[PATCH] c++: Quash bogus -Wunused-value with new [PR107797]

[PATCH] c++: Quash bogus -Wunused-value with new [PR107797]

[Marek Polacek]

We shouldn't emit "right operand of comma operator has no effect"
when that comma operator was created by the compiler for "new int{}".
convert_to_void/COMPOUND_EXPR already checks warning_suppressed_p so
we can just suppress -Wunused-value.

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

	PR c++/107797

gcc/cp/ChangeLog:

	* cvt.cc (ocp_convert): copy_warning when creating a new
	COMPOUND_EXPR.
	* init.cc (build_new_1): Suppress -Wunused-value on
	compiler-generated COMPOUND_EXPRs.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wunused-value-1.C: New test.
---
 gcc/cp/cvt.cc                               |  6 ++++--
 gcc/cp/init.cc                              |  2 ++
 gcc/testsuite/g++.dg/warn/Wunused-value-1.C | 12 ++++++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wunused-value-1.C

diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index 0cbfd8060cb..17827d06a4a 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -711,8 +711,10 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
 	return error_mark_node;
       if (e == TREE_OPERAND (expr, 1))
 	return expr;
-      return build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
-			 TREE_OPERAND (expr, 0), e);
+      e = build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
+		      TREE_OPERAND (expr, 0), e);
+      copy_warning (e, expr);
+      return e;
     }
 
   complete_type (type);
diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index f816c474cef..52e96fbe590 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -3800,6 +3800,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
   if (cookie_expr)
     rval = build2 (COMPOUND_EXPR, TREE_TYPE (rval), cookie_expr, rval);
 
+  suppress_warning (rval, OPT_Wunused_value);
+
   if (rval == data_addr && TREE_CODE (alloc_expr) == TARGET_EXPR)
     /* If we don't have an initializer or a cookie, strip the TARGET_EXPR
        and return the call (which doesn't need to be adjusted).  */
diff --git a/gcc/testsuite/g++.dg/warn/Wunused-value-1.C b/gcc/testsuite/g++.dg/warn/Wunused-value-1.C
new file mode 100644
index 00000000000..2ba5587fce0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wunused-value-1.C
@@ -0,0 +1,12 @@
+// PR c++/107797
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wunused" }
+
+void
+g ()
+{
+  (long) new int{};
+  long(new int{});
+  (long) new int();
+  long(new int());
+}

base-commit: 86caab6c5d1e26e1c54c3dceacc873d6e27bfc09
-- 
2.39.0

Re: [PATCH] c++: Quash bogus -Wunused-value with new [PR107797]

[Jason Merrill]

On 1/19/23 21:03, Marek Polacek wrote:
> We shouldn't emit "right operand of comma operator has no effect"
> when that comma operator was created by the compiler for "new int{}".
> convert_to_void/COMPOUND_EXPR already checks warning_suppressed_p so
> we can just suppress -Wunused-value.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/107797
> 
> gcc/cp/ChangeLog:
> 
> 	* cvt.cc (ocp_convert): copy_warning when creating a new
> 	COMPOUND_EXPR.
> 	* init.cc (build_new_1): Suppress -Wunused-value on
> 	compiler-generated COMPOUND_EXPRs.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/warn/Wunused-value-1.C: New test.
> ---
>   gcc/cp/cvt.cc                               |  6 ++++--
>   gcc/cp/init.cc                              |  2 ++
>   gcc/testsuite/g++.dg/warn/Wunused-value-1.C | 12 ++++++++++++
>   3 files changed, 18 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wunused-value-1.C
> 
> diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> index f816c474cef..52e96fbe590 100644
> --- a/gcc/cp/init.cc
> +++ b/gcc/cp/init.cc
> @@ -3800,6 +3800,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
>     if (cookie_expr)
>       rval = build2 (COMPOUND_EXPR, TREE_TYPE (rval), cookie_expr, rval);
>   
> +  suppress_warning (rval, OPT_Wunused_value);

This makes sense, but IIUC since rval is built with no location, this 
just sets nowarning_flag?

