[PATCH] Adding warning for constexpr's

[PATCH] Adding warning for constexpr's

[Andres Tiraboschi]

Hi, this patch is for adding a warning when a constexpr cannot be evaluated at compile time.
This is a single case:
type var = fun(args...), with fun declared as a constexpr.

diff --git a/gcc/common.opt b/gcc/common.opt
index b49ac46..88374b1 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -581,6 +581,10 @@ Winline
 Common Var(warn_inline) Warning
 Warn when an inlined function cannot be inlined
 
+Wconstexpr
+Common Var(warn_constexpr) Warning
+Warn when a constexpr function is not evaluated at compile time
+
 Winvalid-memory-model
 Common Var(warn_invalid_memory_model) Init(1) Warning
 Warn when an atomic memory model parameter is known to be outside the valid range.
diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index 80a6939..e25c240 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -842,6 +842,15 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags)
       DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl) = const_init;
       TREE_CONSTANT (decl) = const_init && decl_maybe_constant_var_p (decl);
     }
+  else if (TREE_CODE(init) == CALL_EXPR)
+    {
+      tree fn = TREE_OPERAND(CALL_EXPR_FN(init), 0);
+      if (DECL_DECLARED_CONSTEXPR_P(fn) && warn_constexpr)
+        {
+          warning (OPT_Wconstexpr, "function %q+F cannot be evaluated at compile time", fn);
+          warning (OPT_Wconstexpr, "called from here");
+        }
+    }
 
   if (cxx_dialect >= cxx14)
     /* Handle aggregate NSDMI in non-constant initializers, too.  */

Re: [PATCH] Adding warning for constexpr's

[Joseph Myers]

On Fri, 12 Jun 2015, Andres Tiraboschi wrote:

> Hi, this patch is for adding a warning when a constexpr cannot be evaluated at compile time.
> This is a single case:
> type var = fun(args...), with fun declared as a constexpr.

All options need documenting in invoke.texi.  All diagnostics need 
testcases added to the testsuite.  C++-specific options go in c.opt and 
should be listed as C++ ObjC++, not Common.  All new diagnostics should 
use warning_at etc. with explicit locations passed, unless there is some 
strong reason it's hard to get the relevant location when the warning is 
given.

-- 
Joseph S. Myers
joseph@codesourcery.com

Re: [PATCH] Adding warning for constexpr's

[Andres Tiraboschi]

Hi, thanks for your answer. I was trying with warning_at this way
instead that I was doing before:

+  else if (TREE_CODE(init) == CALL_EXPR)
+    {
+      tree fn = TREE_OPERAND(CALL_EXPR_FN(init), 0);
+      if (DECL_DECLARED_CONSTEXPR_P(fn) && warn_constexpr)
+          warning_at (DECL_SOURCE_LINE(decl), OPT_Wconstexpr,
"function %q+F cannot be evaluated at compile time", fn);
+

where "decl" is the declaration that is being initializated. For some
reason this isn't working. Checking with gdb I realised that
warning_at in this case is returnin false. I don't know if I am
missing something.

2015-06-12 18:35 GMT-03:00 Joseph Myers <joseph@codesourcery.com>:
> On Fri, 12 Jun 2015, Andres Tiraboschi wrote:
>
>> Hi, this patch is for adding a warning when a constexpr cannot be evaluated at compile time.
>> This is a single case:
>> type var = fun(args...), with fun declared as a constexpr.
>
> All options need documenting in invoke.texi.  All diagnostics need
> testcases added to the testsuite.  C++-specific options go in c.opt and
> should be listed as C++ ObjC++, not Common.  All new diagnostics should
> use warning_at etc. with explicit locations passed, unless there is some
> strong reason it's hard to get the relevant location when the warning is
> given.
>
> --
> Joseph S. Myers
> joseph@codesourcery.com

Re: [PATCH] Adding warning for constexpr's

[Andres Tiraboschi]

2015-06-12 18:35 GMT-03:00 Joseph Myers <joseph@codesourcery.com>:
> On Fri, 12 Jun 2015, Andres Tiraboschi wrote:
>
>> Hi, this patch is for adding a warning when a constexpr cannot be evaluated at compile time.
>> This is a single case:
>> type var = fun(args...), with fun declared as a constexpr.
>
> All options need documenting in invoke.texi.  All diagnostics need
> testcases added to the testsuite.  C++-specific options go in c.opt and
> should be listed as C++ ObjC++, not Common.  All new diagnostics should
> use warning_at etc. with explicit locations passed, unless there is some
> strong reason it's hard to get the relevant location when the warning is
> given.
>
> --
> Joseph S. Myers
> joseph@codesourcery.com

Hi, do you know where is the .exp file for the tests in
.../gcc/testsuite/g++.dg/warn?
I can't find it. Thanks again

Re: [PATCH] Adding warning for constexpr's

[Mike Stump]

On Jun 15, 2015, at 12:55 PM, Andres Tiraboschi <andres.tiraboschi@tallertechnologies.com> wrote:
> 
> Hi, do you know where is the .exp file for the tests in
> .../gcc/testsuite/g++.dg/warn?
> I can't find it.

find srcdir -name \*.exp -print will show you all of them.  You’ll discover that a .exp file can run the entire tree under it.  In this case, look in the parent directory.