Re: [RFC PATCH v1 36/43] target/hexagon: Add temporary vector storage

[Brian Cain]

On 12/3/2024 12:56 PM, Anton Johansson via wrote:
> On 22/11/24, Richard Henderson wrote:
> > On 11/20/24 19:49, Anton Johansson wrote:
> > > Temporary vectors in helper-to-tcg generated code are allocated from an
> > > array of bytes in CPUArchState, specified with --temp-vector-block.
> > >
> > > This commits adds such a block of memory to CPUArchState, if
> > > CONFIG_HELPER_TO_TCG is set.
> > >
> > > Signed-off-by: Anton Johansson <anjo@rev.ng>
> > > ---
> > >    target/hexagon/cpu.h | 4 ++++
> > >    1 file changed, 4 insertions(+)
> > >
> > > diff --git a/target/hexagon/cpu.h b/target/hexagon/cpu.h
> > > index 7be4b5769e..fa6ac83e01 100644
> > > --- a/target/hexagon/cpu.h
> > > +++ b/target/hexagon/cpu.h
> > > @@ -97,6 +97,10 @@ typedef struct CPUArchState {
> > >        MMVector future_VRegs[VECTOR_TEMPS_MAX] QEMU_ALIGNED(16);
> > >        MMVector tmp_VRegs[VECTOR_TEMPS_MAX] QEMU_ALIGNED(16);
> > > +#ifdef CONFIG_HELPER_TO_TCG
> > > +    uint8_t tmp_vmem[4096] QEMU_ALIGNED(16);
> > > +#endif
> > > +
> > >        MMQReg QRegs[NUM_QREGS] QEMU_ALIGNED(16);
> > >        MMQReg future_QRegs[NUM_QREGS] QEMU_ALIGNED(16);
> > Wow.  Do you really require 4k in temp storage?
> No, 4k is overkill used during testing.  But consider that Hexagon uses
> 128- and 256-byte vectors in some cases so if the emitted code uses say
> 5 temporaries in its computation we end up at 1280 bytes as an upper
> bound.

Per-packet there should be a maximum of one temporary.  But per-TB it's 
unbound.  Could we/should we have some guidance to put the brakes on 
translation early if we encounter ~N temp references?

But maybe that's not needed since the temp space can be reused within a 
TB among packets.

> Two ideas here, we can:
>
>    1. Allow users to specify an upper bound on vector memory, and abort
>       translation of helpers that surpass this, and;
>
>    2. Emit maximum number of bytes used for vector temporaries to a
>       macro.
>
> //Anton