[ccache] cache hides compiler warnings about comments

[Joel Rosdahl]

Hi Akim,

> Compilers such as gcc/clang support -C/-CC to leave comments
> in the preprocessed output.  Wouldn't it make sense to use it
> in ccache?

Perhaps yes, but one problem with that is that ccache (currently) can't
assume that the compiler accepts -C/-CC.

Also, there is this caveat in GCC documentation: "You should be prepared
for side effects when using -C; it causes the preprocessor to treat
comments as tokens in their own right.  For example, comments appearing at
the start of what would be a directive line have the effect of turning that
line into an ordinary source line, since the first token on the line is no
longer a #."

By the way, a workaround is to set CCACHE_CPP2 when compiling.

-- Joel


On 22 October 2013 15:59, Akim Demaille <akim at lrde.epita.fr> wrote:

> Hi!
>
> Since the comments are removed by the preprocessor, they
> don't make it to the compiler, which in turn cannot make
> warnings about comments.
>
> This source file has issues in genuine code, and in comments:
>
>     $ cat foo.cc
>     /// \param foo
>     static int i;
>     int main() {}
>
> The compiler complains about both (-Wdocumentation is really
> all about comments):
>
>     $ clang++-mp-3.4 -Wdocumentation -Wunused -c foo.cc
>     foo.cc:1:6: warning: '\param' command used in a comment that is not
> attached to a function declaration [-Wdocumentation]
>     /// \param foo
>          ^~~~~
>     foo.cc:1:14: warning: empty paragraph passed to '\param' command
> [-Wdocumentation]
>     /// \param foo
>         ~~~~~~~~~^
>     foo.cc:2:12: warning: unused variable 'i' [-Wunused-variable]
>     static int i;
>                ^
>     3 warnings generated.
>
> But ccache shows just one kind of issue:
>
>     $ ccache clang++-mp-3.4 -Wdocumentation -Wunused -c foo.cc
>     foo.cc:2:12: warning: unused variable 'i' [-Wunused-variable]
>     static int i;
>                ^
>     1 warning generated.
>
> Compilers such as gcc/clang support -C/-CC to leave comments
> in the preprocessed output.  Wouldn't it make sense to use it
> in ccache?  That would keep such warnings alive:
>
>     $ ccache clang++-mp-3.4 -Wdocumentation -Wunused -c -C foo.cc
>     clang: warning: argument unused during compilation: '-C'
>     foo.cc:1:6: warning: '\param' command used in a comment that is not
> attached to a function declaration [-Wdocumentation]
>     /// \param foo
>          ^~~~~
>     foo.cc:1:14: warning: empty paragraph passed to '\param' command
> [-Wdocumentation]
>     /// \param foo
>         ~~~~~~~~~^
>     foo.cc:2:12: warning: unused variable 'i' [-Wunused-variable]
>     static int i;
>                ^
>     3 warnings generated.
>
> Thanks!
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