Is < faster than <=?

Question

Is if( a < 901 ) faster than if( a <= 900 ).

Not exactly as in this simple example, but there are slight performance changes on loop

Answer 1

For floating point code, the <= comparison may indeed be slower (by one instruction) even on modern architectures. Here's the first function:

int compare_strict(double a, double b) { return a < b; }

On PowerPC, first this performs a floating point comparison (which updates cr, the condition register), then moves the condition register to a GPR, shifts the "compared less than" bit into place, and then returns. It takes four instructions.

Now consider this function instead:

int compare_loose(double a, double b) { return a <= b; }

This requires the same work as compare_strict above, but now there's two bits of interest: "was less than" and "was equal to." This requires an extra instruction (cror - condition register bitwise OR) to combine these two bits into one. So compare_loose requires five instructions, while compare_strict requires four.

You might think that the compiler could optimize the second function like so:

int compare_loose(double a, double b) { return ! (a > b); }

However this will incorrectly handle NaNs. NaN1 <= NaN2 and NaN1 > NaN2 need to both evaluate to false.