[Bug gas/14775] New: [2.23] gas build failure for aarch64-linux: set but not used variable in tc-aarch64.c:first_error_fmt

[mikpe at it dot uu.se]

http://sourceware.org/bugzilla/show_bug.cgi?id=14775

             Bug #: 14775
           Summary: [2.23] gas build failure for aarch64-linux: set but
                    not used variable in tc-aarch64.c:first_error_fmt
           Product: binutils
           Version: 2.23
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gas
        AssignedTo: address@hidden
        ReportedBy: address@hidden
    Classification: Unclassified


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.

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