I talked about this a bit in the #talos-workstation channel over on Libera.Chat, but here goes: the short answer is that GZDoom works. All of the many libraries GZDoom takes advantage of run natively in Power, save for
Asmjit, which is used to speed up execution of ZScript. I was wrong in the crispy-doom thread: enforcing -mcpu=power9 made a significant performance difference over the stock build options, but -DNO_WARN_X86_INTRINSICS makes no difference because of a prior
commit that disables intrinsics outright for non-x86 architectures (apparently tied to disabling AsmJit for those platforms). With that limitation in mind, the biggest single issue for GZDoom speed comes down to a lack of SIMD optimization, which apparently makes a small difference in the hardware renderer and a
very big difference in the true color software renderer. From what I've read the 8-bit software renderer doesn't leverage SSE2, and it flies at 1440p or higher on my eight core machine.
GZDoom currently detects Power9 as ppc64. Defining ppc64le as a platform and then forcing -DNO_WARN_X86_INTRINSICS specifically for it would probably be a lot less demanding than generating hand-tuned Power assembly, but I do not know whether that would break the current method of disabling AsmJit. Whether this would work on Power8 is up for debate - I don't know if its lack of VSX-3 would impact SSE translation. I need to allocate more time to scrutinize the source and figure out what needs patching, but if anybody wants to beat me to the punch, they are welcome to.
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