[cfe-dev] Silencing "unused argument" for injected default arguments

[Martin Storsjö via cfe-dev]

On Fri, 27 Sep 2019, James Y Knight wrote:

> On Fri, Sep 13, 2019, 7:02 AM Martin Storsjö via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
>       Hi,
>
>       In my setups with clang, I'm using a thin frontend wrapper which
>       passes a
>       bunch of custom defaults (allowing using any stock clang
>       executable,
>       without requiring it to be built with CLANG_DEFAULT_* set to my
>       desired
>       state). The main set of flags I currently use is this:
>
>       -target $TARGET -rtlib=compiler-rt -stdlib=libc++ -fuse-ld=lld
>       -fuse-cxa-atexit
> 
>
>       Many of them are silent (like -fuse-ld=lld despite only
>       compiling, not
>       linking), but e.g. -stdlib=libc++ is considered unused if
>       compiling C
>       code. (Just looking at the driver name, clang vs clang++, isn't
>       enough for
>       deciding to omit it; it could also be "clang -x c++" etc, and
>       I'd prefer
>       not to try to deduce that in the wrapper.)
>
>       In some cases it also adds a few -D and -Wl,-l<lib>, where the
>       -Wl,-l also
>       warns while compiling.
>
>       To mitigate this, I'm also adding -Qunused-arguments, which
>       works fine,
>       but is a bit too overreaching.
>
>       Would it make sense to add some mechanism to flag these
>       arguments as
>       default ones that clang shouldn't warn about being unused?
>
>       That could be something like --start-no-unused-arguments <args>
>       --end-no-unused-arguments.
>
>       I know about the clang config files, which also allows setting
>       default
>       flags, which are exempt from unused warnings, and I've tried
>       that, but for
>       various reasons I'm still more comfortable with setting them via
>       a
>       wrapper. (E.g. on windows, the config file setup would require
>       having N
>       copies of clang.exe, one for each triple combination, while I
>       currently
>       can do with just copies of the wrapper and one single
>       clang.exe.)
> 
> 
> 
> Why not just have the wrapper script use a config file? E.g. have it call
> `clang --config whatever.conf`?

That could work, although it's a little bit less convenient than my 
current setup. It ends up as quite a few files, as there's currently 4 
different archs, times two environment variants (plain desktop or the UWP 
environment), giving currently 8 different files to keep in sync (or 
automate generation of).

// Martin