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Re: RFC: Should x86-64 support arbitrary calling conventions?

On 03/24/2017 07:31 PM, Florian Weimer wrote:

* H. J. Lu:


I compared time for "make check" in glibc.  On Nehalem and Skylake,
the time differences are within noises.  On Knights Landing, xsave
is about 1% slower.


Thanks for doing this benchmarking.

What's the increase in stack usage?


I don't expect xsave will make any differences for long running
benchmarks.  Its impact may only show up on short programs which
call external functions a few times with lazy binding.

Should we consider it at all?


I think the main benefit is that we don't have to adjust the dynamic
linker trampoline for each new microarchitecture, and applications can
safely start using new CPU features once the kernel indicates support.
Not quite true, at least as written. The STATE_SAVE_MASK define selects which components get saved. This would have to be changed for additional cpu bits that could be modified.
One *could* set EAX:EDX = -1 and store everything, and then, yes, we'd be done with changes to glibc for all cpu changes. 


r~
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