Performance – Is 1 < 10 Comparison Less Expensive Than 1 < 1000000?

cpuperformance

I just used ~1 billion as the count for a z-index in CSS, and was thinking about the comparisons that must go on. Is there a difference in performance on the ALU level in comparisons between very large numbers vs very small ones?

For example, would one of these two snippets be more expensive than the other?

snippet 1

for (int i = 0; i < 10000000; i++){
    if (i < 10000000000000) {
        //do nothing
    }
}

snippet 2

for (int i = 0; i < 10000000; i++){
    if (i < 1000) {
        //do nothing
    }
}

Best Answer

Every processor I've worked on does comparison by subtracting one of the operands from the other, discarding the result and leaving the processor's flags (zero, negative, etc.) alone. Because subtraction is done as a single operation, the contents of the operands don't matter.

The best way to answer the question for sure is to compile your code into assembly and consult the target processor's documentation for the instructions generated. For current Intel CPUs, that would be the Intel 64 and IA-32 Architectures Software Developer’s Manual.

The description of the CMP ("compare") instruction is in volume 2A, page 3-126, or page 618 of the PDF, and describes its operation as:

temp ← SRC1 − SignExtend(SRC2);
ModifyStatusFlags; (* Modify status flags in the same manner as the SUB instruction*)

This means the second operand is sign-extended if necessary, subtracted from the first operand and the result placed in a temporary area in the processor. Then the status flags are set the same way as they would be for the SUB ("subtract") instruction (page 1492 of the PDF).

There's no mention in the CMP or SUB documentation that the values of the operands have any bearing on latency, so any value you use is safe.

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