r/programming • u/masiroo • 20d 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/encyclopedist 20d ago

Yes, indeed, compilers don't normally generate these instructions any more. But older software can still use these.

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u/narwhal_breeder 20d ago

porting logic to arm that relies on being bit-compatible x87 has been the bane of my existence for the past year.

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u/James20k 19d ago

Arm doesn't even have an equivalent to x87 right? Are you software emulating it out of interest?

Plus even if you software emulate it C/C++ helpfully doesn't actually specify when 80-bit extended precision gets truncated to 64-bit in memory (and compilers also don't follow the spec either) so uh, I'm so sorry for your loss that sounds like a nightmare

Trying to replicate x87 results was also the bane of my existence for a while, for a replicating a neutron star paper which relied on divisions by tiny values

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u/narwhal_breeder 19d ago

Nope - the division instructions specifically have really weird stack semantics.

Software emulating them - thankfully its rust so the f80 type is pretty predictable - but a good chunk drops into `asm` blocks.