Re: [Qemu-devel] [PATCH RFC 00/11] AREG0 elimination

[Blue Swirl]

On Sun, May 15, 2011 at 4:02 PM, Aurelien Jarno <address@hidden> wrote:
> On Sun, May 15, 2011 at 03:37:00PM +0300, Blue Swirl wrote:
>> On Sun, May 15, 2011 at 3:14 PM, Laurent Desnogues
>> <address@hidden> wrote:
>> > On Sun, May 15, 2011 at 1:33 PM, Blue Swirl <address@hidden> wrote:
>> > [...]
>> >>> x86_64 uses r14 as TCG_AREG0. Despite the instructions being quite
>> >>> simple (only 2 movi_i32), the resulting code makes 2 access to env to
>> >>> save the two registers. Having to reload the env pointer each time to a
>> >>> register would clearly increase the size of this TB.
>> >>
>> >> I don't think TCG would be that simple, instead the pointer would be
>> >> loaded only once in this case.
>> >
>> > Assuming TCG was able to allocate a register for that,
>> > it would be live at most for one TB, so you'd have to
>> > load it at least once per TB, and with block chaining
>> > that wouldn't be efficient as you'd keep on reloading it.
>>
>> Yes, but if there are better uses, the register can be flushed. Now
>> this is not possible since the register is always unavailable.
>>
>
> What are the better uses, that justify to flush a register that is going
> to be used three or four host asm later?

It would obviously replace something else determined by TCG.

> In the current generated code, roughly one every four instruction
> reference TCG_AREG0, so this register is really needed very often.
>
> If you think TCG will be faster by having one more register in between
> I suggest you to first optimize tcg_reg_alloc(), which simply spill
> a random register, even if they are some allocated register that won't
> be used until the end of the TB. You should also should check how often
> TCG spills a register (in which case it would have benefit from one more
> register). It happens less than 2000 times when booting an emulated mips
> system on x86_64, while more than 160000 TB are generated.

Right, on a modern CPU with lots of registers, one additional register
won't be helpful, but on i386 the situation should be very different,
there are very few registers.