Re: [PATCH] RISC-V: Teach liveness computation loop invariant shift amount[Dynamic LMUL]

[Robin Dapp <rdapp.gcc@gmail.com>]

> 1). We not only have vashl_optab,vashr_optab,vlshr_optab which vectorize shift with vector shift amount,
> that is, vectorization of 'a[i] >> x[i]', the shift amount is loop variant.
> 2). But also, we have ashl_optab, ashr_optab, lshr_optab which can vectorize shift with scalar shift amount,
> that is, vectorization of 'a[i] >> x', the shift amount is loop invariant.
> 

> +static bool
> +loop_invariant_op_p (class loop *loop,
> +		     tree op)
> +{
> +  if (is_gimple_min_invariant (op))
> +    return true;
> +  if (SSA_NAME_IS_DEFAULT_DEF (op)
> +      || !flow_bb_inside_loop_p (loop, gimple_bb (SSA_NAME_DEF_STMT (op))))
> +    return true;
> +  return gimple_uid (SSA_NAME_DEF_STMT (op)) & 1;
> +}
> +

Looks like this is straight from tree-ssa-loop-ch.  Do we need
is_gimple_min_invariant (is_gimple_constant could be sufficient?)
and DEFAULT_DEF for our case?  The rhs of a shift should never contain
a default def?

I'm not entirely happy about the "loop invariant" heuristic/proxy
of the shift amount being vectorizable.  That seems like something
that could bite us in the future in case we do slp-like vectorization
on loop-invariant (but varying) data.

As it helps for now and is not a correctness issue I'd still tend to
go forward with it.

Regards
 Robin