Re: [RFC PATCH 1/2] Marker probes in futex.c

[Arjan van de Ven]

> >     4631:       b0 00                   mov    $0x0,%al
> >     4633:       84 c0                   test   %al,%al
> >     4635:       0f 85 c6 00 00 00       jne    4701

the use of partial registers here is unfortunate and probably quite expensive ;(


> > If we want to support NMI context and have the ability to
> > instrument preemptable code without too much headache, we must
> > insure that every modification will leave the code in a "correct"
> > state and that we do not grow the size of any reachable
> > instruction.  Also, we must insure gcc did not put code between
> > these instructions. Modifying non-relocatable instructions would
> > also be a pain, since we would have to deal with instruction
> > pointer relocation in the breakpoint code when the code
> > modification is being done.

you also need to make sure no cpu is executing that code ever.. 
but you already deal with that right?

> > 
> > Luckily, gcc almost never place any code between the mov, test and
> > jne instructions. But since we cannot we sure, we could dynamically
> > check for this code pattern after the mov instruction. If we find
> > it, then we play with it as if it was a single asm block, but if we
> > don't find what we expect, then we use standard immediate values
> > for that. I expect the heavily optimised version will be usable
> > almost all the time.

I expect gcc to start using the macro-fusion capable ones more and more over time at least,
and for that the compare and jmp need to be consecutive.


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