MIPS: Fixing MathPowHalf on ARM.

src/mips/lithium-codegen-mips.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
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@@ skipping 2978 matching lines @@
 void LCodeGen::DoMathSqrt(LUnaryMathOperation* instr) {
   DoubleRegister input = ToDoubleRegister(instr->InputAt(0));
   DoubleRegister result = ToDoubleRegister(instr->result());
   __ sqrt_d(result, input);
 }
 
 
 void LCodeGen::DoMathPowHalf(LUnaryMathOperation* instr) {
   DoubleRegister input = ToDoubleRegister(instr->InputAt(0));
   DoubleRegister result = ToDoubleRegister(instr->result());
-  DoubleRegister double_scratch = double_scratch0();
+  DoubleRegister temp = ToDoubleRegister(instr->TempAt(0));
+

[Yang, 2011/12/06 16:21:53]

From what I can see, you don't need to reserve an additional temp register (and
change register allocation in lithium-mips.cc) if you can already take advantage
of a double_scratch0().
Or is this double_scratch0() somehow related to kDoubleRegZero?

[kalmard, 2011/12/08 13:19:27]

I generally try to stick close to the ARM version because in my experience it
makes long-term maintenance easier. However if this has a performance impact or
you feel that the code is better the other way, I will obviously change it.
The change is not related to kDoubleRegZero.

+  ASSERT(!input.is(result));
+
+  // Note that according to ECMA-262 15.8.2.13:
+  // Math.pow(-Infinity, 0.5) == Infinity
+  // Math.sqrt(-Infinity) == NaN
+  Label done;
+  __ Move(temp, -V8_INFINITY);
+  __ BranchF(USE_DELAY_SLOT, &done, NULL, eq, temp, input);
+  // Set up Infinity in the delay slot.
+  // result is overwritten if the branch is not taken.
+  __ neg_d(result, temp);
 
   // Add +0 to convert -0 to +0.
-  __ mtc1(zero_reg, double_scratch.low());
-  __ mtc1(zero_reg, double_scratch.high());
-  __ add_d(result, input, double_scratch);
+  __ add_d(result, input, kDoubleRegZero);
   __ sqrt_d(result, result);
+  __ bind(&done);
 }
 
 
 void LCodeGen::DoPower(LPower* instr) {
   LOperand* left = instr->InputAt(0);
   LOperand* right = instr->InputAt(1);
   Register scratch = scratch0();
   DoubleRegister result_reg = ToDoubleRegister(instr->result());
   Representation exponent_type = instr->hydrogen()->right()->representation();
   if (exponent_type.IsDouble()) {
@@ skipping 1629 matching lines @@
   ASSERT(!environment->HasBeenRegistered());
   RegisterEnvironmentForDeoptimization(environment, Safepoint::kNoLazyDeopt);
   ASSERT(osr_pc_offset_ == -1);
   osr_pc_offset_ = masm()->pc_offset();
 }
 
 
 #undef __
 
 } }  // namespace v8::internal