[PATCH] D17289: [X86] Fix False Data Dependency in popcnt

[David Kreitzer via llvm-commits]

DavidKreitzer added a comment.

Thanks for the comments, Steven! We are still working on fixing the popcnt false dependence problem, but we are planning to abandon this patch.

Our plan is to first fix the register allocator to bias register assignment choices to hide false dependences. If there is a true dependence on the destination register, then there is no additional cost for the false dependence. So, for example, we will strive to generate

  popcnt %rax, %rax
  popcnt (%rcx), %rcx

rather than

  xor %rdx, %rdx
  popcnt %rax, %rdx
  xor %rbx, %rbx
  popcnt (%rcx), %rbx

The second step will be to enhance the ExecutionDependencyFix pass to support popcnt, which will require us to add support for integer instructions in general. You are right that unconditionally adding an xor (excluding the cases where it is not legal to do so) is better than doing nothing. But since we already have a pass that is designed exactly for the purpose of deciding when it is or isn't profitable to add the xor, we ought to use it.

Note that the RA enhancements will also improve many of the instructions that are currently supported by the ExecutionDependencyFix pass, e.g.

  cvtss2sd %xmm1, %xmm0
  sqrtss %xmm2, %xmm3


Repository:
  rL LLVM

http://reviews.llvm.org/D17289