Implemented the Word64Clz TurboFan operator for x64, arm64, and mips64.

src/x64/macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/bootstrapper.h"
 #include "src/codegen.h"
@@ skipping 3141 matching lines @@
   }
   Label not_zero_src;
   bsrl(dst, src);
   j(not_zero, &not_zero_src, Label::kNear);
   Set(dst, 63);  // 63^31 == 32
   bind(&not_zero_src);
   xorl(dst, Immediate(31));  // for x in [0..31], 31^x == 31 - x
 }
 
 
+void MacroAssembler::Lzcntq(Register dst, Register src) {
+  if (CpuFeatures::IsSupported(LZCNT)) {
+    CpuFeatureScope scope(this, LZCNT);
+    lzcntq(dst, src);
+    return;
+  }
+  Label not_zero_src;
+  bsrq(dst, src);
+  j(not_zero, &not_zero_src, Label::kNear);
+  Set(dst, 127);  // 127^63 == 64
+  bind(&not_zero_src);
+  xorl(dst, Immediate(63));  // for x in [0..63], 63^x == 63 - x
+}
+
+
+void MacroAssembler::Lzcntq(Register dst, const Operand& src) {
+  if (CpuFeatures::IsSupported(LZCNT)) {
+    CpuFeatureScope scope(this, LZCNT);
+    lzcntq(dst, src);
+    return;
+  }
+  Label not_zero_src;
+  bsrq(dst, src);
+  j(not_zero, &not_zero_src, Label::kNear);
+  Set(dst, 127);  // 127^63 == 64
+  bind(&not_zero_src);
+  xorl(dst, Immediate(63));  // for x in [0..63], 63^x == 63 - x
+}
+
+
 void MacroAssembler::Tzcntl(Register dst, Register src) {
   if (CpuFeatures::IsSupported(BMI1)) {
     CpuFeatureScope scope(this, BMI1);
     tzcntl(dst, src);
     return;
   }
   Label not_zero_src;
   bsfl(dst, src);
   j(not_zero, &not_zero_src, Label::kNear);
   Set(dst, 32);  // The result of tzcnt is 32 if src = 0.
@@ skipping 2200 matching lines @@
   movl(rax, dividend);
   shrl(rax, Immediate(31));
   addl(rdx, rax);
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64