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Chapter 6 Branch Optimizations 133
Software Optimization Guide for AMD64 Processors
25112 Rev. 3.06 September 2005
6.5 Recursive Functions
Optimization
Use care when writing recursive functions.
Application
This optimization applies to:
• 32-bit software
• 64-bit software
Rationale
Returns are predicted as described in “Pairing CALL and RETURN,” so recursive functions should
be written carefully. If there are only recursive function calls within the function as shown in the
following example, the return address for each iteration of the recursive function is properly
predicted.
Preferred
long fac(long a)
{
if (a == 0) {
return (1);
} else {
return (a * fac(a – 1));
}
}
If there are any other calls within the recursive function (except to itself) as shown in the next
example, some returns can be mispredicted. If the number of recursive function calls plus the number
of nonrecursive function calls within the recursive function is greater than 12, the return stack does
not predict the correct return address for some of the returns once the recursion begins to unwind.