[PATCH] D146973: [Clang] Implicitly include LLVM libc headers for the GPU

[Artem Belevich via Phabricator via cfe-commits]

tra added a subscriber: echristo.
tra added inline comments.


================
Comment at: clang/lib/Driver/ToolChains/Clang.cpp:1230
+      llvm::sys::path::append(P, "llvm-libc");
+      CmdArgs.push_back("-c-isystem");
+      CmdArgs.push_back(Args.MakeArgString(P));
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jhuber6 wrote:
> tra wrote:
> > Ensuring the right include order will be tricky. Interaction between the headers provided by llvm-libc vs the system headers will be interesting if we end up mixing the headers. It may be less problematic compared to the C++ standard library, but I doubt that mixing two implementations would work well here, either. 
> > 
> > So, the major question I have -- does adding include path here guarantees that we're not picking up the host headers? I do not see any changes that would exclude system headers from include paths.
> > If we do have include paths leading to both llvm-libc and the host headers, what's expected to happen if user code ends up including a header that's not present in  llvm-libc? Is that possible?
> > 
> Right now I'm just kind of relying on an expectation that since this will be the first `c-isystem` path set, then it will pull in these libraries first if they exist. It's not captured by the tests, but compiling with `-v` shows this path being used first in my experience. So, theoretically, if there is an implementation of said header in this location, it will be picked up before anything else. Otherwise it'll just search the other standard locations.
I think this will be a problem. We're cross-compiling here and for that to work reliably we need to make sure that only target headers are in effect. The problem is that we likely just do not have sufficiently complete set of headers for the GPU. Do we? I have no idea what exactly llvm-libc provides and whether it is sufficient for normal user code to cross-compile for a GPU. 

It would be interesting to try to compile some C++ code which would include commonly used, but generally target-agnostic, headers like <vector> <complex> <algorithm>, etc and check whether we end up pulling in any system headers. Then check what happens if we do not have system headers available at all.


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Comment at: clang/test/Driver/gpu-libc-headers.c:14
+// RUN:     FileCheck %s --check-prefix=CHECK-HEADERS
+// CHECK-HEADERS: "-cc1"{{.*}}"-c-isystem" "{{.*}}include/llvm-libc"
+
----------------
jhuber6 wrote:
> tra wrote:
> > I think here we want to test for not just the presence of `include/llvm-libc`, but also that it's in the correct position relative to other include paths. 
> > 
> > We may want something similar to what we have in clang/test/Driver/hip-include-path.hip
> Yeah, I wasn't sure if there was a good way to guarantee a certain path since those can change based on the system. Maybe `--sysroot`?
I do not have a good answer. 

@echristo -- when we need to cross-compile for some target, who/where/how tells clang where to get target-specific headers?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D146973/new/

https://reviews.llvm.org/D146973