Re: [Qemu-devel] [PATCH 5/7] tcg-i386: Implement deposit operation.

[Richard Henderson]

On 01/31/2011 12:33 AM, Aurelien Jarno wrote:
> This week-end I have tested it emulating an x86-64 machine on x86-64,
> with all the patch series applied. I have measured the boot time from
> the bootloader up to the graphical environment of a Debian installation
> I used -snapshot to make sure the host hard-drive is not introducing any
> noise in the measurement (so that the whole image is in the host cache),
> and did the measurement 10 times. The machine is a Core 2 Q9650, nothing
> else was running on the machine except the few standard daemons.
> 
> I have found that the boot time is roughly 1.8% faster with the patch
> series applied. It's undoubtedly an improvement, but still close to the
> measurement noise. This is a bit disappointing...

It's also not terribly surprising, with that test scenario.  GCC tries
not to generate partial register stores, except when (as here) it's 
really a bitfield insert.

A test that might show off the deposit code more would be booting a
16-bit OS.  Either FreeDOS, or Windows 3.1 (if anyone still has a copy).
In that case, the translator will be emitting a deposit op for almost
every guest instruction.

(Which is probably a mistake from a translator point of view -- there's
no reason we can't emulate 16-bit operations with 32-bit operations given
that the high bits are ignorable.)


r~