* Denys Vlasenko <dvlas...@redhat.com> wrote:
> Old code was trying to avoid having three branch insns,
> but instead it has a chain of six insns where each insn
> depends on previos one.
>
> And it was touching PT_OLDSS(%esp) unconditionally, even when it may
> contain bogus data. Elsewhere we have to jump thru hoops
> just to make sure here PT_OLDSS(%esp) is at least in a valid page.
>
> All this just to have one branch instead of three?
>
> The new code simply checks each condition.
> All three checks can run in parallel on an out-of-order CPU.
> Most of the time, none of branches will be taken.
>
> Comparison of object code:
> Old:
> 1e6: 8b 44 24 38 mov 0x38(%esp),%eax
> 1ea: 8a 64 24 40 mov 0x40(%esp),%ah
> 1ee: 8a 44 24 34 mov 0x34(%esp),%al
> 1f2: 25 03 04 02 00 and $0x20403,%eax
> 1f7: 3d 03 04 00 00 cmp $0x403,%eax
> 1fc: 74 0f je 20d <ldt_ss>
> New:
> 1e6: f6 44 24 3a 02 testb $0x2,0x3a(%esp)
> 1eb: 75 0e jne 1fb <restore_nocheck>
> 1ed: f6 44 24 34 03 testb $0x3,0x34(%esp)
> 1f2: 74 07 je 1fb <restore_nocheck>
> 1f4: f6 44 24 40 04 testb $0x4,0x40(%esp)
> 1f9: 75 0f jne 20a <ldt_ss>
Please do some benchmarking of this: a tight loop of getpid or getppid
syscalls ought to be enough to be able to time this accurately.
Thanks,
Ingo
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