Re: [Xen-devel] x86: PIE support and option to extend KASLR randomization

[Ingo Molnar <mingo@xxxxxxxxxx>]

* H. Peter Anvin <hpa@xxxxxxxxx> wrote:

> On 09/22/17 09:32, Ingo Molnar wrote:
> > 
> > BTW., I think things improved with ORC because with ORC we have RBP as an 
> > extra 
> > register and with PIE we lose RBX - so register pressure in code generation 
> > is 
> > lower.
> > 
> 
> We lose EBX on 32 bits, but we don't lose RBX on 64 bits - since x86-64
> has RIP-relative addressing there is no need for a dedicated PIC register.

Indeed, but we'd use a new register _a lot_ for constructs, transforming:

  mov    r9,QWORD PTR [r11*8-0x7e3da060] (8 bytes)

into:

  lea    rbx,[rip+<off>] (7 bytes)
  mov    r9,QWORD PTR [rbx+r11*8] (6 bytes)

... which I suppose is quite close to (but not the same as) 'losing' RBX.

Of course the compiler can pick other registers as well, not that it matters 
much 
to register pressure in larger functions in the end. Plus if the compiler has 
to 
pick a callee-saved register there's the additional saving/restoring overhead 
of 
that as well.

Right?

> I'm somewhat confused how we can have as much as almost 1% overhead.  I 
> suspect 
> that we end up making a GOT and maybe even a PLT for no good reason.

So the above transformation alone would explain a good chunk of the overhead I 
think.

Thanks,

        Ingo

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