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

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. It is based on the assumption that throughput bound
code is likely to be vectorized.

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.

Differential Revision: https://reviews.llvm.org/D21379

llvm-svn: 277725

2 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
index a014549297f..b628d2a4c26 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
@@ -446,8 +446,6 @@ AMDGPUTargetLowering::AMDGPUTargetLowering(const TargetMachine &TM,
   setSelectIsExpensive(false);
   PredictableSelectIsExpensive = false;
 
-  setFsqrtIsCheap(true);
-
   // We want to find all load dependencies for long chains of stores to enable
   // merging into very wide vectors. The problem is with vectors with > 4
   // elements. MergeConsecutiveStores will attempt to merge these because x8/x16
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
index 206c93cd0bc..20775df1b10 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
@@ -166,6 +166,9 @@ public:
 
   const char* getTargetNodeName(unsigned Opcode) const override;
 
+  bool isFsqrtCheap(SDValue Operand, SelectionDAG &DAG) const override {
+    return true;
+  }
   SDValue getRsqrtEstimate(SDValue Operand,
                            DAGCombinerInfo &DCI,
                            unsigned &RefinementSteps,