[cfe-dev] Controlling usage of libc++ headers installed alongside the compiler

[Manoj Gupta via cfe-dev]

Hi Shoaib,

We have a similar problem in Chrome OS SDK (
https://bugs.chromium.org/p/chromium/issues/detail?id=948440 )
where we have libc++ headers installed for use in host while there are
target specific libc++ headers installed in
different locations. The host headers are picked even though we specify
--sysroot to point to target builds.

This hasn't been a major problem for us though since libc++ headers are
identical as long we
use the same version of libc++.

As for the choices: My preference is for a new flag but don't have a naming
preference.

Thanks,
Manoj

On Wed, Jun 26, 2019 at 2:26 PM Shoaib Meenai via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> If you’re building with -stdlib=libc++, the Linux toolchain (which is used
> for e.g. Android) prefers libc++ headers installed alongside the compiler
> to those in the sysroot:
> https://github.com/llvm/llvm-project/blob/d5f7196feb7da50564c5ad444f5fd3fecf08f340/clang/lib/Driver/ToolChains/Linux.cpp#L892.
> I have a use case where this doesn’t work: the Android NDK’s version of
> libc++ uses a different inline namespace, so I want to pair it with the
> corresponding headers from the sysroot, even though my toolchain also
> contains libc++ headers (for use with other platforms).
>
>
>
> I’m looking for a way to turn off searching for libc++ headers installed
> alongside the compiler but still search for them in the sysroot. The
> closest I can get right now is building with -nostdinc++ and adding a
> manual -isystem for the sysroot’s libc++ header directory, but that breaks
> compiles that want to use -nostdinc++ themselves and specify an alternate
> libc++ header location (since my -isystem will always be present, so you’ll
> end up with two copies of the libc++ headers in your search paths and
> everything will break).
>
>
>
> I can think of two ways to accomplish this. One is to add a new flag (and
> I’m looking for naming suggestions for this flag) to turn off searching for
> libc++ headers installed alongside the compiler. The other is to promote
> -cxx-isystem to a driver option, inhibit any -cxx-isystem paths from being
> added to the search paths when -nostdinc++ is passed, and make sure
> -cxx-isystem gets put on the include path before everything else (since C++
> headers usually #include_next into other system headers). What’s the best
> option here?
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