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

[Jason Merrill <jason@redhat.com>]

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