I've previously built cvs head snapshots for aarch64-linux.  Attempting to do so with the binutils-2.23 release fails as follows:

gcc -DHAVE_CONFIG_H -I. -I/tmp/binutils-2.23/gas  -I. -I/tmp/binutils-2.23/gas -I../bfd -I/tmp/binutils-2.23/gas/config -I/tmp/binutils-2.23/gas/../include -I/tmp/binutils-2.23/gas/.. -I/tmp/binutils-2.23/gas/../bfd -DLOCALEDIR="\"/tmp/install/share/locale\""  -W -Wall -Wstrict-prototypes -Wmissing-prototypes -Wshadow -Werror -g -O2 -MT tc-aarch64.o -MD -MP -MF .deps/tc-aarch64.Tpo -c -o tc-aarch64.o `test -f 'config/tc-aarch64.c' || echo '/tmp/binutils-2.23/gas/'`config/tc-aarch64.c
/tmp/binutils-2.23/gas/config/tc-aarch64.c: In function 'first_error_fmt':
/tmp/binutils-2.23/gas/config/tc-aarch64.c:646:11: error: variable 'ret' set but not used [-Werror=unused-but-set-variable]

The code in question is:

    {
      int ret;
      va_start (args, format);
      ret = vsnprintf (buffer, size, format, args);
      know (ret <= size - 1 && ret >= 0);
      va_end (args);
      set_syntax_error (buffer);
    }

That is, the variable ret is only used in the assert-like know call.  However, know is a macro that expands to a dummy statement (do{}while(0)), so the actual parameter expression disappears and ret is unused-but-set.

This doesn't happen when building from cvs head, presumably because know then expands to gas_assert which does evaluate the expression.

I don't want to use --disable-werror by default.  Is there a reason why know(p) can't use the standard technique of expanding to (void)(p)?  That change fixes the problem for me.