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SubjectRe: 2.2.18Pre Lan Performance Rocks!

On Tue, 31 Oct 2000, Jeff V. Merkey wrote:

> One more optimization it has. NetWare never "calls" functions in the
> kernel. There's a template of register assignments in between kernel
> modules that's very strict (esi contains a WTD head, edi has the target
> thread, etc.) and all function calls are jumps in a linear space.

this might be a win on a i486, but is a loss with any branch predicting,
large-pipeline CPUs (think Pentium IV), which are optimized for CALLs, not
for JMP *EAX instructions. This is the problem with assembly optimizations
that try to compete with the compiler's work: hand-made assembly can only
get worse over time (stay constant in the best case), while compilers are
known to improve slowly but steadily. Plus hand-made assembly is a huge
stone tied to your legs if you try to swim to other architectures. Eg. we
quite often make use of GCC's register-based function parameter passing
optimization. We do use hand-made assembly in a number of cases in Linux
as well, and double-check GCC's assembly output in critical code paths,
but we try to not make it an essential facility.


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