At -O2, CorrelatedValuePropagation and SimplifyCFG fold a translated
shader's repeated tests of one value into switches whose cases jump past
their own join. Apple's GPU backend then runs out of control-flow IDs
("Max Unstruct ID 758 ... exceeds supported limit", a note: metal-tt
still exits 0) and lowers the function a far slower way. Odyssey's
alRenderMaterialUber took 24.6 minutes and 17.3 MB of applegpu_g13s code;
at -O1 it takes 31 s and 1.3 MB.
Sampled archive bases from seven games compile to code within a few
percent of -O2. Odyssey's full g13s archive builds in 15.6 minutes
(exit 0), its Cap Kingdom frames match -O2 within 3/255 on scattered
pixels, and GPU time per frame is within run-to-run noise.
Document the mechanism, and correct the Odyssey note: the 758 line never
failed the build; the reporter's exit 1 came from the BAHS link errors
fixed in ff44c59.
Refs #276
seedds
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