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

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

On Thu, Jan 14, 2016 at 12:30 PM, Richard Biener <richard.guenther@gmail.com> wrote:

>
> The vuse test is not necessary
>
>> +              && (gimple_assign_rhs_code (def) == SSA_NAME
>> +                  || is_gimple_min_invariant (gimple_assign_rhs1 (def))))
>
> and the is_gimple_min_invariant (rhs1) test is not sufficient if you
> consider - (-INT_MIN) with -ftrapv for example.

Thanks, I didn't realize gimple_min_invariant would allow such cases.

>> +      /* Keep symbolic form, but look through obvious copies for constants.  */
>
> You're also looing for SSA names copied so the comment is sligntly wrong.

So it occurs to me that we do only need to look for constants: picking one SSA
name instead of another (when both are outside the loop) doesn't really help
SCEV; and it seems safer/less change to avoid fiddling around with which names
are used. Moreover, given we are in LC _SSA_, I think we can look through
degenerate phi's even of SSA_NAMES, without violating loop-closed-ness,
so long as we only use cases where we eventually reach a constant.

Therefore, how about this version?

(Bootstrapped + check-gcc + check-g++ on x86_64, AArch64, ARM; again, fixing
vectorization of pr65947-2.c).

If this is OK - that leaves only the first patch to SRA,
https://gcc.gnu.org/ml/gcc-patches/2015-12/msg02117.html (the "extra" fixes
I claimed in the guality testsuite were a testing mistake, but this
version still fixes the regressions in guality pr54970.c from the original
"ok with the comment added and a testcase (or two)"
https://gcc.gnu.org/ml/gcc-patches/2015-11/msg01948.html, as well as the
comment + 2*testcase). I think I'd still like to propose these as a stage 3/4
fix (with --param sra-max-scalarization-size-Ospeed=32) of PR/63679, a
regression against gcc 4.9.

Thanks, Alan

gcc/ChangeLog:

	* tree-scalar-evolution.c (follow_copies_to_constant): New.
	(analyze_initial_condition, analyze_scalar_evolution_1): Call previous.
---
 gcc/tree-scalar-evolution.c | 50 ++++++++++++++++++++++++++++++---------------
 1 file changed, 33 insertions(+), 17 deletions(-)

diff --git a/gcc/tree-scalar-evolution.c b/gcc/tree-scalar-evolution.c
index 9b33693..470c3ea 100644
--- a/gcc/tree-scalar-evolution.c
+++ b/gcc/tree-scalar-evolution.c
@@ -1522,6 +1522,34 @@ analyze_evolution_in_loop (gphi *loop_phi_node,
   return evolution_function;
 }
 
+/* Looks to see if VAR is a copy of a constant (via straightforward assignments
+   or degenerate phi's).  If so, returns the constant; else, returns VAR.  */
+
+static tree
+follow_copies_to_constant (tree var)
+{
+  tree res = var;
+  while (TREE_CODE (res) == SSA_NAME)
+    {
+      gimple *def = SSA_NAME_DEF_STMT (res);
+      if (gphi *phi = dyn_cast <gphi *> (def))
+	{
+	  if (tree rhs = degenerate_phi_result (phi))
+	    res = rhs;
+	  else
+	    break;
+	}
+      else if (gimple_assign_single_p (def))
+	/* Will exit loop if not an SSA_NAME.  */
+	res = gimple_assign_rhs1 (def);
+      else
+	break;
+    }
+  if (CONSTANT_CLASS_P (res))
+    return res;
+  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 +1602,9 @@ 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_to_constant (init_cond);
 
   if (dump_file && (dump_flags & TDF_SCEV))
     {
@@ -1968,8 +1984,8 @@ 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_to_constant (var);
       goto set_and_end;
     }
 
-- 
1.9.1