Re: [PATCH] Define std::byte for C++17 (P0298R3)

[Jason Merrill]

On Thu, Mar 9, 2017 at 10:47 AM, Jonathan Wakely <jwak...@redhat.com> wrote:
> This is a new type for C++17, with no impact on anything in non-C++17
> dialects. This is intentionally only defined in <cstddef> and not
> <stddef.h>.

And this patch adds the aliasing semantics:

Tested x86_64-pc-linux-gnu, applying to trunk.

commit cc516c1009e5a68dadeb29bd74f1d2fa13e64e95
Author: Jason Merrill <ja...@redhat.com>
Date:   Thu Mar 16 13:37:08 2017 -0400

            * decl.c (start_enum): std::byte aliases anything.

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 0ecd30b..61ecf81 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -14079,6 +14079,12 @@ start_enum (tree name, tree enumtype, tree 
underlying_type,
        {
          enumtype = cxx_make_type (ENUMERAL_TYPE);
          enumtype = pushtag (name, enumtype, /*tag_scope=*/ts_current);
+
+         /* std::byte aliases anything.  */
+         if (enumtype != error_mark_node
+             && TYPE_CONTEXT (enumtype) == std_node
+             && !strcmp ("byte", TYPE_NAME_STRING (enumtype)))
+           TYPE_ALIAS_SET (enumtype) = 0;
        }
       else
          enumtype = xref_tag (enum_type, name, /*tag_scope=*/ts_current,
diff --git a/gcc/testsuite/g++.dg/cpp1z/byte1.C 
b/gcc/testsuite/g++.dg/cpp1z/byte1.C
new file mode 100644
index 0000000..51c1a33
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/byte1.C
@@ -0,0 +1,31 @@
+// Test for std::byte aliasing properties.
+// { dg-options "-std=c++1z -O3" }
+
+#include <cstddef>
+
+using byte = std::byte;
+
+enum class notbyte: unsigned char {} *np;
+
+int main()
+{
+  int x;
+
+  /* Stores through byte* can alias int, so the compiler can't optimize
+     "x != 0".  */
+  byte *p = (byte*)&x;
+  x = 42;
+  for (int i = 0; i < 4; ++i)
+    p[i] = byte(0);
+  if (x != 0)
+    __builtin_abort();
+
+  /* Stores through notbyte* mustn't alias int, so at -O3 the compiler should
+     optimize "x != 42" to false.  */
+  notbyte *np = (notbyte*)&x; 
+  x = 42;
+  for (int i = 0; i < 4; ++i)
+    np[i] = notbyte(0);
+  if (x != 42)
+    __builtin_abort();
+}