>     if (rval == data_addr && TREE_CODE (alloc_expr) == TARGET_EXPR)
>       /* If we don't have an initializer or a cookie, strip the TARGET_EXPR
>          and return the call (which doesn't need to be adjusted).  */
> diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
> index 0cbfd8060cb..17827d06a4a 100644
> --- a/gcc/cp/cvt.cc
> +++ b/gcc/cp/cvt.cc
> @@ -711,8 +711,10 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
>  	return error_mark_node;
>        if (e == TREE_OPERAND (expr, 1))
>  	return expr;
> -      return build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
> -			 TREE_OPERAND (expr, 0), e);
> +      e = build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
> +		      TREE_OPERAND (expr, 0), e);
> +      copy_warning (e, expr);

And so I don't know what effect this would have; copy_warning doesn't 
seem to propagate nowarning_flag, which seems like a bug.

> +      return e;
>      }
>  
>    complete_type (type);

> diff --git a/gcc/testsuite/g++.dg/warn/Wunused-value-1.C b/gcc/testsuite/g++.dg/warn/Wunused-value-1.C
> new file mode 100644
> index 00000000000..2ba5587fce0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wunused-value-1.C
> @@ -0,0 +1,12 @@
> +// PR c++/107797
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wunused" }
> +
> +void
> +g ()
> +{
> +  (long) new int{};
> +  long(new int{});
> +  (long) new int();
> +  long(new int());
> +}
> 
> base-commit: 86caab6c5d1e26e1c54c3dceacc873d6e27bfc09

Re: [PATCH] c++: Quash bogus -Wunused-value with new [PR107797]

[Marek Polacek]

On Fri, Jan 20, 2023 at 02:37:19PM -0500, Jason Merrill wrote:
> On 1/19/23 21:03, Marek Polacek wrote:
> > We shouldn't emit "right operand of comma operator has no effect"
> > when that comma operator was created by the compiler for "new int{}".
> > convert_to_void/COMPOUND_EXPR already checks warning_suppressed_p so
> > we can just suppress -Wunused-value.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > 	PR c++/107797
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* cvt.cc (ocp_convert): copy_warning when creating a new
> > 	COMPOUND_EXPR.
> > 	* init.cc (build_new_1): Suppress -Wunused-value on
> > 	compiler-generated COMPOUND_EXPRs.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/warn/Wunused-value-1.C: New test.
> > ---
> >   gcc/cp/cvt.cc                               |  6 ++++--
> >   gcc/cp/init.cc                              |  2 ++
> >   gcc/testsuite/g++.dg/warn/Wunused-value-1.C | 12 ++++++++++++
> >   3 files changed, 18 insertions(+), 2 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/warn/Wunused-value-1.C
> > 
> > diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> > index f816c474cef..52e96fbe590 100644
> > --- a/gcc/cp/init.cc
> > +++ b/gcc/cp/init.cc
> > @@ -3800,6 +3800,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
> >     if (cookie_expr)
> >       rval = build2 (COMPOUND_EXPR, TREE_TYPE (rval), cookie_expr, rval);
> > +  suppress_warning (rval, OPT_Wunused_value);
> 
> This makes sense, but IIUC since rval is built with no location, this just
> sets nowarning_flag?

Correct (I didn't realize this when writing the patch).  There's another
suppress_warning on a location-less expr just a few lines above.
 
> >     if (rval == data_addr && TREE_CODE (alloc_expr) == TARGET_EXPR)
> >       /* If we don't have an initializer or a cookie, strip the TARGET_EXPR
> >          and return the call (which doesn't need to be adjusted).  */
> > diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
> > index 0cbfd8060cb..17827d06a4a 100644
> > --- a/gcc/cp/cvt.cc
> > +++ b/gcc/cp/cvt.cc
> > @@ -711,8 +711,10 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
> >  	return error_mark_node;
> >        if (e == TREE_OPERAND (expr, 1))
> >  	return expr;
> > -      return build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
> > -			 TREE_OPERAND (expr, 0), e);
> > +      e = build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
> > +		      TREE_OPERAND (expr, 0), e);
> > +      copy_warning (e, expr);
> 
> And so I don't know what effect this would have; copy_warning doesn't seem
> to propagate nowarning_flag, which seems like a bug.

