vect: Avoid divide by zero for permutes of extern VLA vectors

My recent VLA SLP patches caused a regression with cross compilers
in gcc.dg/torture/neon-sve-bridge.c.  There we have a VEC_PERM_EXPR
created from two BIT_FIELD_REFs, with the child node being an
external VLA vector:

note:   node 0x3704a70 (max_nunits=1, refcnt=2) vector(2) long int
note:   op: VEC_PERM_EXPR
note:          stmt 0 val1Return_9 = BIT_FIELD_REF <sveReturn_8, 64, 0>;
note:          stmt 1 val2Return_10 = BIT_FIELD_REF <sveReturn_8, 64, 64>;
note:          lane permutation { 0[0] 0[1] }
note:          children 0x3704b08
note:   node (external) 0x3704b08 (max_nunits=1, refcnt=1) svint64_t
note:          { }

For this kind of external node, the SLP_TREE_LANES is normally
the total number of lanes in the vector, but it is zero if the
vector has variable length:

      auto nunits = TYPE_VECTOR_SUBPARTS (SLP_TREE_VECTYPE (vnode));
      unsigned HOST_WIDE_INT const_nunits;
      if (nunits.is_constant (&const_nunits))
	SLP_TREE_LANES (vnode) = const_nunits;

This led to division by zero in:

      /* Check whether the output has N times as many lanes per vector.  */
      else if (constant_multiple_p (SLP_TREE_LANES (node) * op_nunits,
				    SLP_TREE_LANES (child) * nunits,
				    &this_unpack_factor)
	       && (i == 0 || unpack_factor == this_unpack_factor))
	unpack_factor = this_unpack_factor;

No repetition takes place for this kind of external node, so this
patch goes with Richard's suggestion to check for external nodes
that have no scalar statements.

This didn't show up for my native testing since division by zero
doesn't trap on AArch64.

Bootstrapped & regreesion-tested on aarch64-linux-gnu and spot-checked
with a cross compiler.  OK to install?

gcc/
	* tree-vect-slp.cc (vectorizable_slp_permutation_1): Set repeating_p
	to false if we have an external node for a pre-existing vector.
---
 gcc/tree-vect-slp.cc | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

On Thu, 10 Oct 2024, Richard Sandiford wrote:

> My recent VLA SLP patches caused a regression with cross compilers
> in gcc.dg/torture/neon-sve-bridge.c.  There we have a VEC_PERM_EXPR
> created from two BIT_FIELD_REFs, with the child node being an
> external VLA vector:
> 
> note:   node 0x3704a70 (max_nunits=1, refcnt=2) vector(2) long int
> note:   op: VEC_PERM_EXPR
> note:          stmt 0 val1Return_9 = BIT_FIELD_REF <sveReturn_8, 64, 0>;
> note:          stmt 1 val2Return_10 = BIT_FIELD_REF <sveReturn_8, 64, 64>;
> note:          lane permutation { 0[0] 0[1] }
> note:          children 0x3704b08
> note:   node (external) 0x3704b08 (max_nunits=1, refcnt=1) svint64_t
> note:          { }
> 
> For this kind of external node, the SLP_TREE_LANES is normally
> the total number of lanes in the vector, but it is zero if the
> vector has variable length:
> 
>       auto nunits = TYPE_VECTOR_SUBPARTS (SLP_TREE_VECTYPE (vnode));
>       unsigned HOST_WIDE_INT const_nunits;
>       if (nunits.is_constant (&const_nunits))
> 	SLP_TREE_LANES (vnode) = const_nunits;
> 
> This led to division by zero in:
> 
>       /* Check whether the output has N times as many lanes per vector.  */
>       else if (constant_multiple_p (SLP_TREE_LANES (node) * op_nunits,
> 				    SLP_TREE_LANES (child) * nunits,
> 				    &this_unpack_factor)
> 	       && (i == 0 || unpack_factor == this_unpack_factor))
> 	unpack_factor = this_unpack_factor;
> 
> No repetition takes place for this kind of external node, so this
> patch goes with Richard's suggestion to check for external nodes
> that have no scalar statements.
> 
> This didn't show up for my native testing since division by zero
> doesn't trap on AArch64.
> 
> Bootstrapped & regreesion-tested on aarch64-linux-gnu and spot-checked
> with a cross compiler.  OK to install?

OK.

Thanks,
Richard.

> gcc/
> 	* tree-vect-slp.cc (vectorizable_slp_permutation_1): Set repeating_p
> 	to false if we have an external node for a pre-existing vector.
> ---
>  gcc/tree-vect-slp.cc | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
> index 9bb765e2cba..1991fb1d3b6 100644
> --- a/gcc/tree-vect-slp.cc
> +++ b/gcc/tree-vect-slp.cc
> @@ -10288,10 +10288,19 @@ vectorizable_slp_permutation_1 (vec_info *vinfo, gimple_stmt_iterator *gsi,
>  	}
>        auto op_nunits = TYPE_VECTOR_SUBPARTS (op_vectype);
>        unsigned int this_unpack_factor;
> +      /* Detect permutations of external, pre-existing vectors.  The external
> +	 node's SLP_TREE_LANES stores the total number of units in the vector,
> +	 or zero if the vector has variable length.
> +
> +	 We are expected to keep the original VEC_PERM_EXPR for such cases.
> +	 There is no repetition to model.  */
> +      if (SLP_TREE_DEF_TYPE (child) == vect_external_def
> +	  && SLP_TREE_SCALAR_OPS (child).is_empty ())
> +	repeating_p = false;
>        /* Check whether the input has twice as many lanes per vector.  */
> -      if (children.length () == 1
> -	  && known_eq (SLP_TREE_LANES (child) * nunits,
> -		       SLP_TREE_LANES (node) * op_nunits * 2))
> +      else if (children.length () == 1
> +	       && known_eq (SLP_TREE_LANES (child) * nunits,
> +			    SLP_TREE_LANES (node) * op_nunits * 2))
>  	pack_p = true;
>        /* Check whether the output has N times as many lanes per vector.  */
>        else if (constant_multiple_p (SLP_TREE_LANES (node) * op_nunits,
>