gdb: recognize 64 bits Windows executables as Cygwin osabi

If I generate two Windows PE executables, one 32 bits and one 64 bits:

    $ x86_64-w64-mingw32-gcc test.c -g3 -O0 -o test_64
    $ i686-w64-mingw32-gcc test.c -g3 -O0 -o test_32
    $ file test_64
    test_64: PE32+ executable (console) x86-64, for MS Windows
    $ file test_32
    test_32: PE32 executable (console) Intel 80386, for MS Windows

When I load the 32 bits binary in my GNU/Linux-hosted GDB, the osabi is
correctly recognized as "Cygwin":

    $ ./gdb --data-directory=data-directory -nx test_32
    (gdb) show osabi
    The current OS ABI is "auto" (currently "Cygwin").

When I load the 64 bits binary in GDB, the osabi is incorrectly
recognized as "GNU/Linux":

    $ ./gdb --data-directory=data-directory -nx test_64
    (gdb) show osabi
    The current OS ABI is "auto" (currently "GNU/Linux").

The 32 bits one gets recognized by the i386_cygwin_osabi_sniffer
function, by its target name:

    if (strcmp (target_name, "pei-i386") == 0)
      return GDB_OSABI_CYGWIN;

The target name for the 64 bits binaries is "pei-x86-64".  It doesn't
get recognized by any osabi sniffer, so GDB falls back on its default
osabi, "GNU/Linux".

This patch adds an osabi sniffer function for the Windows 64 bits
executables in amd64-windows-tdep.c.  With it, the osabi is recognized
as "Cygwin", just like with the 32 bits binary.

Note that it may seems strange to have a binary generated by MinGW
(which has nothing to do with Cygwin) be recognized as a Cygwin binary.
This is indeed not accurate, but at the moment GDB uses the Cygwin for
everything Windows.  Subsequent patches will add a separate "Windows" OS
ABI for Windows binaries that are not Cygwin binaries.

gdb/ChangeLog:

	* amd64-windows-tdep.c (amd64_windows_osabi_sniffer): New
	function.
	(_initialize_amd64_windows_tdep): Register osabi sniffer.

2 files changed, 20 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index a0d9758..1b83f42 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2020-03-16  Simon Marchi  <simon.marchi@efficios.com>
+
+	* amd64-windows-tdep.c (amd64_windows_osabi_sniffer): New
+	function.
+	(_initialize_amd64_windows_tdep): Register osabi sniffer.
+
 2020-03-14  Tom Tromey  <tom@tromey.com>
 
 	* c-typeprint.c (cp_type_print_method_args): Print "__restrict__"
diff --git a/gdb/amd64-windows-tdep.c b/gdb/amd64-windows-tdep.c
index d4d7968..2ca9795 100644
--- a/gdb/amd64-windows-tdep.c
+++ b/gdb/amd64-windows-tdep.c
@@ -1244,10 +1244,24 @@ amd64_windows_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
   set_gdbarch_auto_wide_charset (gdbarch, amd64_windows_auto_wide_charset);
 }
 
+static gdb_osabi
+amd64_windows_osabi_sniffer (bfd *abfd)
+{
+  const char *target_name = bfd_get_target (abfd);
+
+  if (strcmp (target_name, "pei-x86-64") == 0)
+    return GDB_OSABI_CYGWIN;
+
+  return GDB_OSABI_UNKNOWN;
+}
+
 void _initialize_amd64_windows_tdep ();
 void
 _initialize_amd64_windows_tdep ()
 {
   gdbarch_register_osabi (bfd_arch_i386, bfd_mach_x86_64, GDB_OSABI_CYGWIN,
                           amd64_windows_init_abi);
+
+  gdbarch_register_osabi_sniffer (bfd_arch_i386, bfd_target_coff_flavour,
+				  amd64_windows_osabi_sniffer);
 }