tree-optimization/107618 - enhance copy propagation of constants

The following enhances copy propagation of constants to also see
through simple operations like conversions but also operations with
otherwise constant operands.  That's required to fulfill the promise

      /* Copy propagation also copy-propagates constants, this is necessary
         to forward object-size and builtin folding results properly.  */
      NEXT_PASS (pass_copy_prop);

and avoid false diagnostics as shown in the testcase.  We're
using gimple_fold_stmt_to_constant_1 with not following SSA edges
and accordingly adjust what stmts we simulate during SSA propagation.

	PR tree-optimization/107618
	* tree-ssa-copy.cc (stmt_may_generate_copy): Simulate all
	assignments with a single SSA use.
	(copy_prop_visit_assignment): Use gimple_fold_stmt_to_constant_1
	to perform simple constant folding.
	(copy_prop::visit_stmt): Visit all assignments.

	* gcc.dg/pr107618.c: New testcase.

2 files changed, 35 insertions, 24 deletions

diff --git a/gcc/testsuite/gcc.dg/pr107618.c b/gcc/testsuite/gcc.dg/pr107618.c
new file mode 100644
index 0000000..9e73cc1
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr107618.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-Og" } */
+
+void a(void) __attribute__((__warning__("")));
+int main(void)
+{
+  unsigned long b = __builtin_object_size(0, 0);
+  if (__builtin_expect(b < 1, 0))
+    a(); /* { dg-bogus "warning" } */
+}
diff --git a/gcc/tree-ssa-copy.cc b/gcc/tree-ssa-copy.cc
index 782ceb5..811161c 100644
--- a/gcc/tree-ssa-copy.cc
+++ b/gcc/tree-ssa-copy.cc
@@ -33,6 +33,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "cfgloop.h"
 #include "tree-scalar-evolution.h"
 #include "tree-ssa-loop-niter.h"
+#include "gimple-fold.h"
 
 
 /* This file implements the copy propagation pass and provides a
@@ -99,12 +100,16 @@ stmt_may_generate_copy (gimple *stmt)
   if (gimple_vuse (stmt))
     return false;
 
+  /* If the assignment is from a constant it generates a useful copy.  */
+  if (gimple_assign_single_p (stmt)
+      && is_gimple_min_invariant (gimple_assign_rhs1 (stmt)))
+    return true;
+
   /* Otherwise, the only statements that generate useful copies are
-     assignments whose RHS is just an SSA name that doesn't flow
-     through abnormal edges.  */
-  return ((gimple_assign_rhs_code (stmt) == SSA_NAME
-	   && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (gimple_assign_rhs1 (stmt)))
-	  || is_gimple_min_invariant (gimple_assign_rhs1 (stmt)));
+     assignments whose single SSA use doesn't flow through abnormal
+     edges.  */
+  tree rhs = single_ssa_tree_operand (stmt, SSA_OP_USE);
+  return (rhs && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rhs));
 }
 
 
@@ -197,26 +202,24 @@ dump_copy_of (FILE *file, tree var)
 static enum ssa_prop_result
 copy_prop_visit_assignment (gimple *stmt, tree *result_p)
 {
-  tree lhs, rhs;
-
-  lhs = gimple_assign_lhs (stmt);
-  rhs = valueize_val (gimple_assign_rhs1 (stmt));
-
-  if (TREE_CODE (lhs) == SSA_NAME)
+  tree lhs = gimple_assign_lhs (stmt);
+  tree rhs = gimple_fold_stmt_to_constant_1 (stmt, valueize_val);
+  if (rhs
+      && (TREE_CODE (rhs) == SSA_NAME
+	  || is_gimple_min_invariant (rhs)))
     {
-      /* Straight copy between two SSA names.  First, make sure that
+      /* Straight copy between two SSA names or a constant.  Make sure that
 	 we can propagate the RHS into uses of LHS.  */
       if (!may_propagate_copy (lhs, rhs))
-	return SSA_PROP_VARYING;
-
-      *result_p = lhs;
-      if (set_copy_of_val (*result_p, rhs))
-	return SSA_PROP_INTERESTING;
-      else
-	return SSA_PROP_NOT_INTERESTING;
+	rhs = lhs;
     }
+  else
+    rhs = lhs;
 
-  return SSA_PROP_VARYING;
+  *result_p = lhs;
+  if (set_copy_of_val (*result_p, rhs))
+    return SSA_PROP_INTERESTING;
+  return rhs != lhs ? SSA_PROP_NOT_INTERESTING : SSA_PROP_VARYING;
 }
 
 
@@ -282,10 +285,8 @@ copy_prop::visit_stmt (gimple *stmt, edge *taken_edge_p, tree *result_p)
       fprintf (dump_file, "\n");
     }
 
-  if (gimple_assign_single_p (stmt)
-      && TREE_CODE (gimple_assign_lhs (stmt)) == SSA_NAME
-      && (TREE_CODE (gimple_assign_rhs1 (stmt)) == SSA_NAME
-	  || is_gimple_min_invariant (gimple_assign_rhs1 (stmt))))
+  if (is_gimple_assign (stmt)
+      && TREE_CODE (gimple_assign_lhs (stmt)) == SSA_NAME)
     {
       /* If the statement is a copy assignment, evaluate its RHS to
 	 see if the lattice value of its output has changed.  */