Re: [Qemu-devel] [PATCH 09/22] exec-all: shrink tb->invalid to uint8_t

[Richard Henderson]

On 07/08/2017 09:50 PM, Emilio G. Cota wrote:
> To avoid wasting a byte. I don't have any use in mind for this byte,
> but I think it's good to leave this byte explicitly free for future use.
> See this discussion for how the u16 came to be:
>    https://lists.gnu.org/archive/html/qemu-devel/2016-07/msg04564.html
> We could use a bool but in some systems that would take > 1 byte.
>
> Signed-off-by: Emilio G. Cota<address@hidden>
> ---
>   include/exec/exec-all.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)

What I would prefer to do is generalize tb->cflags.  Those values *do* affect 
how we compile the TB, and yet we don't take them into account.  So I think it 
would be a good idea to feed that into the TB hash.

At present we use 18 bits of the uint32_t.  That leaves plenty of room for:

* the compile-time value of parallel_cpus, so we don't have to keep flushing 
the TBs that we create for cpu_exec_step_atomic.

* invalid, which no TB within the hashtable would have set.

* other stuff which may become relevant in the future.


r~