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

[Johannes Doerfert via Phabricator via cfe-commits]

jdoerfert accepted this revision.
jdoerfert added a comment.
This revision is now accepted and ready to land.

LG, with or without the -lm lookup



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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)) {
----------------
arsenm wrote:
> jhuber6 wrote:
> > 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.
> It can be subtley wrong, and also pushes reliance on setting the global subtarget. It's best to ensure the default attributes are propagated to the device libraries
We will build a better solution for this soon, for now this is "good enough".


Repository:
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  https://reviews.llvm.org/D119841/new/

https://reviews.llvm.org/D119841