On Fri, Aug 27, 2021 at 9:31 PM Jeff Law via Gcc-patches
<[email protected]> wrote:
>
>
> I was working some aspects of our port's vector support and stumbled
> across a bit of silly code. We were comparing two vectors that were
> both uniform.
>
> We can simplify a comparison of uniform vectors to a comparison of a
> representative element from each. That in turn may expose const/copy
> propagation opportunities or even jump threading opportunities.
>
> And that's precisely what this patch does inside DOM. At the start of
> statement processing we see if we can reduce a vector comparison to a
> scalar comparison.
>
> You can see this in pr95308.C on x86. As we enter dom3 we have:
>
>
> ;; basic block 2, loop depth 0
> ;; pred: ENTRY
> e.1_1 = e;
> _27 = f_26(D) != 0;
> _163 = e.1_1 != 0;
> _164 = _27 & _163;
> _196 = _164 ? -1 : 0;
> vect_cst__194 = {_196, _196, _196, _196, _196, _196, _196, _196};
>
> [ ... ]
>
> ;; basic block 8, loop depth 1
> ;; pred: 7
> ;; 6
> if (vect_cst__194 == { 0, 0, 0, 0, 0, 0, 0, 0 })
> goto <bb 10>; [100.00%]
> else
> goto <bb 9>; [20.00%]
> ;; succ: 10
> ;; 9
>
>
> There's another similar comparison later. We transform the comparison into:
>
> ;; basic block 8, loop depth 1
> ;; pred: 7
> ;; 6
> if (_196 == 0)
> goto <bb 13>; [100.00%]
> else
> goto <bb 9>; [20.00%]
> ;; succ: 13
> ;; 9
>
> There's nice secondary effects that show up after the transformation as
> well.
It's an interesting case indeed, but I think a match.pd rule would have
been better to address this. Sth like
(for cmp (eq ne)
(simplify
(cmp vec_same_elem_p@0 vec_same_elem_p@1)
(if (INTEGRAL_TYPE_P (type)
&& (TREE_CODE (type) == BOOLEAN_TYPE
|| TYPE_PRECISION (type) == 1))
(cmp { uniform_vector_p (@0); } { uniform_vector_p (@1); }))))
should do the trick. That makes the transform available to more places
(and DOM should also pick it up this way).
Richard.
>
> Bootstrapped and regression tested on x86_64. It'll go through the
> rest of the public targets as well as our internal port over the next
> 24-48 hrs.
>
>
> Committed to the trunk,
>
> Jeff
