[PATCH] [AArch64] support -mfentry feature for arm64

[Alexander Monakov]

On Tue, 19 Apr 2016, AKASHI Takahiro wrote:
> > > But if Szabolcs' two-instruction 
> > > sequence in the adjacent subthread is sufficient, this is moot.
> > 
> > .  It can also be solved by having just one NOP after the function label, 
> > and a number of them before, then no thread can be in the nop pad.  That 
> > seems to indicate that GCC should not try to be too clever and simply 
> > leave the specified number of nops before and after the function label, 
> > leaving safety measures to the patching infrastructure.
> 
> I don't get this idea very well.
> How can the instructions *before* a function label be executed
> after branching into this function?

The single nop after the function label is changed to a short backwards branch
to the instructions just before the function label.

As a result, the last instruction in the pad would have to become a short
forward branch jumping over the backwards branch described above, to the first
real instruction of the function.

Alexander