juzhe.zh...@rivai.ai writes:
> + /* If we're using decrement IV approach in loop control, we can use output
> of
> + SELECT_VL to adjust IV of loop control and data reference when it
> satisfies
> + the following checks:
> +
> + (a) SELECT_VL is supported by the target.
> + (b) LOOP_VINFO is single-rgroup control.
> + (c) non-SLP.
> + (d) LOOP can not be unrolled.
> +
> + Otherwise, we use MIN_EXPR approach.
> +
> + 1. We only apply SELECT_VL on single-rgroup since:
> +
> + (1). Multiple-rgroup controls N vector loads/stores would need N pointer
> + updates by variable amounts.
> + (2). SELECT_VL allows flexible length (<=VF) in each iteration.
> + (3). For decrement IV approach, we calculate the MAX length of the loop
> + and then deduce the length of each control from this MAX length.
> +
> + Base on (1), (2) and (3) situations, if we try to use SELECT_VL on
> + multiple-rgroup control, we need to generate multiple SELECT_VL to
> + carefully adjust length of each control.
If we use SELECT_VL to refer only to the target-independent ifn, I don't
see why this last bit is true. Like I said in the previous message,
when it comes to determining the length of each control, the approach we
take for MIN_EXPR IVs should work for SELECT_VL IVs. The point is that,
in both cases, any inactive lanes are always the last lanes.
E.g. suppose that, for one particular iteration, SELECT_VL decides that
6 lanes should be active in a loop with VF==8. If there is a 2-control
rgroup with 4 lanes each, the first control must be 4 and the second
control must be 2, just as if a MIN_EXPR had decided that 6 lanes of
the final iteration are active.
I'm not saying the decision itself is wrong. But I think the explanation
could be clearer.
> + Such approach is very inefficient
> + and unprofitable for targets that are using a standalone instruction
> + to configure the length of each operation.
> + E.g. RISC-V vector use 'vsetvl' to configure the length of each
> operation.
What I don't understand is why this isn't also a problem with the
fallback MIN_EXPR approach. That is, with the same example as above,
but using MIN_EXPR IVs, I would have expected:
VF == 8
1-control rgroup "A":
A set by MIN_EXPR IV
2-control rgroup "B1", "B2":
B1 = MIN (A, 4)
B2 = A - B1
and so the vectors controlled by A, B1 and B2 would all have different
lengths.
Is the point that, when using MIN_EXPR, this only happens in the
final iteration? And that you use a tail/epilogue loop for that,
so that the main loop body operates on full vectors only?
Thanks,
Richard
