Branch predictor: How many "if"s are too many? Including x86 and M1 benchmarks! cloudflare.com - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from cloudflare.com Daily Mail and Mail on Sunday newspapers.
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In January 2018, the entire computer industry was put on alert by two new processor vulnerabilities dubbed Meltdown and Spectre that defeated the fundamental OS security boundaries separating kernel and user space memory. The flaws stemmed from a performance feature of modern CPUs known as speculative execution and mitigating them required one of the biggest patch coordination efforts in history, involving CPU makers, device manufacturers and operating system vendors.
Meltdown and Spectre were certainly not the first vulnerabilities to result from a hardware design decision, but their widespread impact sparked the interest of the security research community into such flaws. Since then, many researchers, both from academia and the private sector, have been studying the low-level operation of CPUs and other hardware components and have been uncovering more and more issues.
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Let’s take a closer look at instruction fetch, decode and dispatch in the Cortex-A72 micro-architecture. These are the “front-end” stages of the core pipeline. The “back-end” of the pipeline consists of the register file(s), execution units and retirement (reorder) buffer. Branch prediction is frequently associated with the front-end since it directly affects instruction fetch.
[Update: This post is part 1 of a two part series. Part 2 discusses ARM Cortex-A72 execution and load/store operations.]
The front-end has one major job: Fetch ARMv8 instructions and keep the back-end execution units as busy as possible.
This page is required reading for Raspberry Pi 4 (BCM2711, ARM Cortex-A72) programmers who want to tune their programs for the ARM Cortex-A72. It is also necessary background information for programmers doing performance measurement with PERF (Performance Events for Linux) on Raspberry Pi 4.