as should never give a warning or error message when
assembling compiler output. But programs written by people often
cause as to give a warning that a particular assumption was
made. All such warnings are directed to the standard error file.
If you use the -W and --no-warn options, no warnings are issued.
This only affects the warning messages: it does not change any particular of
how as assembles your file. Errors, which stop the assembly,
are still reported.
If you use the --fatal-warnings option, as considers
files that generate warnings to be in error.
You can switch these options off again by specifying --warn, which causes warnings to be output as usual.