Comment by chadaustin
3 months ago
https://dougallj.wordpress.com/2022/11/09/why-is-rosetta-2-f...
> Apple M1 has an undocumented extension that, when enabled, ensures instructions like ADDS, SUBS and CMP compute PF and AF and store them as bits 26 and 27 of NZCV respectively, providing accurate emulation with no performance penalty.
Perhaps another interesting aspect of this is that it’ll be Apple with their vertical stack that will decide when to physically remove this logic from the chips.
macOS 26 is the last OS with an Intel build. Presumably this means that in all likelihood, M6 chips will remove this functionality.
Why do you assume that dropping support for Intel hardware from the OS will coincide with dropping hardware features that help support for x86 applications? Have you not seen Apple's documentation that states they plan to retain some Rosetta functionality beyond macOS 27 for the sake of x86 games?
I think that documentation essentially demonstrates how Apple wants to put as little resources into it as possible without making users of popular applications mad.
They might even decide that they will be moving that functionality to software and decide to also leverage FEX.
I think that Apple’s overall mentality has traditionally been that they provide enough time for developers to transition applications but that they are not interested in maintaining support for unmaintained apps. That seems to be a very clear pattern of behavior.
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I am not sure I consider it that likely.
First, these features are a big draw for developers (a key macOS audience).
Second, the ability to run Windows games is not getting less valuable.
I don’t think Apple is catering to developers at all. If that were the case they wouldn’t be charging them $100 a year just to exist. They have the marketshare especially in iOS to force developers on to their platform.
I believe that running Windows games is something that Apple does not care about at all. Their efforts to create the game porting tool kit are 100% about getting more Windows games ported to the macOS App Store.
Oh yeah, maybe that one was too obscure for me. I don't think I've ever seen something use PF/AF…
You do want FEAT_AFP though, so you do want ARMv8.6+.
SETP is used rarely to compute parity, though it doesn't really save anything if you can use POPCNT. PF is also used by floating point comparisons with a different meaning though that is not useful for the Arm extension from Apple.
AF indeed is basically unused. The problem for both is that you need them for accurate emulation "just in case".
You can eliminate flag generation almost all the time with a little optimization (slash deoptimizing if you hit an unexpected use) but it certainly is less convenient to have to implement an optimizer.
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