X87: [turbofan] Implemented the optional Float32RoundDown operator.

src/compiler/x87/code-generator-x87.cc

Index: src/compiler/x87/code-generator-x87.cc
diff --git a/src/compiler/x87/code-generator-x87.cc b/src/compiler/x87/code-generator-x87.cc
index c68d848f74ee1312d7d0e881bd3afa12500fb947..59832b43ff24cc94a821500292f8718b6db39196 100644
--- a/src/compiler/x87/code-generator-x87.cc
+++ b/src/compiler/x87/code-generator-x87.cc
@@ -834,6 +834,23 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
       __ lea(esp, Operand(esp, kFloatSize));
       break;
     }
+    case kX87Float32Round: {
+      RoundingMode mode =
+          static_cast<RoundingMode>(MiscField::decode(instr->opcode()));
+      // Set the correct round mode in x87 control register
+      __ X87SetRC((mode << 10));
+
+      if (!instr->InputAt(0)->IsDoubleRegister()) {
+        InstructionOperand* input = instr->InputAt(0);
+        USE(input);
+        DCHECK(input->IsDoubleStackSlot());
+        __ fstp(0);
+        __ fld_s(i.InputOperand(0));
+      }
+      __ frndint();
+      __ X87SetRC(0x0000);
+      break;
+    }
     case kX87Float64Add: {
       __ X87SetFPUCW(0x027F);
       __ fstp(0);