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Quadrupling code performance with a "useless" if

A deep dive into a counterintuitive performance optimization: adding a seemingly redundant if statement to a tight loop quadruples its speed. The loop's bottleneck is a loop-carried dependency on j through a memory access, making it latency-bound. By introducing a branch that the CPU predicts as not-taken, the processor can speculatively execute iterations without waiting for the previous memory load to resolve. Since compilers optimize away such 'useless' branches, a volatile cast is used to prevent CSE elimination and preserve the branch in the generated code. A synthetic benchmark shows a 4x speedup, with more modest gains in realistic scenarios.

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#performance #hardware #c

Nguồn: https://purplesyringa.moe/blog/quadrupling-code-performance-with-a-useless-if. 8sync News chỉ tóm tắt và dẫn link; bản quyền nội dung thuộc tác giả và nguồn gốc.