branch-prediction | 易学教程

Can branch prediction cause illegal instruction?

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In the following pseudo-code: if (rdtscp supported by hardware) { Invoke "rdtscp" instruction } else { Invoke "rdtsc" instruction } Let's say the CPU does not support the rdtscp instruction and so we fallback to the else statement. If CPU mispredicts the branch, is it possible for the instruction pipeline to try to execute rdtscp and throw an Illgal Instruction error? It is explicitly documented for the #UD trap (Invalid Opcode Execution) in the Intel Processor Manuals, Volume 3A, chapter 6.15: In Intel 64 and IA-32 processors that implement out-of-order execution microarchitectures, this

How to measure mispredictions for a single branch on Linux?

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问题 I know that I can get the total percentage of branch mispredictions during the execution of a program with perf stat . But how can I get the statistics for a specific branch ( if or switch statement in C code)? 回答1: You can sample on the branch-misses event: sudo perf record -e branch-misses <yourapp> and then report it (and even selecting the function you're interested in): sudo perf report -n --symbols=<yourfunction> There you can access the annotated code and get some statistics for a

Performance of branch prediction in a loop

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Would there be any noticeable speed difference between these two snippets of code? Naively, I think the second snippet would be faster because branch instructions are encountered a lot less, but on the other hand the branch predictor should solve this problem. Or will it have a noticeable overhead despite the predictable pattern? Assume that no conditional move instruction is used. Snippet 1: for (int i = 0; i < 100; i++) { if (a == 3) output[i] = 1; else output[i] = 0; } Snippet 2: if (a == 3) { for (int i = 0; i < 100; i++) output[i] = 1; } else { for (int i = 0; i < 100; i++) output[i] = 0;

How to measure mispredictions for a single branch on Linux?

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I know that I can get the total percentage of branch mispredictions during the execution of a program with perf stat . But how can I get the statistics for a specific branch ( if or switch statement in C code)? You can sample on the branch-misses event: sudo perf record -e branch-misses <yourapp> and then report it (and even selecting the function you're interested in): sudo perf report -n --symbols=<yourfunction> There you can access the annotated code and get some statistics for a given branch. Or directly annotate it with the perf command with --symbol option. 来源： https://stackoverflow.com

How to deal with branch prediction when using a switch case in CPU emulation

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问题 I recently read the question here Why is it faster to process a sorted array than an unsorted array? and found the answer to be absolutely fascinating and it has completely changed my outlook on programming when dealing with branches that are based on Data. I currently have a fairly basic, but fully functioning interpreted Intel 8080 Emulator written in C, the heart of the operation is a 256 long switch-case table for handling each opcode. My initial thought was this would obviously be the

Performance of “conditional call” on amd64

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问题 When considering a conditional function call in a critical section of code I found that both gcc and clang will branch around the call. For example, for the following (admittedly trivial) code: int32_t __attribute__((noinline)) negate(int32_t num) { return -num; } int32_t f(int32_t num) { int32_t x = num < 0 ? negate(num) : num; return 2*x + 1; } Both GCC and clang compile to essentially the following: .global _f _f: cmp edi, 0 jg after_call call _negate after_call: lea rax, [rax*2+1] ret

Can I use GCC's __builtin_expect() with ternary operator in C

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The GCC manual only shows examples where __builtin_expect() is placed around the entire condition of an 'if' statement. I also noticed that GCC does not complain if I use it, for example, with a ternary operator, or in any arbitrary integral expression for that matter, even one that is not used in a branching context. So, I wonder what the underlying constraints of its usage actually are. Will it retain its effect when used in a ternary operation like this: int foo(int i) { return __builtin_expect(i == 7, 1) ? 100 : 200; } And what about this case: int foo(int i) { return __builtin_expect(i, 7

Branch target prediction in conjunction with branch prediction?

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问题 EDIT: My confusion arises because surely by predicting which branch is taken, you are effectively doing the target prediction too?? This question is intrinsically linked to my first question on the topic: branch prediction vs branch target prediction Looking at the accepted answer: Unconditional branch, fixed target Infinite loop goto statement break or continue statement End of the 'then' clause of an if/else statement (to jump past the else clause) Non-virtual function call Unconditional

Can I measure branch-prediction failures on a modern Intel Core CPU?

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问题 This question and its answer, which was recently tagged as an Epic Answer, has prompted me to wonder; Can I measure the performance of a running application in Windows in terms of its CPU branch prediction failures? I know that some static analysis tools exist, that might help with optimizing code for good performance in branch-prediction situations, and that manual techniques could help by simply making changes and re-testing, but I'm looking for some automatic mechanism that can report a

Intel x86 0x2E/0x3E Prefix Branch Prediction actually used?

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问题 In the latest Intel software dev manual it describes two opcode prefixes: Group 2 > Branch Hints 0x2E: Branch Not Taken 0x3E: Branch Taken These allow for explicit branch prediction of Jump instructions (opcodes like Jxx ) I remember reading a couple of years ago that on x86 explicit branch prediction was essentially a no-op in the context of gccs branch prediciton intrinsics. I am now unclear if these x86 branch hints are a new feature or whether they are essentially no-ops in practice. Can