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.