[C/C++ PATCH] PR c++/66572. Fix Wlogical-op false positive

[C/C++ PATCH] PR c++/66572. Fix Wlogical-op false positive

[Mikhail Maltsev]

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Hi.
In current implementation we avoid giving "logical and/or of equal
expressions" warning for literal constant operands. The attached patch
fixes the check to make it treat const-qualified VAR_DECLs with constant
initializers the same way.

Bootstrapped and regtested on x86_64-linux. OK for trunk?

-- 
Regards,
    Mikhail Maltsev


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gcc/c-family/ChangeLog:

2015-06-19  Mikhail Maltsev  <maltsevm@gmail.com>

	PR c++/66572
	* c-common.c (warn_logical_operator): Treat constant-initialized
	VAR_DECLs like literal constants.

gcc/testsuite/ChangeLog:

2015-06-19  Mikhail Maltsev  <maltsevm@gmail.com>

	PR c++/66572
	* c-c++-common/Wlogical-op-2.c: New test.
	* g++.dg/warn/Wlogical-op-2.C: New test.



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diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index dc2bf00..38c7be9 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -1766,9 +1766,12 @@ warn_logical_operator (location_t location, enum tree_code code, tree type,
       return;
     }
 
-  /* We do not warn for constants because they are typical of macro
-     expansions that test for features.  */
-  if (CONSTANT_CLASS_P (op_left) || CONSTANT_CLASS_P (op_right))
+  /* We do not warn for literal constants because they are typical of macro
+     expansions that test for features.  Likewise, we do not warn for
+     const-qualified and constexpr variables which are initialized by constant
+     expressions, because they can come from e.g. <type_traits> or similar user
+     code.  */
+  if (TREE_CONSTANT (op_left) || TREE_CONSTANT (op_right))
     return;
 
   /* This warning only makes sense with logical operands.  */
diff --git a/gcc/testsuite/c-c++-common/Wlogical-op-2.c b/gcc/testsuite/c-c++-common/Wlogical-op-2.c
new file mode 100644
index 0000000..47f5c28
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wlogical-op-2.c
@@ -0,0 +1,24 @@
+/* PR c++/66572 */
+/* { dg-do compile } */
+/* { dg-options "-Wlogical-op" } */
+
+#ifndef __cplusplus
+# define bool _Bool
+# define true 1
+# define false 0
+#endif
+
+void
+no_warn ()
+{
+  const bool cst_a = true;
+  const bool cst_b = false;
+
+  if (cst_a || cst_b) {}
+  if (cst_a && cst_b) {}
+  if (true && cst_a) {}
+  if (true || cst_a) {}
+  if (false && cst_a) {}
+  if (false || cst_a) {}
+}
+
diff --git a/gcc/testsuite/g++.dg/warn/Wlogical-op-2.C b/gcc/testsuite/g++.dg/warn/Wlogical-op-2.C
new file mode 100644
index 0000000..252592c6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wlogical-op-2.C
@@ -0,0 +1,59 @@
+// PR c++/66572
+// { dg-do compile }
+// { dg-options "-Wlogical-op" }
+
+#if __cplusplus >= 201103L
+# define CONSTEXPR constexpr
+#else
+# define CONSTEXPR const
+#endif
+
+struct true_type
+{
+  static CONSTEXPR bool value = true;
+};
+
+struct false_type
+{
+  static CONSTEXPR bool value = true;
+};
+
+template<typename T>
+struct is_unsigned : false_type { };
+
+template<>
+struct is_unsigned<unsigned int> : true_type { };
+
+template<typename T1, typename T2>
+bool both_are_unsigned ()
+{
+  return is_unsigned <T1>::value && is_unsigned <T2>::value;
+}
+
+template<typename T1, typename T2>
+bool one_of_is_unsigned ()
+{
+  return is_unsigned <T1>::value || is_unsigned <T2>::value;
+}
+
+void
+foo ()
+{
+  both_are_unsigned <unsigned int, unsigned int> ();
+  both_are_unsigned <int, unsigned int> ();
+  both_are_unsigned <int, int> ();
+  one_of_is_unsigned <unsigned int, unsigned int> ();
+  one_of_is_unsigned <int, unsigned int> ();
+  one_of_is_unsigned <int, int> ();
+}
+
+void
+bar (const int parm_a)
+{
+  const bool a = parm_a;
+  if (a && a) {}    /* { dg-warning "logical .and. of equal expressions" } */
+  if (a || a) {}    /* { dg-warning "logical .or. of equal expressions" } */
+  if (parm_a && parm_a) {}  /* { dg-warning "logical .and. of equal expressions" } */
+  if (parm_a || parm_a) {}  /* { dg-warning "logical .or. of equal expressions" } */
+}
+

