branch-prediction

问题 I am trying to understand how does a branch prediction unit work in a CPU. I have used papi and also linux's perf-events but both of them do not give accurate results (for my case). This is my code: void func(int* arr, int sequence_len){ for(int i = 0; i < sequence_len; i++){ // region starts if(arr[i]){ do_sth(); } // region ends } } My array consists of 0's and 1's. It has a pattern with a size of sequence_len . For example, if my size is 8, then it has a pattern of 0 1 0 1 0 0 1 1 or

问题 Alright, so I know that if a particular conditional branch has a condition that takes time to compute (memory access, for instance), the CPU assumes a condition result and speculatively executes along that path. However, what would happen if, along that path, yet another slow conditional branch pops up (assuming, of course, that the first condition hasn't been resolved yet and the CPU can't just commit the changes)? Does the CPU just speculate inside the speculation? What happens if the last

问题 Alright, so I know that if a particular conditional branch has a condition that takes time to compute (memory access, for instance), the CPU assumes a condition result and speculatively executes along that path. However, what would happen if, along that path, yet another slow conditional branch pops up (assuming, of course, that the first condition hasn't been resolved yet and the CPU can't just commit the changes)? Does the CPU just speculate inside the speculation? What happens if the last

问题 Alright, so I know that if a particular conditional branch has a condition that takes time to compute (memory access, for instance), the CPU assumes a condition result and speculatively executes along that path. However, what would happen if, along that path, yet another slow conditional branch pops up (assuming, of course, that the first condition hasn't been resolved yet and the CPU can't just commit the changes)? Does the CPU just speculate inside the speculation? What happens if the last

问题 My question is how they co-exist and work together in modern CPU architecture? 回答1: You've got it slightly reversed. On every fetch you index into your branch predictor, which tells you whether the instruction that you have just received will be decoded into a taken branch. If not, you fetch the next sequential address. But if your branch predictor says that it will be a taken branch, you don't know which instruction to fetch next, since you haven't decoded this instruction yet. So in order

问题 My question is how they co-exist and work together in modern CPU architecture? 回答1: You've got it slightly reversed. On every fetch you index into your branch predictor, which tells you whether the instruction that you have just received will be decoded into a taken branch. If not, you fetch the next sequential address. But if your branch predictor says that it will be a taken branch, you don't know which instruction to fetch next, since you haven't decoded this instruction yet. So in order

问题 My question is how they co-exist and work together in modern CPU architecture? 回答1: You've got it slightly reversed. On every fetch you index into your branch predictor, which tells you whether the instruction that you have just received will be decoded into a taken branch. If not, you fetch the next sequential address. But if your branch predictor says that it will be a taken branch, you don't know which instruction to fetch next, since you haven't decoded this instruction yet. So in order

问题 My question is how they co-exist and work together in modern CPU architecture? 回答1: You've got it slightly reversed. On every fetch you index into your branch predictor, which tells you whether the instruction that you have just received will be decoded into a taken branch. If not, you fetch the next sequential address. But if your branch predictor says that it will be a taken branch, you don't know which instruction to fetch next, since you haven't decoded this instruction yet. So in order

来源： https://stackoverflow.com/questions/56412615/what-does-it-mean-by-a-branch-penalty

来源： https://stackoverflow.com/questions/58399395/how-does-branch-prediction-affect-performance-in-r