Re: [Qemu-devel] Re: [PATCH 01/12] TCG "sync" op

[Paul Brook <paul@codesourcery.com>]

On Thursday 22 October 2009, Aurelien Jarno wrote:
> On Wed, Oct 21, 2009 at 03:52:22PM +0200, Ulrich Hecht wrote:
> > sync allows concurrent accesses to locations in memory through different
> > TCG variables. This comes in handy when you are emulating CPU registers
> > that can be used as either 32 or 64 bit, as TCG doesn't know anything
> > about aliases. See the s390x target for an example.
> >
> > Fixed sync_i64 build failure on 32-bit targets.
> 
> Looking more in details to the use case of this patch, I think it can be
> useful in QEMU. However I don't feel very comfortable in merging it
> without having the opinion of more persons. Paul, Malc Blue Swirl or
> others, any opinion?

I don't think this is the right solution.

IIUC the basic problem is that we have a register file where adjacent pairs of 
32-bit registers are also accessed as a 64-bit value.  This is something many 
other targets need to do (at least ARM, PPC, MIPS and SPARC).

While sync appears attractive as a quick hack to achieve this, I think it is 
liable to be abused, and cause us serious pain long-term. If you need an easy 
solution then use ld/st (as with ARM VFP registers). If you want a good 
solution then fix whichever bit of TCG makes accessing a pair of registers 
horribly slow. We already have some support for this (concat_i32_i64).

Paul