[PATCH] D21379: [X86] Heuristic to selectively build Newton-Raphson SQRT estimation

[Nikolai Bozhenov via llvm-commits]

n.bozhenov created this revision.
n.bozhenov added reviewers: bogner, hfinkel, andreadb, spatel, nadav.
n.bozhenov added subscribers: zansari, DavidKreitzer, zinovy.nis, llvm-commits.
Herald added a reviewer: tstellarAMD.
Herald added a subscriber: arsenm.

On modern Intel processors hardware SQRT in many cases is faster than RSQRT
followed by Newton-Raphson refinement. The patch introduces a simple heuristic
to choose between hardware SQRT instruction and Newton-Raphson software
estimation.

The patch treats scalars and vectors differently. The heuristic is that for
scalars the compiler should optimize for latency while for vectors it should
optimize for throughput.

Basically, the patch disables scalar NR for big cores and disables NR completely
for Skylake. Firstly, scalar SQRT has shorter latency than NR code in big cores.
Secondly, vector SQRT has been greatly improved in Skylake and has better
throughput compared to NR.


http://reviews.llvm.org/D21379

Files:
  include/llvm/Target/TargetLowering.h
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  lib/CodeGen/TargetLoweringBase.cpp
  lib/Target/AMDGPU/AMDGPUISelLowering.cpp
  lib/Target/AMDGPU/AMDGPUISelLowering.h
  lib/Target/X86/X86.td
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  lib/Target/X86/X86Subtarget.cpp
  lib/Target/X86/X86Subtarget.h
  test/CodeGen/X86/sqrt-fastmath-tune.ll

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