Popcount CPU instruction

Re:Popcount CPU instruction by userbinator on Sunday September 08, 2019 @11:52PM (#20915100)

The question of whether the CPU can identify an implementation of it and perform it quicker in dedicated hardware is more interesting, and leads to the argument for CISC --- if POPCNT was there since early x86 but microcoded, it would only get faster once the microarchitecture provides dedicated hardware, and there would be no need to recompile or detect CPU features. On older CPUs without the hardware, it wouldn't be any slower, since the CPU would internally execute the equivalent of a manual implementation, only without needing to fetch and decode each individual macro-instruction of it.

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Re:Popcount CPU instruction by xenadu02 on Monday September 09, 2019 @02:14AM (#20915605)

The argument for RISC was that the CPU was simpler but could be made much faster. That proved to be overly simplistic. Like the rest of computing it was solved with another layer of abstraction, where CISC processors break instructions into µ-ops.

It also turned out that memory latency would not scale so CISC can be thought of as an instruction stream compression mechanism. Specifically x86 decode is more or less a fixed size block but we have ever more transistors available.

Then RISC ended up adding in a bunch of the complexity it was supposedly going to avoid because in the real world we just need to make the damn thing fast, purity be damned. It didn't help that some architectures made really bad design decisions (hello branch delay slots). Also many RISC cpus have microcode and break down some instructions, though not nearly to the degree a classic CISC cpu does.

It's all very messy and all the armchair theorizing in the world turns out not to matter. What matters is taking the silicon and measuring your workload on it.

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Re:Popcount CPU instruction by zaarn on Monday September 09, 2019 @03:02AM (#20915844)

It's not that fast; IIRC the microcode implementation is either just a loop over all bits or a nibble/byte-wide lookup table for the result. It's not that fast, from my experience, atleast compared to manually doing it.

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