Intel® 64 and IA-32 Architectures Optimization Reference Manual

GENERAL OPTIMIZATION GUIDELINESthrough; therefore, the BTB does not issue a prediction. The static predictor,however, will predict the branch to be taken, so a misprediction will not occur.Example 3-6. Static Taken PredictionBegin: mov eax, mem32and eax, ebximul eax, edxshld eax, 7jc BeginThe first branch instruction (JC BEGIN) in Example 3-7 is a conditional forwardbranch. It is not in the BTB the first time through, but the static predictor will predictthe branch to fall through. The static prediction algorithm correctly predicts that theCALL CONVERT instruction will be taken, even before the branch has any branchhistory in the BTB.Example 3-7. Static Not-Taken Predictionmov eax, mem32and eax, ebximul eax, edxshld eax, 7jc Beginmov eax, 0Begin: call ConvertThe Intel Core microarchitecture does not use the static prediction heuristic.However, to maintain consistency across Intel 64 and IA-32 processors, softwareshould maintain the static prediction heuristic as the default.3.4.1.4 Inlining, Calls and ReturnsThe return address stack mechanism augments the static and dynamic predictors tooptimize specifically for calls and returns. It holds 16 entries, which is large enoughto cover the call depth of most programs. If there is a chain of more than 16 nestedcalls and more than 16 returns in rapid succession, performance may degrade.The trace cache in Intel NetBurst microarchitecture maintains branch predictioninformation for calls and returns. As long as the trace with the call or return remainsin the trace cache and the call and return targets remain unchanged, the depth limitof the return address stack described above will not impede performance.To enable the use of the return stack mechanism, calls and returns must be matchedin pairs. If this is done, the likelihood of exceeding the stack depth in a manner thatwill impact performance is very low.3-11