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Microbenchmarking return address branch prediction (2018)
processors use branch predictors to predict a program’s control flow in order to execute further ahead in the instruction stream, for better performance. Function return instructions usually use a specialized branch predictor called a Return Address Stack (RAS), Return Stack Buffer (RSB), return stack, or other various names.
To test the basic operation of the return address predictor, we’ll start with a simple microbenchmark (shown in Figure 1) that simply calls an empty function repeatedly. In the following code listing ( Figure 3), the function get_current_ip appears to have an extra call that would cause the RAS predictor to be misaligned (and also mispredict the return) if call-to-next-instruction were not handled as a special case. In an attempt to test the repair abilities of the RAS, the mispredicted microbenchmark from Figure 4 is modified to encourage the processor to execute many calls and returns speculatively that will then be squashed due to a pipeline flush.
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