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Is < Faster than <=?

In a typical if statement using the comparison operator < or <=, it might be assumed that < would be faster because it has a single step or bound. However, this assumption is incorrect. The performance impact is negligible.

Machine Code Generation

On most architectures, including x86, integral comparisons employ the following machine instructions:

• A test or cmp instruction that modifies the EFLAGS register
• A jump instruction (Jcc) based on the comparison type

The jump instructions include jne (Jump if not equal), jz (Jump if zero), jg (Jump if greater), and others.

Example Code

Consider the following example:

if (a < b) {
    // Do something 1
}

and

if (a <= b) {
    // Do something 2
}

Comparing the assembly code generated by a compiler, we see a difference between the jump instructions used: jge versus jg.

if (a < b)
mov eax, [a]
cmp eax, [b]
jge .L2
// Do something 1

if (a <= b)
mov eax, [a]
cmp eax, [b]
jg .L5
// Do something 2

Therefore, the only difference in machine code is the jump instruction, indicating that the comparison itself takes the same amount of time.

Floating Point

The same principle applies to x87 floating-point comparisons. The compiler generates the same number of instructions for both < and <= comparisons.

Conclusion

Despite the difference in the number of operators (< versus <=), the performance difference in integral comparisons is insignificant on most architectures. The jump instructions employed (jge and jg) take the same amount of time to execute. The same holds true for floating-point comparisons.

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