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[x86] Zen processors now marked with CPUX86_HAS_FAST_BT_MEM flag

J. Gareth "Kit" Moreton requested to merge CuriousKit/optimisations:zen-flags-update into main

Summary

This merge request adds the CPUX86_HAS_FAST_BT_MEM feature flag for Zen processors (-CpZEN and -OpZEN), since the BT instruction reading a memory operand is known to be efficient on this processor.

System

  • Processor architecture: i386, x86_64

What is the current bug behavior?

N/A

What is the behavior after applying this patch?

Future optimisations that check the CPUX86_HAS_FAST_BT_MEM flag will now be active on programs complied for Zen processors.

Additional notes

According to Agner Fog's Instruction Tables (https://www.agner.org/optimize/instruction_tables.pdf), page 90, BT mem, imm instructions (Intel notation) run in a single clock cycle. Annoyingly, this slows down to 2 clock cycles on Zen3 and later.

It is admittedly difficult to interpret some of the tables because the format differs between processors. It does seem to be the case though that BTx mem, reg (i.e. a variable bit index), including simple BT, are very inefficient no matter what the processor is.

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