[C++ PATCH] Avoid code generation changes with -Wnonnull-compare vs. -Wno-nonnull-compare (PR c++/86569)

[Jakub Jelinek]

Hi!

A couple of -fcompare-debug=-Wsomething PRs have been filed recently, this
fixes one of them.  Changing code generation based on TREE_NO_WARNING flag
which usually gets set only in the warning code, or on warn_* option is
undesirable.  This patch prevents that kind of folding regardless of the
warning options, the GIMPLE optimizers can handle it fine (but it makes a
difference with -O0).

Bootstrapped/regtested on x86_64-linux (only, i686-linux bootstrap is
broken), ok for trunk?

2018-07-22  Jakub Jelinek  <ja...@redhat.com>

        PR c++/86569
        * cp-gimplify.c (cp_fold): Don't fold comparisons into other kind
        of expressions other than INTEGER_CST regardless of TREE_NO_WARNING
        or warn_nonnull_compare.

        * g++.dg/warn/Wnonnull-compare-9.C: New test.

--- gcc/cp/cp-gimplify.c.jj     2018-07-16 09:43:03.237023048 +0200
+++ gcc/cp/cp-gimplify.c        2018-07-19 11:44:24.881759294 +0200
@@ -2381,21 +2381,26 @@ cp_fold (tree x)
       else
        x = fold (x);
 
-      if (TREE_NO_WARNING (org_x)
-         && warn_nonnull_compare
-         && COMPARISON_CLASS_P (org_x))
+      /* This is only needed for -Wnonnull-compare and only if
+        TREE_NO_WARNING (org_x), but to avoid that option affecting code
+        generation, we do it always.  */
+      if (COMPARISON_CLASS_P (org_x))
        {
          if (x == error_mark_node || TREE_CODE (x) == INTEGER_CST)
            ;
          else if (COMPARISON_CLASS_P (x))
-           TREE_NO_WARNING (x) = 1;
+           {
+             if (TREE_NO_WARNING (org_x) && warn_nonnull_compare)
+               TREE_NO_WARNING (x) = 1;
+           }
          /* Otherwise give up on optimizing these, let GIMPLE folders
             optimize those later on.  */
          else if (op0 != TREE_OPERAND (org_x, 0)
                   || op1 != TREE_OPERAND (org_x, 1))
            {
              x = build2_loc (loc, code, TREE_TYPE (org_x), op0, op1);
-             TREE_NO_WARNING (x) = 1;
+             if (TREE_NO_WARNING (org_x) && warn_nonnull_compare)
+               TREE_NO_WARNING (x) = 1;
            }
          else
            x = org_x;
--- gcc/testsuite/g++.dg/warn/Wnonnull-compare-9.C.jj   2018-07-19 
11:57:41.909727597 +0200
+++ gcc/testsuite/g++.dg/warn/Wnonnull-compare-9.C      2018-07-19 
11:57:14.627696497 +0200
@@ -0,0 +1,11 @@
+// PR c++/86569
+// { dg-do compile }
+// { dg-options "-fcompare-debug=-Wnonnull-compare" }
+
+bool b;
+
+int
+main ()
+{
+  return ((!b) != 0);
+}

        Jakub