branch-prediction

问题 I have this memchr code that I'm trying to make non-branching: .globl memchr memchr: mov %rdx, %rcx mov %sil, %al cld repne scasb lea -1(%rdi), %rax test %rcx, %rcx cmove %rcx, %rax ret I'm unsure whether or not cmove is a branching instruction. Is it? If so, how do I rearrange my code so it doesn't branch? 回答1: No, it's not a branch, that's the whole point of cmovcc . It's an ALU select that has a data dependency on both inputs, not a control dependency . (With a memory source, it

问题 For those that have already measured or have deep knowledge about this kind of considerations, assume that you have to do the following (just to pick any for the example) floating-point operator: float calc(float y, float z) { return sqrt(y * y + z * z) / 100; } Where y and z could be denormal numbers, let's assume two possible situations where just y, just z, or maybe both, in a totally random manner, can be denormal numbers 50% of the time <1% of the time And now assume I want to avoid the

问题 CPU's use branch prediction to speed up code, but only if the first branch is actually taken. Why not simply take both branches? That is, assume both branches will be hit, cache both sides, and the take the proper one when necessary. The cache does not need to be invalidated. While this requires the compiler to load both branches before hand(more memory, proper layout, etc), I imagine that proper optimization could streamline both so that one can get near optimal results from a single

问题 The issue of the repz ret has been covered here [1] as well as in other sources [2, 3] quite satisfactorily. However, reading neither of these sources, I found answers to the following: What is the actual penalty in a quantitative comparison with ret or nop; ret ? Especially in the latter case – is decoding one extra instruction (and an empty one at that!) really relevant, when most functions either have 100+ of those or get inlined? Why did this never get fixed in AMD K8, and even made its

问题 I am trying to understand when branch predictor entries are invalidated. Here are the experiments I have done: Code1: start_measure_branch_mispred() while(X times): if(something something): do_useless() endif endwhile end_measurement() store_difference() So, I am running this code a number of times. I can see that after the first run, the misprediction rates go lower. The branch predictor learns how to predict correctly. But, if I run this experiment again and again (i.e. by writing .

问题 I am trying to understand when branch predictor entries are invalidated. Here are the experiments I have done: Code1: start_measure_branch_mispred() while(X times): if(something something): do_useless() endif endwhile end_measurement() store_difference() So, I am running this code a number of times. I can see that after the first run, the misprediction rates go lower. The branch predictor learns how to predict correctly. But, if I run this experiment again and again (i.e. by writing .

问题 I am trying to understand when branch predictor entries are invalidated. Here are the experiments I have done: Code1: start_measure_branch_mispred() while(X times): if(something something): do_useless() endif endwhile end_measurement() store_difference() So, I am running this code a number of times. I can see that after the first run, the misprediction rates go lower. The branch predictor learns how to predict correctly. But, if I run this experiment again and again (i.e. by writing .

问题 In Volume 3 of the Intel Manuals it contains the description of a hardware event counter: BACLEAR_FORCE_IQ Counts number of times a BACLEAR was forced by the Instruction Queue. The IQ is also responsible for providing conditional branch prediction direction based on a static scheme and dynamic data provided by the L2 Branch Prediction Unit. If the conditional branch target is not found in the Target Array and the IQ predicts that the branch is taken, then the IQ will force the Branch Address

问题 I stumbled upon this question/answer which mentions that in most languages, logical operators such as: x == y && doSomething(); can be faster than doing the same thing with an if branch: if(x == y) { doSomething(); } Similarly, it says that the ternary operator: x = y == z ? 0 : 1 is usually faster than using an if branch: if(y == z) { x = 0; } else { x = 1; } This got me Googling, which led me to this fantastic answer which explains branch prediction. Basically, what it says is that the CPU

问题 I stumbled upon this question/answer which mentions that in most languages, logical operators such as: x == y && doSomething(); can be faster than doing the same thing with an if branch: if(x == y) { doSomething(); } Similarly, it says that the ternary operator: x = y == z ? 0 : 1 is usually faster than using an if branch: if(y == z) { x = 0; } else { x = 1; } This got me Googling, which led me to this fantastic answer which explains branch prediction. Basically, what it says is that the CPU