Inline Doubles truncate and round.

runtime/vm/assembler_ia32.cc

Index: runtime/vm/assembler_ia32.cc
===================================================================
--- runtime/vm/assembler_ia32.cc	(revision 16607)
+++ runtime/vm/assembler_ia32.cc	(working copy)
@@ -809,6 +809,19 @@
 }
 
 
+void Assembler::roundsd(XmmRegister dst, XmmRegister src, RoundingMode mode) {
+  ASSERT(CPUFeatures::sse4_1_supported());
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0x66);
+  EmitUint8(0x0F);
+  EmitUint8(0x3A);
+  EmitUint8(0x0B);
+  EmitXmmRegisterOperand(dst, src);
+  // Mask precision exeption.
+  EmitUint8(static_cast<uint8_t>(mode) | 0x8);
+}
+
+
 void Assembler::fldl(const Address& src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitUint8(0xDD);
@@ -1728,6 +1741,28 @@
 }
 
 
+void Assembler::DoubleRound(XmmRegister dst, XmmRegister src, XmmRegister tmp) {
+  ASSERT(tmp != src);
+  static double kZeroFiveConst = 0.5;
+  static double kNegZeroFiveConst = -0.5;
+  Label is_negative, round;
+  if (src != dst) {
+    movsd(dst, src);
+  }
+  pxor(tmp, tmp);
+  comisd(src, tmp);
+  j(BELOW, &is_negative, Assembler::kNearJump);
+  addsd(dst,
+      Address::Absolute(reinterpret_cast<uword>(&kZeroFiveConst)));
+  jmp(&round, Assembler::kNearJump);
+  Bind(&is_negative);
+  addsd(dst,
+      Address::Absolute(reinterpret_cast<uword>(&kNegZeroFiveConst)));
+  Bind(&round);
+  roundsd(dst, dst, Assembler::kRoundToZero);

[Florian Schneider, 2013/01/08 14:24:04]

truncate(x+0.5) does not work for some corner cases like x=0.49999999999999994
where expected result of x.round() is 0.0.

Does roundsd with mode round-to-nearest not work for us otherwise?

[srdjan, 2013/01/10 22:35:52]

No, it doesn't. Round to nearest rounds 0.5 to 0 instead of 1. Wrote more
elaborate code (similar to V8) to handle special cases, added tests.

+}
+
+
 void Assembler::EnterFrame(intptr_t frame_size) {
   if (prologue_offset_ == -1) {
     prologue_offset_ = CodeSize();