[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
diff --git a/llvm/lib/Target/X86/X86Subtarget.h b/llvm/lib/Target/X86/X86Subtarget.h
index a274b79..c1f862d 100644
--- a/llvm/lib/Target/X86/X86Subtarget.h
+++ b/llvm/lib/Target/X86/X86Subtarget.h
@@ -199,6 +199,14 @@
/// of a YMM register without clearing the upper part.
bool HasFastPartialYMMWrite;
+ /// True if hardware SQRTSS instruction is at least as fast (latency) as
+ /// RSQRTSS followed by a Newton-Raphson iteration.
+ bool HasFastScalarFSQRT;
+
+ /// True if hardware SQRTPS/VSQRTPS instructions are at least as fast
+ /// (throughput) as RSQRTPS/VRSQRTPS followed by a Newton-Raphson iteration.
+ bool HasFastVectorFSQRT;
+
/// True if 8-bit divisions are significantly faster than
/// 32-bit divisions and should be used when possible.
bool HasSlowDivide32;
@@ -434,6 +442,8 @@
bool hasCmpxchg16b() const { return HasCmpxchg16b; }
bool useLeaForSP() const { return UseLeaForSP; }
bool hasFastPartialYMMWrite() const { return HasFastPartialYMMWrite; }
+ bool hasFastScalarFSQRT() const { return HasFastScalarFSQRT; }
+ bool hasFastVectorFSQRT() const { return HasFastVectorFSQRT; }
bool hasSlowDivide32() const { return HasSlowDivide32; }
bool hasSlowDivide64() const { return HasSlowDivide64; }
bool padShortFunctions() const { return PadShortFunctions; }