Re: [Qemu-devel] Re: branches are expensive

[Paul Brook <paul@codesourcery.com>]

> The ratio is quite bad. Do you have any documentation on when Qemu does the
> chaining and more important, when it does not. For example are
> unconditional jumps always chained, or only in one direction (forward or
> backward).

Direct jumps[1] within the same page are chained (including ). Indirect 
jumps[2] and direct jumps to a different page are not chained. Chaining jumps 
between pages would require breaking TB chains every time a TLB flush occurs.

Unchained jumps just a two stage lookup to cache frequently used entries.

Paul

[1] b, b<cc> and bl
[2] bx, mov pc, ldr pc, pop {pc}