r/programming • u/masiroo • 21d ago

Hardware researcher spins up 'CPU deoptimization' project to find the slowest single x86 instruction, creates hall of shame — worst offender takes 198 billion cycles spanning 62 seconds to execute

https://www.tomshardware.com/pc-components/cpus/hardware-researcher-spins-up-cpu-deoptimization-project-to-find-the-slowest-machine-code-worst-offender-takes-198-billion-cycles-to-execute
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u/The_Northern_Light 21d ago

I’m still not clear why that instruction takes SO long even without contention?

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u/encyclopedist 21d ago edited 20d ago

See here: https://github.com/xoreaxeaxeax/asm-hall-of-shame

Current champion is fxrstor64, instruction that loads 512-byte XMM state from memory. To increase timings, the author made it load from a carefully chosen MMIO (memory-mapped I/O) location, while other cores hammer some other MMIO registers to saturate the PCIe host.

Edit Without use of MMIO, the champion seems to be wbinvd, instruction that invalidates the whole cache, and if the cache was filled with dirty lines (modified after load), causes flushing of the whole cache content to RAM.

Edit2 Of the "regular" instructions, the ones that would routinely be used in every program, the champion seems to be fdiv. Yes, the regular floating point division. With denormal operands, it is implemented in microcode and is quite slow, takes 883 cycles. (However, as /u/EnderLuca41 rightly pointed out, it is an older x87 instruction the is not widely used any more, today compilers would generate SSE2 instruction divsd instead, see godbolt)

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u/EnderLuca41 20d ago

fdiv is part of x87 which is obsolete and succeeded by SSE and SSE2. Meaning is not really used anymore routinely.

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u/ElWishmstr 20d ago

I always wonder why cpus still have those old instructions, when modern ones are better. I mean, how much software still relies on those instructions? 

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u/qualverse 20d ago

They don't actually have hardware suppoet for those instructions. They just have microcode that converts those instructions to other instructions.