Re: [PATCH 01/28] tcg: Add flags argument to bswap opcodes

[Richard Henderson]

On 6/21/21 6:51 AM, Peter Maydell wrote:
> On Mon, 21 Jun 2021 at 14:41, Peter Maydell <peter.maydell@linaro.org> wrote:
> > On Mon, 14 Jun 2021 at 09:43, Richard Henderson
> > <richard.henderson@linaro.org> wrote:
> > > This will eventually simplify front-end usage, and will allow
> > > backends to unset TCG_TARGET_HAS_MEMORY_BSWAP without loss of
> > > optimization.
> > >
> > > The argument is added during expansion, not currently exposed
> > > to the front end translators.  Non-zero values are not yet
> > > supported by any backends.
> >
> > Here we say non-zero values are not yet supported by the backend...
>
> Looking at the tcg/README docs, I think what you mean is that
> at the moment all the backends assume/require that the caller passes
> one of TCG_BSWAP_IZ or (TCG_BSWAP_IZ | TCG_BSWAP_OZ), since the
> pre-flags implementation requires the top bytes to be zero and leaves
> them that way.

Correct.

> But then the parts of your patch that pass in a zero
> flags word would be wrong...

The parts that pass in a zero flags word are covered by, from the README:

> The flags are ignored -- the argument is present
> for consistency with the smaller bswaps.


r~