Re: [C/C++ PATCH] PR c++/66572. Fix Wlogical-op false positive

[Marek Polacek]

On Sat, Jun 20, 2015 at 03:02:06AM +0300, Mikhail Maltsev wrote:
> -  /* We do not warn for constants because they are typical of macro
> -     expansions that test for features.  */
> -  if (CONSTANT_CLASS_P (op_left) || CONSTANT_CLASS_P (op_right))
> +  /* We do not warn for literal constants because they are typical of macro
> +     expansions that test for features.  Likewise, we do not warn for
> +     const-qualified and constexpr variables which are initialized by constant
> +     expressions, because they can come from e.g. <type_traits> or similar user
> +     code.  */
> +  if (TREE_CONSTANT (op_left) || TREE_CONSTANT (op_right))
>      return;

That looks wrong, because with TREE_CONSTANT we'd warn in C but not in C++
for the following:

const int a = 4;
void
f (void)
{
  const int b = 4;
  static const int c = 5;
  if (a && a) {}
  if (b && b) {}
  if (c && c) {}
}

Note that const-qualified types are checked using TYPE_READONLY.

But I'm not even sure that the warning in the original testcase in the PR
is bogus; you won't get any warning when using e.g.
  foo<unsigned, signed>();
in main().

	Marek

Re: [C/C++ PATCH] PR c++/66572. Fix Wlogical-op false positive

[Mikhail Maltsev]

On 23.06.2015 22:49, Marek Polacek wrote:
> On Sat, Jun 20, 2015 at 03:02:06AM +0300, Mikhail Maltsev wrote:
>> -  /* We do not warn for constants because they are typical of macro
>> -     expansions that test for features.  */
>> -  if (CONSTANT_CLASS_P (op_left) || CONSTANT_CLASS_P (op_right))
>> +  /* We do not warn for literal constants because they are typical of macro
>> +     expansions that test for features.  Likewise, we do not warn for
>> +     const-qualified and constexpr variables which are initialized by constant
>> +     expressions, because they can come from e.g. <type_traits> or similar user
>> +     code.  */
>> +  if (TREE_CONSTANT (op_left) || TREE_CONSTANT (op_right))
>>      return;
> 
> That looks wrong, because with TREE_CONSTANT we'd warn in C but not in C++
> for the following:
> 
> const int a = 4;
> void
> f (void)
> {
>   const int b = 4;
>   static const int c = 5;
>   if (a && a) {}
>   if (b && b) {}
>   if (c && c) {}
> }
> 
Actually for this case the patch silences the warning both for C and
C++. It's interesting that Clang warns like this:

test.c:7:10: warning: use of logical '&&' with constant operand
[-Wconstant-logical-operand]

It does not warn for my testcase with templates. It also does not warn
about:

void
bar (const int parm_a)
{
  const bool a = parm_a;
  if (a && a) {}
  if (a || a) {}
  if (parm_a && parm_a) {}
  if (parm_a || parm_a) {}
}

EDG does not give any warnings at all (in all 3 testcases).

> Note that const-qualified types are checked using TYPE_READONLY.
Yes, but I think we should warn about const-qualified types like in the
example above (and in your recent patch).

> 
> But I'm not even sure that the warning in the original testcase in the PR
> is bogus; you won't get any warning when using e.g.
>   foo<unsigned, signed>();
> in main().

Maybe my snippet does not express clearly enough what it was supposed to
express. I actually meant something like this:

      template<class _U1, class _U2, class = typename
	       enable_if<__and_<is_convertible<_U1, _T1>,
				is_convertible<_U2, _T2>>::value>::type>
	constexpr pair(pair<_U1, _U2>&& __p)
	: first(std::forward<_U1>(__p.first)),
	  second(std::forward<_U2>(__p.second)) { }

(it's std::pair move constructor)
It is perfectly possible that the user will construct an std::pair<T, T>
object from an std::pair<U, U>. In this case we get an "and" of two
identical is_convertible instantiations. The difference is that here
there is a clever __and_ template which helps to avoid warnings. Well,
at least I now know a good way to suppress them in my code :).

