What are the best techniques for optimizing low-level code on x86 CPUs?

1 Know your CPU

The first step to optimize your low-level code is to know your CPU and its features. Different x86 CPUs have different capabilities, such as cache size, instruction sets, pipelines, branch prediction, and hyper-threading. You should be aware of the specifications and limitations of your target CPU, and tailor your code accordingly. For example, you can use CPUID instruction to detect the available instruction sets, such as SSE, AVX, or BMI, and use them to perform vector operations, bit manipulation, or hashing. You can also use cache alignment and prefetching techniques to reduce cache misses and improve memory access.

2 Use registers wisely

The second step to optimize your low-level code is to use registers wisely. Registers are the fastest and most limited memory locations in the CPU, where you can store and manipulate data. You should use registers as much as possible, and avoid unnecessary memory access or spills. You should also follow the calling conventions and register usage conventions of your platform, such as Windows or Linux, and your compiler, such as GCC or MSVC. For example, you should use RAX, RCX, RDX, and R8-R11 for volatile registers, and RBX, RBP, RDI, RSI, and R12-R15 for non-volatile registers. You should also use XMM0-XMM7 for floating-point or vector parameters and return values.

3 Optimize loops and branches

The third step to optimize your low-level code is to optimize loops and branches. Loops and branches are the most common sources of performance bottlenecks, as they can cause pipeline stalls, branch mispredictions, or instruction cache misses. You should try to minimize the number and complexity of loops and branches, and use techniques such as loop unrolling, loop invariant code motion, loop fusion, loop interchange, or loop tiling to improve the efficiency of loops. You should also use techniques such as branch prediction hints, branch fusion, branch alignment, or branch elimination to improve the efficiency of branches.

4 Use inline assembly and intrinsics

The fourth step to optimize your low-level code is to use inline assembly and intrinsics. Inline assembly and intrinsics are ways to embed assembly code or instructions into your high-level code, such as C or C++. They allow you to access the low-level features of the CPU that are not exposed by the high-level language, such as registers, flags, or instructions. They also allow you to bypass the compiler optimizations that may not be optimal for your specific case. You should use inline assembly and intrinsics sparingly, and only when you are sure that they can improve the performance of your code. You should also test and benchmark your code to verify the results.

5 Use compiler flags and options

The fifth step to optimize your low-level code is to use compiler flags and options. Compiler flags and options are ways to control the behavior and output of the compiler, such as optimization level, target architecture, or debug information. They can affect the quality and size of the generated code, as well as the runtime performance and compatibility. You should use compiler flags and options carefully, and understand their implications and trade-offs. For example, you can use -O3 flag to enable the highest level of optimization, but it may increase the code size and compilation time. You can also use -march, -mtune, or -mcpu flags to specify the target CPU and instruction sets, but it may reduce the portability of your code.

6 Learn from the experts

The sixth and final step to optimize your low-level code is to learn from the experts. There are many resources and examples of low-level code optimization on the internet, such as blogs, books, articles, or open-source projects. You can learn from the best practices, tips, tricks, and techniques of the experts, and apply them to your own code. You can also use tools such as disassemblers, debuggers, profilers, or analyzers to inspect, debug, and optimize your code. You can also join communities and forums where you can ask questions, share ideas, and get feedback from other programmers.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?