d: Add `@visibility' and `@hidden' attributes.

The `@visibility' attribute is functionality the same as
`__attribute__((visibility))', and `@hidden' is a convenience alias to
`@visibility("hidden")' defined in the `gcc.attributes' module.

As the visibility of a symbol is also indirectly controlled by the
`export' keyword, the handling of this in the code generation pass has
been improved so that conflicts will be appropriately diagnosed.

gcc/d/ChangeLog:

	* d-attribs.cc (d_langhook_attribute_table): Add visibility.
	(insert_type_attribute): Use decl_attributes instead of
	merge_attributes.
	(insert_decl_attribute): Likewise.
	(apply_user_attributes): Do nothing when no UDAs applied.
	(d_handle_visibility_attribute): New function.
	* d-gimplify.cc (d_gimplify_binary_expr): Adjust.
	* d-tree.h (set_visibility_for_decl): Declare.
	* decl.cc (get_symbol_decl): Move setting of visibility flags to...
	(set_visibility_for_decl): ... here.  New function.
	* types.cc (TypeVisitor::visit (TypeStruct *)): Call
	set_visibility_for_decl().
	(TypeVisitor::visit (TypeClass *)): Likewise.

gcc/testsuite/ChangeLog:

	* gdc.dg/attr_visibility1.d: New test.
	* gdc.dg/attr_visibility2.d: New test.
	* gdc.dg/attr_visibility3.d: New test.

libphobos/ChangeLog:

	* libdruntime/gcc/attributes.d (visibility): Define.
	(hidden): Define.

1 files changed, 34 insertions, 0 deletions

diff --git a/libphobos/libdruntime/gcc/attributes.d b/libphobos/libdruntime/gcc/attributes.d
index 09f684cd44e..ca066cef822 100644
--- a/libphobos/libdruntime/gcc/attributes.d
+++ b/libphobos/libdruntime/gcc/attributes.d
@@ -425,6 +425,30 @@ auto target_clones(A...)(A arguments)
 enum used = attribute("used");
 
 /**
+ * The `@visibility` attribute affects the linkage of the declaration to which
+ * it is attached. It can be applied to variables, types, and functions.
+ *
+ * There are four supported visibility_type values: `default`, `hidden`,
+ * `protected`, or `internal` visibility.
+ *
+ * Example:
+ * ---
+ * import gcc.attributes;
+ *
+ * @visibility("protected") void func() {  }
+ * ---
+ */
+auto visibility(string visibilityName)
+{
+    return attribute("visibility", visibilityName);
+}
+
+auto visibility(A...)(A arguments)
+{
+    assert(false, "visibility attribute argument not a string constant");
+}
+
+/**
  * The `@weak` attribute causes a declaration of an external symbol to be
  * emitted as a weak symbol rather than a global.  This is primarily useful in
  * defining library functions that can be overridden in user code, though it can
@@ -543,6 +567,16 @@ enum dynamicCompileEmit = false;
 enum fastmath = optimize("Ofast");
 
 /**
+ * Sets the visibility of a function or global variable to "hidden".
+ * Such symbols aren't directly accessible from outside the DSO
+ * (executable or DLL/.so/.dylib) and are resolved inside the DSO
+ * during linking. If unreferenced within the DSO, the linker can
+ * strip a hidden symbol.
+ * An `export` visibility overrides this attribute.
+ */
+enum hidden = visibility("hidden");
+
+/**
  * Adds GCC's "naked" attribute to a function, disabling function prologue /
  * epilogue emission.
  * Intended to be used in combination with basic `asm` statement.  While using