It actually does propagate nowarning_flag: in copy_warning we have

219   /* The no-warning bit might be set even if the map has not been consulted, or
220      otherwise if there's no entry in the map.  */
221   set_no_warning_bit (to, supp);

so 'e' ends up with TREE_NO_WARNING set.

I would not think we want to build 'rval' in build_new_1 with a location
or use TREE_NO_WARNING directly instead of suppress_warning so I'm leaving
the patch as-is, but please let me know if you disagree.

Marek

Re: [PATCH] c++: Quash bogus -Wunused-value with new [PR107797]

[Jason Merrill]

On 1/23/23 16:28, Marek Polacek wrote:
> On Fri, Jan 20, 2023 at 02:37:19PM -0500, Jason Merrill wrote:
>> On 1/19/23 21:03, Marek Polacek wrote:
>>> We shouldn't emit "right operand of comma operator has no effect"
>>> when that comma operator was created by the compiler for "new int{}".
>>> convert_to_void/COMPOUND_EXPR already checks warning_suppressed_p so
>>> we can just suppress -Wunused-value.
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> 	PR c++/107797
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* cvt.cc (ocp_convert): copy_warning when creating a new
>>> 	COMPOUND_EXPR.
>>> 	* init.cc (build_new_1): Suppress -Wunused-value on
>>> 	compiler-generated COMPOUND_EXPRs.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/warn/Wunused-value-1.C: New test.
>>> ---
>>>    gcc/cp/cvt.cc                               |  6 ++++--
>>>    gcc/cp/init.cc                              |  2 ++
>>>    gcc/testsuite/g++.dg/warn/Wunused-value-1.C | 12 ++++++++++++
>>>    3 files changed, 18 insertions(+), 2 deletions(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/warn/Wunused-value-1.C
>>>
>>> diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
>>> index f816c474cef..52e96fbe590 100644
>>> --- a/gcc/cp/init.cc
>>> +++ b/gcc/cp/init.cc
>>> @@ -3800,6 +3800,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
>>>      if (cookie_expr)
>>>        rval = build2 (COMPOUND_EXPR, TREE_TYPE (rval), cookie_expr, rval);
>>> +  suppress_warning (rval, OPT_Wunused_value);
>>
>> This makes sense, but IIUC since rval is built with no location, this just
>> sets nowarning_flag?
> 
> Correct (I didn't realize this when writing the patch).  There's another
> suppress_warning on a location-less expr just a few lines above.
>   
>>>      if (rval == data_addr && TREE_CODE (alloc_expr) == TARGET_EXPR)
>>>        /* If we don't have an initializer or a cookie, strip the TARGET_EXPR
>>>           and return the call (which doesn't need to be adjusted).  */
>>> diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
>>> index 0cbfd8060cb..17827d06a4a 100644
>>> --- a/gcc/cp/cvt.cc
>>> +++ b/gcc/cp/cvt.cc
>>> @@ -711,8 +711,10 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
>>>   	return error_mark_node;
>>>         if (e == TREE_OPERAND (expr, 1))
>>>   	return expr;
>>> -      return build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
>>> -			 TREE_OPERAND (expr, 0), e);
>>> +      e = build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
>>> +		      TREE_OPERAND (expr, 0), e);
>>> +      copy_warning (e, expr);
>>
>> And so I don't know what effect this would have; copy_warning doesn't seem
>> to propagate nowarning_flag, which seems like a bug.
> 
> It actually does propagate nowarning_flag: in copy_warning we have
> 
> 219   /* The no-warning bit might be set even if the map has not been consulted, or
> 220      otherwise if there's no entry in the map.  */
> 221   set_no_warning_bit (to, supp);
> 
> so 'e' ends up with TREE_NO_WARNING set.

Ah, yes.  Pesky overloaded functions.

> I would not think we want to build 'rval' in build_new_1 with a location
> or use TREE_NO_WARNING directly instead of suppress_warning so I'm leaving
> the patch as-is, but please let me know if you disagree.

Agreed, the patch is OK.

Jason