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Re: [C++ PATCH] Allow frexp etc. builtins in c++14 constexpr (PR c++/50060)


On Mon, Jul 18, 2016 at 02:07:50PM -0400, Jason Merrill wrote:
> On Mon, Jul 18, 2016 at 2:03 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> > On Mon, Jul 18, 2016 at 01:16:26PM -0400, Jason Merrill wrote:
> > That is reasonable, but not 100% sure what to do if it returns NULL
> > - it should return t, but if I do VERIFY_CONSTANT (t); there or
> > manually
> >     if (!*non_constant_p)
> >       {
> >         if (!allow_non_constant)
> >           error ("%q+E is not a constant expression", t);
> >         *non_constant_p = true;
> >       }
> >     return t;
> > it will print the original expression (without folded arguments).
> > Another option would be to build_call_array_loc if we want to emit
> > the error.  Preferences?
> 
> Hmm, I guess let's build the call for the error.

Ok.

> >    /* Fold away the NOP_EXPR from fold_builtin_n.  */
> >    new_call = fold (new_call);
> >    force_folding_builtin_constant_p = save_ffbcp;
> > +
> > +  /* Folding some math builtins produces e.g. COMPOUND_EXPR etc.  */
> > +  if (cxx_dialect >= cxx14)
> > +    return cxx_eval_constant_expression (&new_ctx, new_call, lval,
> > +                                        non_constant_p, overflow_p);
> 
> If we do this unconditionally, can we drop the fold above?

So I've tried following patch, but on
extern "C" double frexp (double, int *);

struct S
{
#ifdef FREXP
  constexpr S (double a) : y {}, x (frexp (a, &y)) {}
#else
  constexpr S (double a) : y {}, x ((y = 1, 0.8125)) {}
#endif
  double x;
  int y;
};

static_assert (S (6.5).x == 0.8125, "");

it means the testcase is accepted with -DFREXP in both
-std=gnu++11 and -std=gnu++14 modes and without -DFREXP in
-std=gnu++14 mode only.  I can move over the fold call
to the cxx11 block, but I think we need to reject it for C++11.

2016-07-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/50060
	* constexpr.c (cxx_eval_builtin_function_call): Pass false as lval
	when evaluating call arguments.  Use fold_builtin_call_array instead
	of fold_build_call_array_loc, return t if it returns NULL.
	For C++14 and later, pass new_call to if new_call
	cxx_eval_constant_expression.

	* g++.dg/cpp1y/constexpr-50060.C: New test.

--- gcc/cp/constexpr.c.jj	2016-07-16 10:41:04.525652516 +0200
+++ gcc/cp/constexpr.c	2016-07-18 20:18:08.035709874 +0200
@@ -1105,7 +1105,7 @@ cxx_eval_builtin_function_call (const co
   for (i = 0; i < nargs; ++i)
     {
       args[i] = cxx_eval_constant_expression (&new_ctx, CALL_EXPR_ARG (t, i),
-					      lval, &dummy1, &dummy2);
+					      false, &dummy1, &dummy2);
       if (bi_const_p)
 	/* For __built_in_constant_p, fold all expressions with constant values
 	   even if they aren't C++ constant-expressions.  */
@@ -1114,13 +1114,23 @@ cxx_eval_builtin_function_call (const co
 
   bool save_ffbcp = force_folding_builtin_constant_p;
   force_folding_builtin_constant_p = true;
-  new_call = fold_build_call_array_loc (EXPR_LOCATION (t), TREE_TYPE (t),
-					CALL_EXPR_FN (t), nargs, args);
-  /* Fold away the NOP_EXPR from fold_builtin_n.  */
-  new_call = fold (new_call);
+  new_call = fold_builtin_call_array (EXPR_LOCATION (t), TREE_TYPE (t),
+				      CALL_EXPR_FN (t), nargs, args);
   force_folding_builtin_constant_p = save_ffbcp;
-  VERIFY_CONSTANT (new_call);
-  return new_call;
+  if (new_call == NULL)
+    {
+      if (!*non_constant_p && !ctx->quiet)
+	{
+	  new_call = build_call_array_loc (EXPR_LOCATION (t), TREE_TYPE (t),
+					   CALL_EXPR_FN (t), nargs, args);
+	  error ("%q+E is not a constant expression", new_call);
+	}
+      *non_constant_p = true;
+      return t;
+    }
+
+  return cxx_eval_constant_expression (&new_ctx, new_call, lval,
+				       non_constant_p, overflow_p);
 }
 
 /* TEMP is the constant value of a temporary object of type TYPE.  Adjust
--- gcc/testsuite/g++.dg/cpp1y/constexpr-50060.C.jj	2016-07-18 19:45:30.528496123 +0200
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-50060.C	2016-07-18 19:45:30.528496123 +0200
@@ -0,0 +1,100 @@
+// PR c++/50060
+// { dg-do compile { target c++14 } }
+
+// sincos and lgamma_r aren't available in -std=c++14,
+// only in -std=gnu++14.  Use __builtin_* in that case.
+extern "C" void sincos (double, double *, double *);
+extern "C" double frexp (double, int *);
+extern "C" double modf (double, double *);
+extern "C" double remquo (double, double, int *);
+extern "C" double lgamma_r (double, int *);
+
+constexpr double
+f0 (double x)
+{
+  double y {};
+  double z {};
+  __builtin_sincos (x, &y, &z);
+  return y;
+}
+
+constexpr double
+f1 (double x)
+{
+  double y {};
+  double z {};
+  __builtin_sincos (x, &y, &z);
+  return z;
+}
+
+constexpr double
+f2 (double x)
+{
+  int y {};
+  return frexp (x, &y);
+}
+
+constexpr int
+f3 (double x)
+{
+  int y {};
+  frexp (x, &y);
+  return y;
+}
+
+constexpr double
+f4 (double x)
+{
+  double y {};
+  return modf (x, &y);
+}
+
+constexpr double
+f5 (double x)
+{
+  double y {};
+  modf (x, &y);
+  return y;
+}
+
+constexpr double
+f6 (double x, double y)
+{
+  int z {};
+  return remquo (x, y, &z);
+}
+
+constexpr int
+f7 (double x, double y)
+{
+  int z {};
+  remquo (x, y, &z);
+  return z;
+}
+
+constexpr double
+f8 (double x)
+{
+  int y {};
+  return __builtin_lgamma_r (x, &y);
+}
+
+constexpr int
+f9 (double x)
+{
+  int y {};
+  __builtin_lgamma_r (x, &y);
+  return y;
+}
+
+static_assert (f0 (0.0) == 0.0, "");
+static_assert (f1 (0.0) == 1.0, "");
+static_assert (f2 (6.5) == 0.8125, "");
+static_assert (f3 (6.5) == 3, "");
+static_assert (f4 (-7.25) == -0.25, "");
+static_assert (f5 (-7.25) == -7.0, "");
+static_assert (f6 (3.0, 2.0) == -1.0, "");
+static_assert (f7 (3.0, 2.0) == 2, "");
+static_assert (f8 (0.75) >= 0.20 && f8 (0.75) <= 0.21, "");
+static_assert (f8 (0.75) >= 0.20 && f8 (0.75) <= 0.21, "");
+static_assert (f9 (0.75) == 1, "");


	Jakub


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