[PATCH] D119841: [OpenMP] Pass AMDGPU math libraries into the linker wrapper

[Joseph Huber via Phabricator via cfe-commits]

jhuber6 added inline comments.


================
Comment at: clang/lib/Driver/ToolChains/Clang.cpp:8195
+  // Get the AMDGPU math libraries.
+  // FIXME: This method is bad, remove once AMDGPU has a proper math library.
+  for (auto &I : llvm::make_range(OpenMPTCRange.first, OpenMPTCRange.second)) {
----------------
jdoerfert wrote:
> jhuber6 wrote:
> > jdoerfert wrote:
> > > Can you elaborate on this comment, what's bad, how would the better version look
> > It's explained in more detail where this is done for the AMDGPU ToolChain, e.g.
> > ```
> >       // This is not certain to work. The device libs added here, and passed to    
> >       // llvm-link, are missing attributes that they expect to be inserted when    
> >       // passed to mlink-builtin-bitcode. The amdgpu backend does not generate    
> >       // conservatively correct code when attributes are missing, so this may    
> >       // be the root cause of miscompilations. Passing via mlink-builtin-bitcode    
> >       // ultimately hits CodeGenModule::addDefaultFunctionDefinitionAttributes    
> >       // on each function, see D28538 for context.    
> >       // Potential workarounds:    
> >       //  - unconditionally link all of the device libs to every translation    
> >       //    unit in clang via mlink-builtin-bitcode    
> >       //  - build a libm bitcode file as part of the DeviceRTL and explictly    
> >       //    mlink-builtin-bitcode the rocm device libs components at build time    
> >       //  - drop this llvm-link fork in favour or some calls into LLVM, chosen    
> >       //    to do basically the same work as llvm-link but with that call first    
> >       //  - write an opt pass that sets that on every function it sees and pipe    
> >       //    the device-libs bitcode through that on the way to this llvm-link
> > ```
> > Should I copy the gist here?
> Is it still relevant? We don't use llvm-link here, do we?
> 
> @arsenm, the backend is (almost) OK with the lack of attributes, is it not? 
Linking is done using LTO now, I don't know exactly how they merge bitcode compared to llvm-link but I'm assuming it's similar.


================
Comment at: clang/lib/Driver/ToolChains/Clang.cpp:8205
+    if (llvm::find(LibraryArgs, "m") == LibraryArgs.end() && !D.CCCIsCXX())
+      continue;
+
----------------
jdoerfert wrote:
> I'd switch the conditions.
> 
> More importantly, does this require that the user passes -lm to the linker invocation? I'm not convinced we should not always link these in.
Yes, would save some time assuming most codes are C++

So I figured I'd copy the same semantics of how `-lm` works where you need to specify it for C but not C++. We could just pass this in all the time, but since linking it in currently required `-lm` I copied that.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D119841/new/

https://reviews.llvm.org/D119841