On 1/5/21 10:26 AM, Jakub Jelinek wrote:
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?
OK.
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
