Understanding compiler warning options

[Manuel López-Ibáñez]

On 30/09/15 03:57, Martin Sebor wrote:
> On 09/29/2015 05:41 PM, Edward Diener wrote:
>> I am trying to understand how the compiler decides which warnings are
>> issued when compiling a translation unit.
>>
>> What is the default warnings that are issued if no compiler warning
>> options is set on the command line ?
[...]
> An example might be the -Wdiv-by-zero warning that's defined like
> this:
>
>    Wdiv-by-zero
>    C ObjC C++ ObjC++ Var(warn_div_by_zero) Init(1) Warning
>    Warn about compile-time integer division by zero

Unfortunately, many options are enabled under some conditions and this cannot 
(yet) be encoded in the .opt files: https://gcc.gnu.org/PR53063

Any contribution in that direction would be welcome.

> You may be able to tell (probably less reliably) from the manual
> by searching for warning options listed in the negative -Wno-xxx
> form. For instance:
>
>    -Wno-endif-labels
>        Do not warn whenever an #else or an #endif are followed by text.
>
> means the option is enabled by default. Other options may say
> when (e.g., in what language) they're enabled by default.

I think the consensus nowadays is that options that are enabled by default 
under some condition should be documented as such directly. Documenting the 
-Wno- form is not really useful as a substitute for this because the "default" 
depends on other options (e.g., -std=), language, target, etc. Documentation 
patches welcome!

>> If I set a general option, such as -w, -Wall, or -Wextra, do these get
>> overridden by more specific compiler options no matter where in the
>> command line they are encountered, or does the order of the compiler
>> warnings on the command line matter in such a case ? As a specific
>> practical example if the command line has "-Wno-unused-local-typedef
>> -Wall ..." does the "-Wno-unused-local-typedef" override the "-Wall"
>> option ?
>
> Unfortunately, this is neither documented nor necessarily intuitive
> (YMMV). IME, the best way to find out is by experimenting.

It is documented here: 
https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#Warning-Options

but perhaps it could be better documented. Again, unless someone gives it a try 
to improve the documentation, it will remain the same.

In particular: "You can request many specific warnings with options beginning 
with ‘-W’, for example -Wimplicit to request warnings on implicit declarations. 
Each of these specific warning options also has a negative form beginning 
‘-Wno-’ to turn off warnings; for example, -Wno-implicit. This manual lists 
only one of the two forms, whichever is not the default.

Some options, such as -Wall and -Wextra, turn on other options, such as 
-Wunused, which may turn on further options, such as -Wunused-value. The 
combined effect of positive and negative forms is that more specific options 
have priority over less specific ones, independently of their position in the 
command-line. For options of the same specificity, the last one takes effect."

The above behavior is handled automatically nowadays such that all options 
behave the same. If it doesn't work, something is wrong in the .opt file or the 
option was never converted to the automatic way of handling options 
dependencies and it is being set explicitly somewhere, which is also a latent bug.

Cheers,

Manuel.