MIPS: Implement flooring division by a constant via truncating division by a constant.

Port r20123 (31ab416)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/206373004

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20135 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/lithium-codegen-mips.cc

@@ -1367,8 +1367,31 @@ void LCodeGen::DoFlooringDivByConstI(LFlooringDivByConstI* instr) {
     DeoptimizeIf(eq, instr->environment(), dividend, Operand(zero_reg));
   }
 
-  // TODO(svenpanne) Add correction terms.
-  __ TruncatingDiv(result, dividend, divisor);
+  // Easy case: We need no dynamic check for the dividend and the flooring
+  // division is the same as the truncating division.
+  if ((divisor > 0 && !hdiv->CheckFlag(HValue::kLeftCanBeNegative)) ||
+      (divisor < 0 && !hdiv->CheckFlag(HValue::kLeftCanBePositive))) {
+    __ TruncatingDiv(result, dividend, Abs(divisor));
+    if (divisor < 0) __ Subu(result, zero_reg, result);
+    return;
+  }
+
+  // In the general case we may need to adjust before and after the truncating
+  // division to get a flooring division.
+  Register temp = ToRegister(instr->temp());
+  ASSERT(!temp.is(dividend) && !temp.is(result));
+  Label needs_adjustment, done;
+  __ Branch(&needs_adjustment, divisor > 0 ? lt : gt,
+            dividend, Operand(zero_reg));
+  __ TruncatingDiv(result, dividend, Abs(divisor));
+  if (divisor < 0) __ Subu(result, zero_reg, result);
+  __ jmp(&done);
+  __ bind(&needs_adjustment);
+  __ Addu(temp, dividend, Operand(divisor > 0 ? 1 : -1));
+  __ TruncatingDiv(result, temp, Abs(divisor));
+  if (divisor < 0) __ Subu(result, zero_reg, result);
+  __ Subu(result, result, Operand(1));
+  __ bind(&done);
 }
 
 

src/mips/lithium-mips.cc

@@ -1337,20 +1337,25 @@ LInstruction* LChunkBuilder::DoFlooringDivByConstI(HMathFloorOfDiv* instr) {
   ASSERT(instr->right()->representation().Equals(instr->representation()));
   LOperand* dividend = UseRegister(instr->left());
   int32_t divisor = instr->right()->GetInteger32Constant();
-  LInstruction* result =
-      DefineAsRegister(new(zone()) LFlooringDivByConstI(dividend, divisor));
-  bool can_deopt =
-      divisor == 0 ||
-      (instr->CheckFlag(HValue::kBailoutOnMinusZero) && divisor < 0);
-  return can_deopt ? AssignEnvironment(result) : result;
+  LOperand* temp =
+      ((divisor > 0 && !instr->CheckFlag(HValue::kLeftCanBeNegative)) ||
+       (divisor < 0 && !instr->CheckFlag(HValue::kLeftCanBePositive))) ?
+      NULL : TempRegister();
+  LInstruction* result = DefineAsRegister(
+      new(zone()) LFlooringDivByConstI(dividend, divisor, temp));
+  if (divisor == 0 ||
+      (instr->CheckFlag(HValue::kBailoutOnMinusZero) && divisor < 0)) {
+    result = AssignEnvironment(result);
+  }
+  return result;
 }
 
 
 LInstruction* LChunkBuilder::DoMathFloorOfDiv(HMathFloorOfDiv* instr) {
   if (instr->RightIsPowerOf2()) {
     return DoFlooringDivByPowerOf2I(instr);
-  } else if (false && instr->right()->IsConstant()) {
-    return DoFlooringDivByConstI(instr);  // TODO(svenpanne) Fix and re-enable.
+  } else if (instr->right()->IsConstant()) {
+    return DoFlooringDivByConstI(instr);
   } else {
     return DoDivI(instr);
   }

src/mips/lithium-mips.h

@@ -741,16 +741,17 @@ class LFlooringDivByPowerOf2I V8_FINAL : public LTemplateInstruction<1, 1, 0> {
 };
 
 
-class LFlooringDivByConstI V8_FINAL : public LTemplateInstruction<1, 1, 0> {
+class LFlooringDivByConstI V8_FINAL : public LTemplateInstruction<1, 1, 2> {
  public:
-  LFlooringDivByConstI(LOperand* dividend, int32_t divisor) {
+  LFlooringDivByConstI(LOperand* dividend, int32_t divisor, LOperand* temp) {
     inputs_[0] = dividend;
     divisor_ = divisor;
+    temps_[0] = temp;
   }
 
   LOperand* dividend() { return inputs_[0]; }
   int32_t divisor() const { return divisor_; }
-  LOperand* temp1() { return temps_[0]; }
+  LOperand* temp() { return temps_[0]; }
 
   DECLARE_CONCRETE_INSTRUCTION(FlooringDivByConstI, "flooring-div-by-const-i")
   DECLARE_HYDROGEN_ACCESSOR(MathFloorOfDiv)