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

[Anton Johansson]

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.

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