-	  tree res = degenerate_phi_result (phi);
-	  if (res != NULL_TREE
-	      /* Only allow invariants here, otherwise we may break
-		 loop-closed SSA form.  */
-	      && is_gimple_min_invariant (res))
-	    init_cond = res;
-	}
-    }
+  /* We may not have fully constant propagated IL.  Handle degenerate PHIs here
+     to not miss important early loop unrollings.  */
+  init_cond = follow_copies (init_cond);
 
   if (dump_file && (dump_flags & TDF_SCEV))
     {
@@ -1968,8 +1987,8 @@ analyze_scalar_evolution_1 (struct loop *loop, tree var, tree res)
   if (bb == NULL
       || !flow_bb_inside_loop_p (loop, bb))
     {
-      /* Keep the symbolic form.  */
-      res = var;
+      /* Keep symbolic form, but look through obvious copies for constants.  */
+      res = follow_copies (var);
       goto set_and_end;
     }
 
-- 
1.9.1