I went down a rabbit hole and pulled myself back out. I narrowed it down to the
following.
We are using cmake to check if the linker supports ‘-Wl,—as-needed'. The test
compile (and link?) command cmake builds using ‘cmake -G Xcode’ does not fail.
The command from ‘cmake -G’ has been narrowed down to the following:
$
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
\
-Qunused-arguments \
-DWS_LD_FLAG_VALID0 \
-Wl,--as-needed \
-c src.c \
-o src.o
$
The following modified command generates an error:
$
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
\
-Qunused-arguments \
-DWS_LD_FLAG_VALID0 \
-Wl,--as-needed \
src.c
ld: unknown option: --as-needed
clang: error: linker command failed with exit code 1 (use -v to see invocation)
$
It appears the linker isn’t called when the -c and the -o options are present.
The following warning is generated when the ‘-Qunused-arguments’ is removed.
clang: warning: -Wl,--as-needed: 'linker' input unused
The cmake logic in CMakeLists.txt is basically:
SET(CMAKE_REQUIRED_FLAGS "${_FLAG}")
CHECK_C_SOURCE_RUNS(“int main() { return 0;}" ${_RESULT})
where _FLAG is set to ‘-DWS_LD_FLAG_VALID0 -Wl,—-as-needed’
So, how/where is the clang compile/link command generated and why isn’t it
being setup for clang to link?
> On Jan 20, 2016, at 3:22 PM, Guy Harris <[email protected]> wrote:
>
> On Jan 20, 2016, at 11:09 AM, David Morsberger <[email protected]> wrote:
>
>> This is what I am also tracking down. It has to do with the way cmake is
>> instantiating the compiler. The --serialized-diagnostics option is what
>> appears to be hiding the error.
>
> --serialize-diagnostics takes an argument, which is the name of a file to
> which the diagnostics are written, in a binary format.
>
> Clang documentation sucks. Srsly. The man page doesn't enumerate the -W
> options; the website:
>
>
> http://clang.llvm.org/docs/UsersManual.html#options-to-control-error-and-warning-messages
>
> does, but I'm not sure how to get the manual for an *arbitrary* version of
> Clang (the GCC folks are kind enough to provide online manuals, complete with
> listings of compiler flags, for multiple versions), and the word "serialize"
> appears nowhere on that page.
>
> It might be that, when the tests are done, ld is being run directly, rather
> than by the gcc/clang driver, and that -as-needed isn't getting passed to it.
> It obviously is getting passed when the actual *build* is done.
>
> Perhaps when CMake was being designed, its designers should have looked at
> the documentation for another configuration system - I think it's called
> "autoconf" or something such as that - and noticed that it offered not only
> macros to try compiling a file ("compiling" as in "turning a source file into
> an object file, without trying to link anything"), but macros to try
> *linking* things, which can come in handy if, say, to pick a purely
> *hypothetical* example, you're trying to see whether a given linker flag
> works.
>
> Or perhaps they mistakenly had the attitude that "we take care of all the
> platform details, so you don't have to".
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