[arm][arm64] Improve CountTrailingZero implementation.

src/arm/simulator-arm.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.
 
 #include <stdarg.h>
 #include <stdlib.h>
 #include <cmath>
 
 #if V8_TARGET_ARCH_ARM
 
@@ skipping 2629 matching lines @@
   } else {
     if (instr->HasL()) {
       set_register(rd, ReadW(addr, instr));
     } else {
       WriteW(addr, get_register(rd), instr);
     }
   }
 }
 
 
+static uint32_t ReverseBits(uint32_t value) {

[titzer, 2016/02/14 11:18:34]

Good candidate for src/base/bits.cc?

[Rodolph Perfetta, 2016/02/15 17:56:01]

Done.

+  uint32_t result = 0;
+  for (unsigned i = 0; i < 32; ++i) {
+    result = (result << 1) | (value & 1);
+    value >>= 1;
+  }
+  return result;
+}
+
+
 void Simulator::DecodeType3(Instruction* instr) {
   int rd = instr->RdValue();
   int rn = instr->RnValue();
   int32_t rn_val = get_register(rn);
   bool shifter_carry_out = 0;
   int32_t shifter_operand = GetShiftRm(instr, &shifter_carry_out);
   int32_t addr = 0;
   switch (instr->PUField()) {
     case da_x: {
       DCHECK(!instr->HasW());
@@ skipping 253 matching lines @@
                         rm_val = (rm_val >> 16) | (rm_val << 16);
                         break;
                       case 3:
                         rm_val = (rm_val >> 24) | (rm_val << 8);
                         break;
                     }
                     set_register(rd, rn_val + (rm_val & 0xFFFF));
                   }
                 }
               } else {
-                UNIMPLEMENTED();
+                // PU == 0b01, BW == 0b11, Bits(9, 6) != 0b0001
+                if ((instr->Bits(20, 16) == 0x1f) &&
+                    (instr->Bits(11, 4) == 0xf3)) {
+                  // Rbit.
+                  uint32_t rm_val = get_register(instr->RmValue());
+                  set_register(rd, ReverseBits(rm_val));
+                } else {
+                  UNIMPLEMENTED();
+                }
               }
               break;
           }
         }
         return;
       }
       break;
     }
     case db_x: {
       if (instr->Bits(22, 20) == 0x5) {
@@ skipping 1281 matching lines @@
   set_register(sp, current_sp + sizeof(uintptr_t));
   return address;
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // USE_SIMULATOR
 
 #endif  // V8_TARGET_ARCH_ARM