[PATCH] D69785: [OpenMP] Introduce the OpenMP-IR-Builder

[Michael Kruse via Phabricator via cfe-commits]

Meinersbur added inline comments.


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Comment at: llvm/include/llvm/IR/OpenMPKinds.def:186
+///{
+
+#ifndef OMP_IDENT_FLAG
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JonChesterfield wrote:
> jdoerfert wrote:
> > Meinersbur wrote:
> > > jdoerfert wrote:
> > > > JonChesterfield wrote:
> > > > > Sharing constants between the compiler and the runtime is an interesting subproblem. I think the cleanest solution is the one used by libc, where the compiler generates header files containing the constants which the runtime library includes.
> > > > I'd prefer not to tackle this right now but get this done first and revisit the issue later. OK?
> > > I don't think this is a good solution. It means that libomp cannot built built anymore without also building clang. Moreover, the values cannot be changed anyway since it would break the ABI.
> > > 
> > > I'd go the other route: The libomp defines what it's ABI is, the compiler generates code for it. 
> > This patch doesn't change what we do, just where. The numbers are hard coded in clang now. Let's start a discussion on the list and if we come up with a different scheme we do it after this landed.
> Revisit later sounds good.
> 
> @Meinersbur Do you know of an example of a non-llvm compiler using this libomp?
> 
> The usual order is build a compiler, then use it to build the runtime libraries, then the whole package can build other stuff. Provided the compiler doesn't need any of the runtime libraries (compiler-rt, maths libraries, libomp etc) itself the system bootstraps cleanly. Especially important when cross compiling and I suspect the gpu targets in openmp have similarly strict requirements on the first compiler.
> 
> Closely related to that, I tend to assume that the runtime libraries can be rewritten to best serve their only client - the associated compiler - so if libomp is used by out of tree compilers I'd like to know who we are at risk of breaking.
> Do you know of an example of a non-llvm compiler using this libomp?

[[ https://github.com/llvm-project/llvm-project/blob/master/openmp/runtime/src/kmp_gsupport.cpp | gcc  ]] (using libomp's gomp compatibility layer), [[ https://www.openmprtl.org/ | icc  ]] (as libomp was initially donated by Intel).

I don't understand why it even matters if there are other compilers using libomp. Every LLVM runtime library can be built stand-alone. 
With constant values being determined during compiler bootstrapping, programs built on one computer would be potentially ABI-incompatible with a runtime library on another. Think about updating your compiler-rt/libomp/libc++ on you computer causing all existing binaries on the system to crash because constants changed in the updated compiler's bootstrapping process.

The only use case I know that does this is are operating system's syscall tables. Linux's reference is [[ https://github.com/torvalds/linux/blob/master/arch/sh/include/uapi/asm/unistd_64.h | unistd.h ]] which is platform-specific and Windows generates the table during its [[ https://j00ru.vexillium.org/syscalls/nt/64/ | build process ]]. Therefore on Windows, system calls can only be done through ntdll. Even on Linux one should use the system's libc instead of directly invoking a system call.


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