Re: [PATCH PR69848/partial]Propagate comparison into VEC_COND_EXPR if target supports

[Richard Biener <richard dot guenther at gmail dot com>]

On May 13, 2016 6:02:27 PM GMT+02:00, Bin Cheng <Bin.Cheng@arm.com> wrote:
>Hi,
>As PR69848 reported, GCC vectorizer now generates comparison outside of
>VEC_COND_EXPR for COND_REDUCTION case, as below:
>
>  _20 = vect__1.6_8 != { 0, 0, 0, 0 };
>  vect_c_2.8_16 = VEC_COND_EXPR <_20, { 0, 0, 0, 0 }, vect_c_2.7_13>;
>  _21 = VEC_COND_EXPR <_20, ivtmp_17, _19>;
>
>This results in inefficient expanding.  With IR like:
>
>vect_c_2.8_16 = VEC_COND_EXPR <vect__1.6_8 != { 0, 0, 0, 0 }, { 0, 0,
>0, 0 }, vect_c_2.7_13>;
>  _21 = VEC_COND_EXPR <vect__1.6_8 != { 0, 0, 0, 0 }, ivtmp_17, _19>;
>
>We can do:
>1) Expanding time optimization, for example, reverting comparison
>operator by switching VEC_COND_EXPR operands.  This is useful when
>backend only supports some comparison operators.
>2) For backend not supporting vcond_mask patterns, saving one LT_EXPR
>instruction which introduced by expand_vec_cond_expr.
>
>This patch fixes this by propagating comparison into VEC_COND_EXPR even
>if it's used multiple times.  For now, GCC does single_use_only
>propagation.  Ideally, we may duplicate the comparison before each use
>statement just before expanding, so that TER can successfully backtrack
>it from each VEC_COND_EXPR.  Unfortunately I didn't find a good pass to
>do this.  Tree-vect-generic.c looks like a good candidate, but it's so
>early that following CSE could undo the transform.  Another possible
>fix is to generate comparison inside VEC_COND_EXPR directly in function
>vectorizable_reduction.

I prefer this for now.

Richard.

>As for possible comparison CSE opportunities, I checked that it's
>simple enough to be handled by RTL CSE.
>
>Bootstrap and test on x86_64 and AArch64.  Any comments?
>
>Thanks,
>bin
>
>2016-05-12  Bin Cheng  <bin.cheng@arm.com>
>
>	PR tree-optimization/69848
>	* optabs-tree.c (expand_vcond_mask_p, expand_vcond_p): New.
>	(expand_vec_cmp_expr_p): Call above functions.
>	* optabs-tree.h (expand_vcond_mask_p, expand_vcond_p): New.
>	* tree-ssa-forwprop.c (optabs-tree.h): Include header file.
>	(forward_propagate_into_cond): Propgate multiple uses for
>	VEC_COND_EXPR.