GCC vs Clang: Same Instructions, Different Performance
This article explores a performance difference between GCC and Clang compilers, even when the generated assembly code is similar. The key factor is the efficiency of address generation and instruction scheduling.
Why it matters
Understanding the impact of compiler optimizations on low-level CPU performance is crucial for writing efficient code, especially in performance-critical applications.
Key Points
- 1GCC generates simpler addressing patterns, reducing AGU (Address Generation Unit) pressure
- 2Clang shows higher AGU pressure, leading to more stalls and less efficient scheduling
- 3It's not just about instruction count, but how efficiently the compiler feeds the CPU pipeline
Details
The article discusses a benchmark where the same code compiled with GCC consistently used fewer CPU cycles than the Clang-compiled version, despite having similar instruction counts and no vectorization. The author explains that the key difference lies in how the compilers handle address computations and instruction scheduling. On x86 CPUs, memory instructions rely on AGUs, and complex addressing patterns can increase AGU pressure, leading to more stalls and less efficient execution. GCC was able to generate simpler addressing patterns, reducing AGU contention and keeping the execution more consistent, while Clang showed higher AGU pressure. The author concludes that in tight loops, factors like AGU pressure, addressing patterns, and instruction scheduling can matter as much as or more than vectorization.
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