X64/Win64: Alternative implementation of fmod in general.

src/x64/disasm-x64.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 842 matching lines @@
           mnem = "fstp_s";
           break;
         default:
           UnimplementedInstruction();
       }
       AppendToBuffer("%s ", mnem);
       int count = PrintRightOperand(data + 1);
       return count + 1;
     }
   } else if (b1 == 0xDD) {
-    if ((b2 & 0xF8) == 0xC0) {
-      AppendToBuffer("ffree st%d", b2 & 0x7);
+    int mod, regop, rm;
+    get_modrm(*(data + 1), &mod, &regop, &rm);
+    if (mod == 3) {
+      switch (regop) {
+        case 0:
+          AppendToBuffer("ffree st%d", rm & 7);
+          break;
+        case 2:
+          AppendToBuffer("fstp st%d", rm & 7);
+          break;
+        default:
+          UnimplementedInstruction();
+          break;
+      }
       return 2;
     } else {
-      int mod, regop, rm;
-      get_modrm(*(data + 1), &mod, &regop, &rm);
       const char* mnem = "?";
       switch (regop) {
         case 0:
           mnem = "fld_d";
           break;
         case 3:
           mnem = "fstp_d";
           break;
         default:
           UnimplementedInstruction();
@@ skipping 703 matching lines @@
       fprintf(f, "%02x", *bp);
     }
     for (int i = 6 - (pc - prev_pc); i >= 0; i--) {
       fprintf(f, "  ");
     }
     fprintf(f, "  %s\n", buffer.start());
   }
 }
 
 }  // namespace disasm