Though I still think that this warning is bogus. Probably the correct
(and the hard) way to check templates is to compare ASTs of the operands
before any substitutions.

But for now I could try to implement an idea, which I mentioned in the
bug report: add a new flag to enum tsubst_flags, and set it when we
check ASTs which depend on parameters of a template being instantiated
(we already have similar checks for macro expansions). What do you think
about such approach?

-- 
Regards,
    Mikhail Maltsev

Re: [C/C++ PATCH] PR c++/66572. Fix Wlogical-op false positive

[Marek Polacek]

Sorry it's taken so long to respond.

On Wed, Jun 24, 2015 at 05:43:05PM +0300, Mikhail Maltsev wrote:
> > That looks wrong, because with TREE_CONSTANT we'd warn in C but not in C++
> > for the following:
> > 
> > const int a = 4;
> > void
> > f (void)
> > {
> >   const int b = 4;
> >   static const int c = 5;
> >   if (a && a) {}
> >   if (b && b) {}
> >   if (c && c) {}
> > }
> > 
> Actually for this case the patch silences the warning both for C and
> C++. It's interesting that Clang warns like this:

That's really not what I see.  With your patch, cc1 warns on that testcase
while cc1plus does not.
 
> test.c:7:10: warning: use of logical '&&' with constant operand
> [-Wconstant-logical-operand]
> 
> It does not warn for my testcase with templates. It also does not warn
> about:
> 
> void
> bar (const int parm_a)
> {
>   const bool a = parm_a;
>   if (a && a) {}
>   if (a || a) {}
>   if (parm_a && parm_a) {}
>   if (parm_a || parm_a) {}
> }

I think we want 4 warnings here, but vanilla cc1 only issues 2 warnings.
 
> Maybe my snippet does not express clearly enough what it was supposed to
> express. I actually meant something like this:
> 
>       template<class _U1, class _U2, class = typename
> 	       enable_if<__and_<is_convertible<_U1, _T1>,
> 				is_convertible<_U2, _T2>>::value>::type>
> 	constexpr pair(pair<_U1, _U2>&& __p)
> 	: first(std::forward<_U1>(__p.first)),
> 	  second(std::forward<_U2>(__p.second)) { }
> 
> (it's std::pair move constructor)
> It is perfectly possible that the user will construct an std::pair<T, T>
> object from an std::pair<U, U>. In this case we get an "and" of two
> identical is_convertible instantiations. The difference is that here
> there is a clever __and_ template which helps to avoid warnings. Well,
> at least I now know a good way to suppress them in my code :).
> 
> Though I still think that this warning is bogus. Probably the correct
> (and the hard) way to check templates is to compare ASTs of the operands
> before any substitutions.
> 
> But for now I could try to implement an idea, which I mentioned in the
> bug report: add a new flag to enum tsubst_flags, and set it when we
> check ASTs which depend on parameters of a template being instantiated
> (we already have similar checks for macro expansions). What do you think
> about such approach?

Ok, in that case I think easiest would the following (I hit the same issue
when writing the -Wtautological-compare patch):

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

2015-07-14  Marek Polacek  <polacek@redhat.com>

	PR c++/66572
	* pt.c (tsubst_copy_and_build): Add warn_logical_op sentinel.

	* g++.dg/warn/Wlogical-op-2.C: New test.

