tweak of Math.abs in its x64 stub

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/22285003

Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16113 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/ia32/stub-cache-ia32.cc

@@ -2479,6 +2479,8 @@ Handle<Code> CallStubCompiler::CompileMathAbsCall(
   STATIC_ASSERT(kSmiTag == 0);
   __ JumpIfNotSmi(eax, &not_smi);
 
+  // Branchless abs implementation, refer to below:
+  // http://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs
   // Set ebx to 1...1 (== -1) if the argument is negative, or to 0...0
   // otherwise.
   __ mov(ebx, eax);

src/x64/lithium-codegen-x64.cc

@@ -3449,7 +3449,7 @@ void LCodeGen::EmitIntegerMathAbs(LMathAbs* instr) {
 }
 
 
-void LCodeGen::EmitInteger64MathAbs(LMathAbs* instr) {
+void LCodeGen::EmitSmiMathAbs(LMathAbs* instr) {
   Register input_reg = ToRegister(instr->value());
   __ testq(input_reg, input_reg);
   Label is_positive;
@@ -3486,16 +3486,14 @@ void LCodeGen::DoMathAbs(LMathAbs* instr) {
   } else if (r.IsInteger32()) {
     EmitIntegerMathAbs(instr);
   } else if (r.IsSmi()) {
-    EmitInteger64MathAbs(instr);
+    EmitSmiMathAbs(instr);
   } else {  // Tagged case.
     DeferredMathAbsTaggedHeapNumber* deferred =
         new(zone()) DeferredMathAbsTaggedHeapNumber(this, instr);
     Register input_reg = ToRegister(instr->value());
     // Smi check.
     __ JumpIfNotSmi(input_reg, deferred->entry());
-    __ SmiToInteger32(input_reg, input_reg);
-    EmitIntegerMathAbs(instr);
-    __ Integer32ToSmi(input_reg, input_reg);
+    EmitSmiMathAbs(instr);
     __ bind(deferred->exit());
   }
 }

src/x64/lithium-codegen-x64.h

@@ -269,7 +269,7 @@ class LCodeGen BASE_EMBEDDED {
       uint32_t additional_index = 0);
 
   void EmitIntegerMathAbs(LMathAbs* instr);
-  void EmitInteger64MathAbs(LMathAbs* instr);
+  void EmitSmiMathAbs(LMathAbs* instr);
 
   // Support for recording safepoint and position information.
   void RecordSafepoint(LPointerMap* pointers,

src/x64/stub-cache-x64.cc

@@ -2246,26 +2246,25 @@ Handle<Code> CallStubCompiler::CompileMathAbsCall(
   Label not_smi;
   STATIC_ASSERT(kSmiTag == 0);
   __ JumpIfNotSmi(rax, &not_smi);
-  __ SmiToInteger32(rax, rax);
 
+  // Branchless abs implementation, refer to below:
+  // http://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs
   // Set ebx to 1...1 (== -1) if the argument is negative, or to 0...0
   // otherwise.
-  __ movl(rbx, rax);
-  __ sarl(rbx, Immediate(kBitsPerInt - 1));
+  __ movq(rbx, rax);
+  __ sar(rbx, Immediate(kBitsPerPointer - 1));
 
   // Do bitwise not or do nothing depending on ebx.
-  __ xorl(rax, rbx);
+  __ xor_(rax, rbx);
 
   // Add 1 or do nothing depending on ebx.
-  __ subl(rax, rbx);
+  __ subq(rax, rbx);
 
   // If the result is still negative, go to the slow case.
   // This only happens for the most negative smi.
   Label slow;
   __ j(negative, &slow);
 
-  // Smi case done.
-  __ Integer32ToSmi(rax, rax);
   __ ret(2 * kPointerSize);
 
   // Check if the argument is a heap number and load its value.