Need to switch to Opaque Pointers

[dbabokin]

Opaque Pointers is a controversial evolution of LLVM with unclear benefits (the declared benefits are more clear/simpler semantics, fewer number of bitcasts), which moves forward on unpredictable schedule (the most likely typed pointers will be deprecated / phased out after LLVM 14 branching). Nevertheless, it's a breaking feature, as it changes the core LLVM IR semantics. And it needs to be properly supported by ISPC in order to keep using new versions of LLVM.

The main concern with opaque pointers is undermined ability of the optimizer to reason about types. This mostly affects high-level optimizations and should have limited effect on the optimizations done by ISPC.

This thread seems to be the kick starter for opaque pointers discussions.

This commit llvm/llvm-project@184591a broke ISPC build with LLVM trunk (which is 14 at the moment), but more fundamental changes will follow.

[dbabokin]

Great post by Chris Lattner on opaque pointers and background of original decision to have high level information in the first place: link. In brief, LLVM IR is considered too low level for this type of information.

[pbrubaker]

Is this still an ongoing issue?

[aneshlya]

Yes, the work to support opaque pointers is in progress.

[aneshlya]

Related PRs so far:
#2357
#2360
#2363
#2365
#2368
#2374
#2377

[aneshlya]

Support of opaque pointers for CPU build was completed. Adjustments for Xe-enabled built will be done when dependencies are ready.