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Re: adding COPTS to shrink kernel binary size

Den 2023-02-09 kl. 15:05, skrev Izumi Tsutsui:
uwe@ wrote:

One concern of using -Os is inline functions.

In netbsd-6 days, m68k ports also used -Os but I noticed some
applications (especially mlterm-fb on slow luna68k framebuffers)
were much slower (~30%) with -Os because inline functions were used
as macro in loops for screen scroll rendering copy ops:
If they are intended to be always inlined, they should be declared
with the always_inline attribute.  -Os doesn't turn off inlining in
general, but since inlining usually does increase code size you need
to tell the compiler.  (E.g. sh3 kernel cache code does this b/c cache
must be accessed by code in uncached P2, so macro-like inline
functions must be inlined, or they will be called via their normal
addresses in the cached space.)
I know the sh3 case.  If the functions need essencially to be inlined,
"always_inline" is mandatory.

On the other hand, if a port has an explicit upper size limit
(like sun2 and sun3 due to bootloaders) -Os is the only choise.

In vax case, not all VAXen have size restrictions, especially
simh users. I'm afraid such users (who are building packages etc.)
rather want proper performance than smaller kernels, so I just
thought -freorder-blocks-agolithm=simple could be a compromise.
(-fno-unwind-functions may be harmless though)

Maybe only the port maintainer can make a decision?
I would assume that -Os helps Vax both on real hardware and using Simh.
I cannot think of anything that would affect performance significantly, even though vax function calls are somewhat expensive.

-- R

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