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.
