[libc-commits] [PATCH] D141426: [libc] Use a prebuilt libc-hdrgen binary if available.

[Siva Chandra via Phabricator via libc-commits]

sivachandra added inline comments.


================
Comment at: libc/CMakeLists.txt:155-156
-  # We need to set up hdrgen first since other targets depend on it.
-  add_subdirectory(utils/LibcTableGenUtil)
-  add_subdirectory(utils/HdrGen)
-endif()
----------------
jhuber6 wrote:
> sivachandra wrote:
> > jhuber6 wrote:
> > > sivachandra wrote:
> > > > jhuber6 wrote:
> > > > > sivachandra wrote:
> > > > > > jhuber6 wrote:
> > > > > > > ```
> > > > > > > CMake Error at /home/jhuber/Documents/llvm/llvm-project/libc/utils/HdrGen/CMakeLists.txt:3 (add_tablegen):
> > > > > > >   Unknown CMake command "add_tablegen".
> > > > > > > ```
> > > > > > > This gives me the following error when I run with `-DLLVM_ENABLE_RUNTIMES=libc`. I remember adding this code to work around it.
> > > > > > Ah, OK. One way to fix that problem is proposed here: https://reviews.llvm.org/D141460. Essentially, libc-hdrgen is a host tool and should be built separately and not as part of the libc build. As a workaround for now, you can try building libc-hdrgen separately and then do a full build with the `LIBC_HDRGEN_EXE` option specified.
> > > > > > 
> > > > > > Also, can you explain why you need full-build for GPU builds? For headers?
> > > > > Yes, it's primarily the headers since the user's system headers are most likely not going to be compatible. For example, on my machine if I try to include the standard headers while compiling for the GPU target it'll think it's a 32-bit system and try to include some stubs which don't exist.
> > > > > 
> > > > > The second reason is that I think in order to run the code on the GPU, we need a "loader" of some kind. Which is usually handled by a language runtime, i.e. `libcuda` or `libomptarget`, so I think that's in-line with a full build.
> > > > > 
> > > > > Is there a reason we don't want to just do what I did above? A runtimes build is somewhat different from a bootstrap since we can still reliably expect to be in-tree.
> > > > > ```
> > > > > if("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
> > > > >   include(TableGen)
> > > > > endif()
> > > > > ```
> > > > > 
> > > > The proposed patch that I linked to above solves this problem for the general case. In the general case, runtimes build/bootstrap build is a crossbuild. So, you cannot build libc-hdrgen for the target and run it at libc build time on the host. It should be built as a host tool separately and then used to do the actual runtimes build.
> > > > 
> > > > If we go with what you are suggesting, then we will build libc-hdrgen for the target as well, which is incorrect.
> > > I see, that makes sense for cross builds. So how do you get the `libc` header generator tool then? The patch linked just seems to error if it hasn't been added as a target. So how do we get the binary in the first place without first building `libc`?
> > The runtimes build for the libc full-build is currently broken in many ways without the proposed patch. So, I actually would say that until it is fixed, you should build clang separately and then do a standalone libc full build. That way, you will mimic the runtimes build but not have to deal with broken runtimes build for the full libc.
> The whole point of `-DLLVM_ENABLE_RUNTIMES=` is to remove the need for two-step builds, so it would be nice if we could get it to work. What's the problem with including the target? So, if I understand this correctly, the problem is that if the user wants to cross-build (e.g. x86_64 host but AArch64 target) then the `HdrGen` executable will be built for `AArch64` and thus not able to be run? Could we override the triple in the compilation?
Ah, if all you want is a host build runtimes build (or a libc GPU build), you can still do it without any changes: Just list the libc project in `LLVM_ENABLE_PROJECTS` and `LLVM_ENABLE_RUNTIMES`.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D141426/new/

https://reviews.llvm.org/D141426