Re: [Qemu-devel] [PATCH v5 00/17] target/arm: Reduce overhead of cpu_get_tb_cpu_state

[Richard Henderson <richard.henderson@linaro.org>]

On 9/4/19 3:48 AM, Peter Maydell wrote:
> On Wed, 21 Aug 2019 at 00:54, Richard Henderson
> <richard.henderson@linaro.org> wrote:
>> However, while working on this patch set, I noticed that we have a lot of
>> unnecessary overlap between A- and M- profile in the TBFLAGs.  Thus point 4
>> above and the completely separate rebuild_hflags_m32().
>>
>> If we rearrange things like the appended, then we recover 4 bits.
> 
> You can't make the THUMB bit A-profile only: we need it in
> M-profile too, so we can correctly generate code that takes
> the InvalidState exception for attempts to execute with the
> Thumb bit not set.

Yes, IIRC I found that when I went to make this work, rather than merely a
sketch through the header file.

> If you want to make VFPEN be A-profile only you need to
> do something so we don't look at it for M-profile: currently
> we set it always-1 for M-profile so we don't trip the code
> that causes us to take an exception if it's 0.
> 
> Otherwise seems reasonable. My overall question is: how bad
> is it if we just start using bits in the cs_base word?

*shrug* Even then we don't want to waste bits; there are only 32 of them
remaining, and after that there's no room at all for expansion.

> If we try to get too tricky with using the same bits for
> different purposes it opens the door for accidentally writing
> code where we use a bit that isn't actually set correctly
> for all the situations where we're using it.

Thankfully, we don't touch these bits in many places.  We set them in the
generator function, and we read them at the beginning of the translator.


r~