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 or --no-warn option, 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.
Warnings are switched on by default. They can be switched off with -W or --no-warn. Specifying the --warn again later on the command line will switch on warnings again, and cause them to be output as usual.
If you use the --fatal-warnings option, as considers files that generate warnings to be in error.