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

[Richard Henderson]

On 6/14/21 4:49 AM, Alex Bennée wrote:
> > +/*
> > + * Flags for the bswap opcodes.
> > + * If IZ, the input is zero-extended, otherwise unknown.
> > + * If OZ or OS, the output is zero- or sign-extended respectively,
> > + * otherwise the high bits are undefined.
> > + */
> > +enum {
> > +    TCG_BSWAP_IZ = 1,
> > +    TCG_BSWAP_OZ = 2,
> > +    TCG_BSWAP_OS = 4,
> > +};
> > +
>
> So is a TCG_BSWAP_IZ only really for cases where we have loaded up a
> narrower width value into the "natural" TCG sized register? We seem to
> assume this is always the case even though the TCG bswap op doesn't have
> visibility of how the arg value was loaded.

All of these flags are for narrower width bswap.  When you get to patch 17, you'll see 
that tcg_gen_bswap32_i32 and bswap64_i64 do not present this argument to the target/ front 
ends at all.

IZ is for when we know that we've used a zero-extending load feeding into the operation. 
I've also added code to the optimizer to *set* this bit when it can prove that the value 
feeding the bswap is zero-extended.


r~