Re: [RFC][PATCH] Reduce generated code by 3% by increasing MMU indices

[Helge Deller]

On 8/5/23 19:58, Richard Henderson wrote:
> On 8/5/23 10:43, Helge Deller wrote:
> > > If there were a way to change no more than two lines of code,
> > > that would be fine.  But otherwise I don't see this as being
> > > worth making the rest of the code base any more complex.
> >
> > Ok. What about that 6-line patch below for x86? It's trivial and
> > all what's needed for x86. Btw, any index which is >= 9 will use
> > the shorter code sequence.
> >
> > Helge
> >
> > diff --git a/target/i386/cpu.h b/target/i386/cpu.h index
> > e0771a1043..3e71e666db 100644 --- a/target/i386/cpu.h +++
> > b/target/i386/cpu.h @@ -2251,11 +2251,11 @@ uint64_t
> > cpu_get_tsc(CPUX86State *env); #define cpu_list x86_cpu_list
> >
> > /* MMU modes definitions */ -#define MMU_KSMAP_IDX   0 -#define
> > MMU_USER_IDX    1 -#define MMU_KNOSMAP_IDX 2 -#define
> > MMU_NESTED_IDX  3 -#define MMU_PHYS_IDX    4 +#define MMU_KSMAP_IDX
> > 11 +#define MMU_USER_IDX    12 +#define MMU_KNOSMAP_IDX 13 +#define
> > MMU_NESTED_IDX  14 +#define MMU_PHYS_IDX    15
>
> No.  The small patch would need to apply to all guests.

Yes.

> Perhaps something to handle indexing of CPUTLBDescFast, e.g.
>
> static inline CPUTLBDescFast cputlb_fast(CPUTLB *tlb, unsigned idx)
> { return &tlb->f[NB_MMU_MODES - 1 - idx]; }
>
> There's already tlb_mask_table_ofs, which handles all tcg backends;
> you just need to adjust that and cputlb.c> Introduce cputlb_fast with
> normal indexing in one patch, and then the second patch to invert the
> indexing may well be exactly two lines.  :-)

You're cheating :-)
But ok, that's an easy one and I can come up with both patches.

One last idea which came into my mind and which may be worth
asking before I start to hack the patch above...:

include/exec/cpu-defs.h:
/* add some comment here why we use this transformation: */
#define MMU_INDEX(nr)   (NB_MMU_MODES - 1 - (x))

target/*/cpu.h:
/* MMU modes definitions */
#define MMU_KSMAP_IDX   MMU_INDEX(0)
#define MMU_USER_IDX    MMU_INDEX(1)
#define MMU_KNOSMAP_IDX MMU_INDEX(2)
#define MMU_NESTED_IDX  MMU_INDEX(3)
...

Downside:
- of course it's a lot more than the 2 lines you asked for
Upsides:
- no additional subtaction at tcg compile time/runtime
- clear indication that this is an MMU index, easy to grep.
- easy to use

?

Helge