[arm][arm64] Improve CountTrailingZero implementation.

src/arm/simulator-arm.cc

Index: src/arm/simulator-arm.cc
diff --git a/src/arm/simulator-arm.cc b/src/arm/simulator-arm.cc
index 563912bf6fa14e5c0d1dfb06fa033c097f288023..2b67c89f27d753414167d1a0dc010f990df90628 100644
--- a/src/arm/simulator-arm.cc
+++ b/src/arm/simulator-arm.cc
@@ -2647,6 +2647,16 @@ void Simulator::DecodeType2(Instruction* 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();
@@ -2920,7 +2930,15 @@ void Simulator::DecodeType3(Instruction* instr) {
                   }
                 }
               } 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;
           }