diff --git gcc/cp/pt.c gcc/cp/pt.c
index 2097963..0c9712a 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -14893,6 +14893,7 @@ tsubst_copy_and_build (tree t,
       {
 	warning_sentinel s1(warn_type_limits);
 	warning_sentinel s2(warn_div_by_zero);
+	warning_sentinel s3(warn_logical_op);
 	tree op0 = RECUR (TREE_OPERAND (t, 0));
 	tree op1 = RECUR (TREE_OPERAND (t, 1));
 	tree r = build_x_binary_op
diff --git gcc/testsuite/g++.dg/warn/Wlogical-op-2.C gcc/testsuite/g++.dg/warn/Wlogical-op-2.C
index e69de29..755db08 100644
--- gcc/testsuite/g++.dg/warn/Wlogical-op-2.C
+++ gcc/testsuite/g++.dg/warn/Wlogical-op-2.C
@@ -0,0 +1,30 @@
+// PR c++/66572
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wlogical-op" }
+
+struct false_type
+{
+    static constexpr bool value = false;
+};
+
+struct true_type
+{
+    static constexpr bool value = true;
+};
+
+template<typename T>
+struct is_unsigned : false_type {};
+
+template<>
+struct is_unsigned<unsigned> : true_type {};
+
+template<typename T1, typename T2>
+bool foo()
+{
+    return is_unsigned<T1>::value && is_unsigned<T2>::value;
+}
+
+int main()
+{
+    foo<unsigned, unsigned>();
+}

	Marek

Re: [C/C++ PATCH] PR c++/66572. Fix Wlogical-op false positive

[Mikhail Maltsev]

On 07/14/2015 07:38 PM, Marek Polacek wrote:
> Ok, in that case I think easiest would the following (I hit the same issue
> when writing the -Wtautological-compare patch):
Thanks for taking care of this issue.

-- 
Regards,
    Mikhail Maltsev

Re: [C/C++ PATCH] PR c++/66572. Fix Wlogical-op false positive

[Marek Polacek]

Ping.

On Tue, Jul 14, 2015 at 06:38:12PM +0200, Marek Polacek wrote:
> Ok, in that case I think easiest would the following (I hit the same issue
> when writing the -Wtautological-compare patch):
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
> 
> 2015-07-14  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/66572
> 	* pt.c (tsubst_copy_and_build): Add warn_logical_op sentinel.
> 
> 	* g++.dg/warn/Wlogical-op-2.C: New test.
> 
> diff --git gcc/cp/pt.c gcc/cp/pt.c
> index 2097963..0c9712a 100644
> --- gcc/cp/pt.c
> +++ gcc/cp/pt.c
> @@ -14893,6 +14893,7 @@ tsubst_copy_and_build (tree t,
>        {
>  	warning_sentinel s1(warn_type_limits);
>  	warning_sentinel s2(warn_div_by_zero);
> +	warning_sentinel s3(warn_logical_op);
>  	tree op0 = RECUR (TREE_OPERAND (t, 0));
>  	tree op1 = RECUR (TREE_OPERAND (t, 1));
>  	tree r = build_x_binary_op
> diff --git gcc/testsuite/g++.dg/warn/Wlogical-op-2.C gcc/testsuite/g++.dg/warn/Wlogical-op-2.C
> index e69de29..755db08 100644
> --- gcc/testsuite/g++.dg/warn/Wlogical-op-2.C
> +++ gcc/testsuite/g++.dg/warn/Wlogical-op-2.C
> @@ -0,0 +1,30 @@
> +// PR c++/66572
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wlogical-op" }
> +
> +struct false_type
> +{
> +    static constexpr bool value = false;
> +};
> +
> +struct true_type
> +{
> +    static constexpr bool value = true;
> +};
> +
> +template<typename T>
> +struct is_unsigned : false_type {};
> +
> +template<>
> +struct is_unsigned<unsigned> : true_type {};
> +
> +template<typename T1, typename T2>
> +bool foo()
> +{
> +    return is_unsigned<T1>::value && is_unsigned<T2>::value;
> +}
> +
> +int main()
> +{
> +    foo<unsigned, unsigned>();
> +}
> 
> 	Marek

	Marek

Re: [C/C++ PATCH] PR c++/66572. Fix Wlogical-op false positive

[Jeff Law]

On 07/21/2015 04:56 AM, Marek Polacek wrote:
> Ping.
>
> On Tue, Jul 14, 2015 at 06:38:12PM +0200, Marek Polacek wrote:
>> Ok, in that case I think easiest would the following (I hit the same issue
>> when writing the -Wtautological-compare patch):
>>
>> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>>
>> 2015-07-14  Marek Polacek  <polacek@redhat.com>
>>
>> 	PR c++/66572
>> 	* pt.c (tsubst_copy_and_build): Add warn_logical_op sentinel.
>>
>> 	* g++.dg/warn/Wlogical-op-2.C: New test.
I realize it's C++, but it's simple enough for me.

Ok for the trunk.

jeff