[PATCH v2] c++: Fix bogus -Wparentheses warning [PR95344]

[Marek Polacek]

On Thu, May 28, 2020 at 05:01:51PM -0400, Jason Merrill wrote:
> On 5/26/20 8:25 PM, Marek Polacek wrote:
> > Since r267272, which added location wrappers, cp_fold loses
> > TREE_NO_WARNING on a MODIFY_EXPR that finish_parenthesized_expr set, and
> > that results in a bogus -Wparentheses warning.
> > 
> > I.e., previously we had "b = 1" but now we have "VIEW_CONVERT_EXPR<bool>(b) = 1"
> > and cp_fold_maybe_rvalue folds away the location wrapper and so we do
> > 2718             x = fold_build2_loc (loc, code, TREE_TYPE (x), op0, op1);
> > in cp_fold and the flag is lost.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/10/9?
> > 
> > 	PR c++/95344
> > 	* cp-gimplify.c (cp_fold) <case MODIFY_EXPR>: Set TREE_NO_WARNING.
> > 
> > 	* c-c++-common/Wparentheses-2.c: New test.
> > ---
> >   gcc/cp/cp-gimplify.c                        |  5 ++++-
> >   gcc/testsuite/c-c++-common/Wparentheses-2.c | 18 ++++++++++++++++++
> >   2 files changed, 22 insertions(+), 1 deletion(-)
> >   create mode 100644 gcc/testsuite/c-c++-common/Wparentheses-2.c
> > 
> > diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
> > index 53d715dcd89..8b505dd878c 100644
> > --- a/gcc/cp/cp-gimplify.c
> > +++ b/gcc/cp/cp-gimplify.c
> > @@ -2745,7 +2745,10 @@ cp_fold (tree x)
> >   	    x = org_x;
> >   	}
> >         if (code == MODIFY_EXPR && TREE_CODE (x) == MODIFY_EXPR)
> > -	TREE_THIS_VOLATILE (x) = TREE_THIS_VOLATILE (org_x);
> > +	{
> > +	  TREE_THIS_VOLATILE (x) = TREE_THIS_VOLATILE (org_x);
> > +	  TREE_NO_WARNING (x) = TREE_NO_WARNING (org_x);
> > +	}
> 
> I wonder if we want to copy these flags lower down for any EXPR_P (x) where
> TREE_CODE (x) == code?

Sounds good; I don't think we want to lose those flags when folding in general,
not just for MODIFY_EXPR.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
Since r267272, which added location wrappers, cp_fold loses
TREE_NO_WARNING on a MODIFY_EXPR that finish_parenthesized_expr set, and
that results in a bogus -Wparentheses warning.

I.e., previously we had "b = 1" but now we have "VIEW_CONVERT_EXPR<bool>(b) = 1"
and cp_fold_maybe_rvalue folds away the location wrapper and so we do
2718             x = fold_build2_loc (loc, code, TREE_TYPE (x), op0, op1);
in cp_fold and the flag is lost.

	PR c++/95344
	* cp-gimplify.c (cp_fold) <case MODIFY_EXPR>: Don't set
	TREE_THIS_VOLATILE here.
	(cp_fold): Set it here along with TREE_NO_WARNING.

	* c-c++-common/Wparentheses-2.c: New test.
---
 gcc/cp/cp-gimplify.c                        |  8 ++++++--
 gcc/testsuite/c-c++-common/Wparentheses-2.c | 18 ++++++++++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/c-c++-common/Wparentheses-2.c

diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 53d715dcd89..d6723e44ec4 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -2744,8 +2744,6 @@ cp_fold (tree x)
 	  else
 	    x = org_x;
 	}
-      if (code == MODIFY_EXPR && TREE_CODE (x) == MODIFY_EXPR)
-	TREE_THIS_VOLATILE (x) = TREE_THIS_VOLATILE (org_x);
 
       break;
 
@@ -2994,6 +2992,12 @@ cp_fold (tree x)
       return org_x;
     }
 
+  if (EXPR_P (x) && TREE_CODE (x) == code)
+    {
+      TREE_THIS_VOLATILE (x) = TREE_THIS_VOLATILE (org_x);
+      TREE_NO_WARNING (x) = TREE_NO_WARNING (org_x);
+    }
+
   if (!c.evaluation_restricted_p ())
     {
       fold_cache->put (org_x, x);
diff --git a/gcc/testsuite/c-c++-common/Wparentheses-2.c b/gcc/testsuite/c-c++-common/Wparentheses-2.c
new file mode 100644
index 00000000000..1aa5d314ae7
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wparentheses-2.c
@@ -0,0 +1,18 @@
+// PR c++/95344 - bogus -Wparentheses warning.
+// { dg-do compile }
+// { dg-options "-Wparentheses" }
+
+#ifndef __cplusplus
+# define bool _Bool
+# define true 1
+# define false 0
+#endif
+
+void
+f (int i)
+{
+  bool b = false;
+  if (i == 99 ? (b = true) : false) // { dg-bogus "suggest parentheses" }
+    {
+    }
+}

base-commit: 3d8d5ddb539a5254c7ef83414377f4c74c7701d4
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA