[clang] Correctly link and optimize device libraries with -mlink-builtin-bitcode (PR #69371)

[Jacob Lambert via cfe-commits]

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@@ -1035,6 +1043,13 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
     }
   }
 
+  // Re-link against any bitcodes supplied via the -mlink-builtin-bitcode option
+  // Some optimizations may generate new function calls that would not have
+  // been linked pre-optimization (i.e. fused sincos calls generated by
+  // AMDGPULibCalls::fold_sincos.)
+  if (ClRelinkBuiltinBitcodePostop)
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lamb-j wrote:

I did look into having a file-specific option to link specific bitcodes post-optimization, and I think it is a good idea overall.

It is possible to come up with a specific set of bitcodes that need to be re-linked, based on the current optimizations we've implemented that introduce undefined functions. And we could then specify that set for a file-specific post-optimization linking.

But there are some downsides:
  - Any users would also need to know the specific set of bitcodes needed for post-optimization linking
  - We may need to hardcode that list in a bunch of different places, both within LLVM and external APIs
  - If a new optimization is implemented that expands the set of bitcodes needed for post-optimization linking, we'd need to go around and update any place that uses the file-specific options

Given that, I think the boolean relink option may serve us better for the time being

https://github.com/llvm/llvm-project/pull/69371