The GNU linker doesn't support a zero-sized global variable that is
dynamically exported, so gccgo generates those global variables with a
size of 1 byte.  Unfortunately, that means that passing a global
variable to a function that takes an argument of a zero-sized type
will actually pass 1 byte, taking up an argument slot, rather than a
zero-sized argument that should be skipped.  This patch fixes the
problem in the Go frontend -> GCC interface by adding a conversion to
the real type for any such global variables, and undoing the
conversion where necessary.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline and gccgo branch.

Ian

2015-12-21  Ian Lance Taylor  <iant@google.com>

* go-gcc.cc (Gcc_backend::global_variable): If type is zero-sized,
add a VIEW_CONVERT_EXPR to the tree.
(Gcc_backend::global_variable_set_init): Remove any
VIEW_CONVERT_EXPR.
(Gcc_backend::write_global_definitions): Likewise.