[PATCH] widening_mul: Fix a > ~b to .ADD_OVERFLOW optimization [PR98256]

[Jakub Jelinek]

Hi!

Unfortunately, my latest tree-ssa-math-opts.c patch broke the following
testcase.  The problem is that the code is adding .ADD_OVERFLOW or
.SUB_OVERFLOW before or after the stmt on which the function has been
called, which is normally a addition or subtraction that has all the
operands.
But in the a > ~b optimization that stmt is the ~b stmt and the other
comparison operand might be defined only after that ~b stmt, so we can't
insert the .ADD_OVERFLOW next to ~b that we want to delete, but need to
insert it before the a > temp comparison that uses it; and in that case
when removing the BIT_NOT_EXPR stmt we need to ensure the caller doesn't do
gsi_next because gsi_remove already points the iterator to the next stmt.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2020-12-13  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98256
	* tree-ssa-math-opts.c (match_uaddsub_overflow): For BIT_NOT_EXPR,
	only handle a single use, and insert .ADD_OVERFLOW before the
	comparison rather than after the BIT_NOT_EXPR.  Return true iff
	it is BIT_NOT_EXPR and it has been removed.
	(math_opts_dom_walker::after_dom_children) <case BIT_NOT_EXPR>:
	If match_uaddsub_overflow returned true, continue instead of break.

	* gcc.c-torture/compile/pr98256.c: New test.

--- gcc/tree-ssa-math-opts.c.jj	2020-12-12 14:48:40.195306781 +0100
+++ gcc/tree-ssa-math-opts.c	2020-12-12 22:42:24.474396562 +0100
@@ -3598,6 +3598,7 @@ match_uaddsub_overflow (gimple_stmt_iter
   gimple *use_stmt;
   gimple *add_stmt = NULL;
   bool add_first = false;
+  gimple *cond_stmt = NULL;
 
   gcc_checking_assert (code == PLUS_EXPR
 		       || code == MINUS_EXPR
@@ -3628,8 +3629,9 @@ match_uaddsub_overflow (gimple_stmt_iter
 		return false;
 	      if (rhs2 == NULL)
 		rhs2 = other;
-	      else if (rhs2 != other)
+	      else
 		return false;
+	      cond_stmt = use_stmt;
 	    }
 	  ovf_use_seen = true;
 	}
@@ -3818,6 +3820,9 @@ match_uaddsub_overflow (gimple_stmt_iter
 	}
     }
 
+  if (code == BIT_NOT_EXPR)
+    *gsi = gsi_for_stmt (cond_stmt);
+
   tree ctype = build_complex_type (type);
   gcall *g = gimple_build_call_internal (code != MINUS_EXPR
 					 ? IFN_ADD_OVERFLOW : IFN_SUB_OVERFLOW,
@@ -3843,7 +3848,10 @@ match_uaddsub_overflow (gimple_stmt_iter
   tree ovf = make_ssa_name (type);
   g2 = gimple_build_assign (ovf, IMAGPART_EXPR,
 			    build1 (IMAGPART_EXPR, type, ctmp));
-  gsi_insert_after (gsi, g2, GSI_NEW_STMT);
+  if (code != BIT_NOT_EXPR)
+    gsi_insert_after (gsi, g2, GSI_NEW_STMT);
+  else
+    gsi_insert_before (gsi, g2, GSI_SAME_STMT);
 
   FOR_EACH_IMM_USE_STMT (use_stmt, iter, lhs)
     {
@@ -3908,11 +3916,12 @@ match_uaddsub_overflow (gimple_stmt_iter
     }
   else if (code == BIT_NOT_EXPR)
     {
-      gimple_stmt_iterator gsi2 = gsi_for_stmt (stmt);
-      gsi_remove (&gsi2, true);
+      *gsi = gsi_for_stmt (stmt);
+      gsi_remove (gsi, true);
       release_ssa_name (lhs);
+      return true;
     }
-  return true;
+  return false;
 }
 
 /* Return true if target has support for divmod.  */
@@ -4238,7 +4247,8 @@ math_opts_dom_walker::after_dom_children
 	      break;
 
 	    case BIT_NOT_EXPR:
-	      match_uaddsub_overflow (&gsi, stmt, code);
+	      if (match_uaddsub_overflow (&gsi, stmt, code))
+		continue;
 	      break;
 
 	    case TRUNC_MOD_EXPR:
--- gcc/testsuite/gcc.c-torture/compile/pr98256.c.jj	2020-12-12 22:44:34.037940761 +0100
+++ gcc/testsuite/gcc.c-torture/compile/pr98256.c	2020-12-12 22:35:54.452778912 +0100
@@ -0,0 +1,9 @@
+/* PR tree-optimization/98256 */
+
+unsigned a, b;
+
+int
+foo (void)
+{
+  return !!(~a / b);
+}

	Jakub