Re: [PATCH v3 6/7] arm64, jump label: optimize jump labelimplementation

[Will Deacon]

Hi guys,

On Fri, Oct 18, 2013 at 04:31:22AM +0100, Jiang Liu (Gerry) wrote:
> On 2013/10/17 23:27, Steven Rostedt wrote:
> > On Thu, 17 Oct 2013 22:40:32 +0800
> > Jiang Liu <liuj97@xxxxxxxxx> wrote:
> >
> >
> >>>>> You could make the code more concise by limiting your patching ability to
> >>>>> branch immediates. Then a nop is simply a branch to the next instruction (I
> >>>>> doubt any modern CPUs will choke on this, whereas the architecture requires
> >>>>> a NOP to take time).
> >>>> I guess a NOP should be more effecient than a "B #4" on real CPUs:)
> >>>
> >>> Well, I was actually questioning that. A NOP *has* to take time (the
> >>> architecture prevents implementations from discaring it) whereas a static,
> >>> unconditional branch will likely be discarded early on by CPUs with even
> >>> simple branch prediction logic.
> >> I naively thought "NOP" is cheaper than a "B" :(
> >> Will use a "B #1" to replace "NOP".
> >>
> >
> > Really?? What's the purpose of a NOP then? It seems to me that an
> > architecture is broken if a NOP is slower than a static branch.

Cheers for making me double-check this: it turns out I was mixing up my
architecture and micro-architecture. The architecture actually states:

  `The timing effects of including a NOP instruction in a program are not
   guaranteed. It can increase execution time, leave it unchanged, or even
   reduce it. Therefore, NOP instructions are not suitable for timing loops.'

however I know of at least one micro-architecture where a NOP is actually
more expensive than some other instructions, hence my original concerns.

> 	I have discussed this with one of our chip design members.
> He thinks "NOP" should be better than "B #1" because jump instruction
> is one of the most complex instructions for microarchitecture, which
> may stall the pipeline. And NOP should be friendly enough for all
> microarchitectures. So I will keep the "NOP" version.

Fine by me, we can't please all micro-architectures and NOP probably makes
more sense. However, I would rather you rework your aarch64_insn_gen_nop
function to actually generate hint instructions (since NOP is a hint alias
in AArch64), where you specify the alias as a parameter.

In other news, the GCC guys have started pushing a patch to add the %c
output template to the AArch64 backend:

  http://gcc.gnu.org/ml/gcc-patches/2013-10/msg01314.html

Cheers, and sorry for the earlier confusion,

Will
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