Fix Math.round in runtime.cc and x64 optimized code.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 202e7a24fcd239a7fe47aacf26ac24f9fce5b33a..9e7e228dadb9138f393f7319128329d8391dffad 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -2721,33 +2721,44 @@ void LCodeGen::DoMathRound(LUnaryMathOperation* instr) {
   Register output_reg = ToRegister(instr->result());
   XMMRegister input_reg = ToDoubleRegister(instr->InputAt(0));
 
+  Label done;
   // xmm_scratch = 0.5
   __ movq(kScratchRegister, V8_INT64_C(0x3FE0000000000000), RelocInfo::NONE);
   __ movq(xmm_scratch, kScratchRegister);
-
+  NearLabel below_half;
+  __ ucomisd(xmm_scratch, input_reg);
+  __ j(above, &below_half);  // If input_reg is NaN, this doesn't jump.
   // input = input + 0.5
+  // This addition might give a result that isn't the correct for
+  // rounding, due to loss of precission, but only for a number that's

[William Hesse, 2011/04/13 09:12:45]

precision

[Lasse Reichstein, 2011/04/13 09:35:23]

Fixed, thanks.

+  // so big that the conversion below will overflow anyway.
   __ addsd(input_reg, xmm_scratch);
+  // Compute Math.floor(input).
+  // Use truncating instruction (OK because input is positive).
+  __ cvttsd2si(output_reg, input_reg);
+  // Overflow is signalled with minint.
+  __ cmpl(output_reg, Immediate(0x80000000));
+  DeoptimizeIf(equal, instr->environment());
+  __ jmp(&done);
 
-  // We need to return -0 for the input range [-0.5, 0[, otherwise
-  // compute Math.floor(value + 0.5).
+  __ bind(&below_half);
   if (instr->hydrogen()->CheckFlag(HValue::kBailoutOnMinusZero)) {
-    __ ucomisd(input_reg, xmm_scratch);
-    DeoptimizeIf(below_equal, instr->environment());
+    // Bailout if negative (including -0).
+    __ movq(output_reg, input_reg);
+    __ testq(output_reg, output_reg);
+    DeoptimizeIf(negative, instr->environment());
   } else {
-    // If we don't need to bailout on -0, we check only bailout
-    // on negative inputs.
-    __ xorpd(xmm_scratch, xmm_scratch);  // Zero the register.
+    // Bailout if below -0.5, otherwise round to (positive) zero, even
+    // if negative.
+    // xmm_scrach = -0.5
+    __ movq(kScratchRegister, V8_INT64_C(0xBFE0000000000000), RelocInfo::NONE);
+    __ movq(xmm_scratch, kScratchRegister);
     __ ucomisd(input_reg, xmm_scratch);
     DeoptimizeIf(below, instr->environment());
   }
+  __ xorl(output_reg, output_reg);
 
-  // Compute Math.floor(value + 0.5).
-  // Use truncating instruction (OK because input is positive).
-  __ cvttsd2si(output_reg, input_reg);
-
-  // Overflow is signalled with minint.
-  __ cmpl(output_reg, Immediate(0x80000000));
-  DeoptimizeIf(equal, instr->environment());
+  __ bind(&done);
 }