x86 Instruction Set Reference

Performs an SIMD compare of the two packed double-precision floating-point values in the source operand (second operand) and the destination operand (first operand) and returns the results of the comparison to the destination operand. The comparison predicate operand (third operand) specifies the type of comparison performed on each of the pairs of packed values. The result of each comparison is a quadword mask of all 1s (comparison true) or all 0s (comparison false). The source operand can be an XMM register or a 128-bit memory location. The destination operand is an XMM register. The comparison predicate operand is an 8-bit immediate the first 3 bits of which define the type of comparison to be made (see the following table); bits 4 through 7 of the immediate are reserved.

The unordered relationship is true when at least one of the two source operands being compared is a NaN; the ordered relationship is true when neither source operand is a NaN.

A subsequent computational instruction that uses the mask result in the destination operand as an input operand will not generate an exception, because a mask of all 0s corresponds to a floating-point value of +0.0 and a mask of all 1s corresponds to a QNaN.

Note that the processor does not implement the greater-than, greater-than-or-equal, not-greaterthan, and not-greater-than-or-equal relations. These comparisons can be made either by using the inverse relationship (that is, use the "not-less-than-or-equal" to make a "greater-than" comparison) or by using software emulation. When using software emulation, the program must swap the operands (copying registers when necessary to protect the data that will now be in the destination), and then perform the compare using a different predicate. The predicate to be used for these emulations is listed in the following table under the heading Emulation.

Compilers and assemblers may implement the following two-operand pseudo-ops in addition to the three-operand CMPPD instruction. See the following table.

The greater-than relations that the processor does not implement require more than one instruction to emulate in software and therefore should not be implemented as pseudo-ops. (For these, the programmer should reverse the operands of the corresponding less than relations and use move instructions to ensure that the mask is moved to the correct destination register and that the source operand is left intact.)

switch(ComparisonPredicate) {
	case 0:
		Operator = OperatorEqual;
		break;
	case 1:
		Operator = OperatorLessThan;
		break;
	case 2:
		Operator = OperatorLessOrEqual;
		break;
	case 3:
		Operator = OperatorUnordered;
		break;
	case 4:
		Operator = OperatorNotEqual;
		break;
	case 5:
		Operator = OperatorNotLessThan;
		break;
	case 6:
		Operator = OperatorNotLessOrEqual;
		break;
	case 7:
		Operator = OperatorOrdered;
		break;
	default:
		Reserved();
}
CMP0 = Destination[0..63] Operator Source[0..63];
CMP1 = Destination[64..127] Operator Source[64..127];
if(CMP0 == true) Destination[0..64] = 0xFFFFFFFFFFFFFFFF;
else Destination[0..63] = 0;
if(CMP1 == true) Destination[64..127] = 0xFFFFFFFFFFFFFFFF;
else Destination[64..127] = 0;