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According to [1] sections 7.4, 8.1, and 8.2, desktop and server
processors support fast unaligned access, but embedded systems likely
don't.
It's important that TUKLIB_FAST_UNALIGNED_ACCESS isn't defined when
-mstrict-align is in use because it will result in slower binaries
even if running on a processor that supports fast unaligned access.
It's because compilers will translate multibyte memcpy() to multiple
byte-by-byte instructions instead of wider loads and stores. The
compression times from [2] show this well:
Unaligned access CFLAGS Compression time
enabled -O2 -mno-strict-align 66.1 s
disabled -O2 -mno-strict-align 79.5 s
disabled -O2 -mstrict-align 79.9 s
enabled -O2 -mstrict-align 129.1 s
There currently (GCC 15.2) is no preprocessor macro on LoongArch
to detect if -mstrict-align or -mno-strict-align is in effect (the
default is -mno-strict-align). Use heuristics to detect which of the
flags is in effect.
[1] https://github.com/loongson/la-softdev-convention/blob/v0.2/la-softdev-convention.adoc
[2] https://github.com/tukaani-project/xz/pull/186#issuecomment-3494570304
Thanks-to: Li Chenggang <lichenggang@deepin.org>
Thanks-to: Xi Ruoyao
See: https://github.com/tukaani-project/xz/pull/186