[X86] Remove LowerToHorizontalOp and update test coverage

[RKSimon]

LowerToHorizontalOp is a x86 DAG lowering that tries to vectorize horizontal add/sub code into SSE HADD/SUB vector instructions.

Its also a lot of X86ISelLowering code that is almost certainly unnecessary now.

SLP + VectorCombine between them should be handling all of this, so the backend is very unlikely to encounter real-world scalar patterns anymore.

So, this task involves:

• Disable LowerToHorizontalOp
• Review all CodeGen/X86 test changes
• Convert the changed CodeGen/X86 tests into similar PhaseOrdering/X86 tests to track the vectorized IR (for the various slow/fast SSE/AVX targets)
• Replace the CodeGen/X86 test coverage with the real vectorized IR
• Remove LowerToHorizontalOp and all its support methods
• Handle any CodeGen regressions (maybe a cost model, transform or backend fix)

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

LowerToHorizontalOp is a x86 DAG lowering that tries to vectorize horizontal add/sub code into SSE HADD/SUB vector instructions.

Its also a lot of X86ISelLowering code that is almost certainly unnecessary now.

SLP + VectorCombine between them should be handling all of this, so the backend is very unlikely to encounter real-world scalar patterns a anymore.

So, this task involves:

• Disable LowerToHorizontalOp
• Review all CodeGen/X86 test changes
• Convert the changed CodeGen/X86 tests into similar PhaseOrdering/X86 tests to track the vectorized IR (for the various slow/fast SSE/AVX targets)
• Replace the CodeGen/X86 test coverage with the real vectorized IR
• Remove LowerToHorizontalOp and all its support methods
• Handle any CodeGen regressions (maybe a cost model, transform or backend fix)

[houngkoungting]

Hi @RKSimon, I'd like to take on this task.

[RKSimon]

Good luck, this one is not going to be straightforward and you should expect a number of yak shaving PRs that will need to be pushed before we can remove it entirely.