On Mon, Jan 04, 2021 at 04:01:25PM -0500, Jason Merrill via Gcc-patches wrote:
> On 1/4/21 3:48 PM, Jakub Jelinek wrote:
> > On Mon, Jan 04, 2021 at 03:44:46PM -0500, Jason Merrill wrote:
> > > This change is OK, but part of the problem is that we're trying to do
> > > overload resolution for an S copy/move constructor, which we shouldn't be
> > > because bit_cast is a prvalue, so in C++17 and up we should use it to
> > > directly initialize the target without any implied constructor call.
> > >
> > > It seems we're mishandling this because the code in
> > > build_special_member_call specifically looks for TARGET_EXPR or
> > > CONSTRUCTOR,
> > > and BIT_CAST_EXPR is neither of those.
> > >
> > > Wrapping a BIT_CAST_EXPR of aggregate type in a TARGET_EXPR would address
> > > this, and any other places that expect a class prvalue to come in the form
> > > of a TARGET_EXPR.
> >
> > I can try that tomorrow. Won't that cause copying through extra temporary
> > in some cases though, or is that guaranteed to be optimized?
>
> It won't cause any extra copying when it's used to initialize another object
> (like the return value of std::bit_cast). Class prvalues are always
> expressed with a TARGET_EXPR in the front end; the TARGET_EXPR melts away
> when used as an initializer, it only creates a temporary when it's used in
> another way.
Ok, this version wraps it into a TARGET_EXPR then, it alone fixes the bug,
but I've kept the constexpr.c change too.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2021-01-05 Jakub Jelinek <ja...@redhat.com>
PR c++/98469
* constexpr.c (cxx_eval_constant_expression) <case BIT_CAST_EXPR>:
Punt if lval is true.
* semantics.c (cp_build_bit_cast): Call get_target_expr_sfinae on
the result if it has a class type.
* g++.dg/cpp2a/bit-cast8.C: New test.
* g++.dg/cpp2a/bit-cast9.C: New test.
--- gcc/cp/constexpr.c.jj 2021-01-04 10:25:48.750121531 +0100
+++ gcc/cp/constexpr.c 2021-01-05 11:41:38.315032636 +0100
@@ -6900,6 +6900,15 @@ cxx_eval_constant_expression (const cons
return t;
case BIT_CAST_EXPR:
+ if (lval)
+ {
+ if (!ctx->quiet)
+ error_at (EXPR_LOCATION (t),
+ "address of a call to %qs is not a constant expression",
+ "__builtin_bit_cast");
+ *non_constant_p = true;
+ return t;
+ }
r = cxx_eval_bit_cast (ctx, t, non_constant_p, overflow_p);
break;
--- gcc/cp/semantics.c.jj 2021-01-04 10:25:48.489124486 +0100
+++ gcc/cp/semantics.c 2021-01-05 11:27:49.327372582 +0100
@@ -10761,6 +10761,10 @@ cp_build_bit_cast (location_t loc, tree
tree ret = build_min (BIT_CAST_EXPR, type, arg);
SET_EXPR_LOCATION (ret, loc);
+
+ if (!processing_template_decl && CLASS_TYPE_P (type))
+ ret = get_target_expr_sfinae (ret, complain);
+
return ret;
}
--- gcc/testsuite/g++.dg/cpp2a/bit-cast8.C.jj 2021-01-05 11:41:38.315032636
+0100
+++ gcc/testsuite/g++.dg/cpp2a/bit-cast8.C 2021-01-05 11:41:38.315032636
+0100
@@ -0,0 +1,11 @@
+// PR c++/98469
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wall" }
+
+struct S { int s; };
+
+S
+foo ()
+{
+ return __builtin_bit_cast (S, 0);
+}
--- gcc/testsuite/g++.dg/cpp2a/bit-cast9.C.jj 2021-01-05 11:41:38.315032636
+0100
+++ gcc/testsuite/g++.dg/cpp2a/bit-cast9.C 2021-01-05 11:41:38.315032636
+0100
@@ -0,0 +1,15 @@
+// PR c++/98469
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wall" }
+
+template<typename T, typename F>
+constexpr T
+bit_cast (const F &f) noexcept
+{
+ return __builtin_bit_cast (T, f);
+}
+struct S { int s; };
+constexpr int foo (const S &x) { return x.s; }
+constexpr int bar () { return foo (bit_cast<S> (0)); }
+constexpr int x = bar ();
+static_assert (!x);
Jakub
