[X86] Take advantage of the lzcnt instruction on btver2 architectures when ORing comparisons to zero.

This change adds transformations such as:
  zext(or(setcc(eq, (cmp x, 0)), setcc(eq, (cmp y, 0))))
  To:
  srl(or(ctlz(x), ctlz(y)), log2(bitsize(x))
This optimisation is beneficial on Jaguar architecture only, where lzcnt has a good reciprocal throughput.
Other architectures such as Intel's Haswell/Broadwell or AMD's Bulldozer/PileDriver do not benefit from it.
For this reason the change also adds a "HasFastLZCNT" feature which gets enabled for Jaguar.

Differential Revision: https://reviews.llvm.org/D23446

llvm-svn: 284248

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86Subtarget.cpp b/llvm/lib/Target/X86/X86Subtarget.cpp
index 156c0b99a2b..91fdc5afe8a 100644
--- a/llvm/lib/Target/X86/X86Subtarget.cpp
+++ b/llvm/lib/Target/X86/X86Subtarget.cpp
@@ -284,6 +284,7 @@ void X86Subtarget::initializeEnvironment() {
   HasFastPartialYMMWrite = false;
   HasFastScalarFSQRT = false;
   HasFastVectorFSQRT = false;
+  HasFastLZCNT = false;
   HasSlowDivide32 = false;
   HasSlowDivide64 = false;
   PadShortFunctions = false;