Re: [PATCH] gimple-match: Do not try UNCOND optimization with COND_LEN.

[Richard Sandiford <richard.sandiford@arm.com>]

Robin Dapp via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> Hi,
>
> as Juzhe noticed in gcc.dg/pr92301.c there was still something missing in
> the last patch.  The attached v2 makes sure we always have a COND_LEN operation
> before returning true and initializes len and bias even if they are unused.
>
> Bootstrapped and regtested on aarch64 and x86.

Sorry for the slow review.  I was hoping Richi would take it,
but I see he was hoping the same from me.

> Regards
>  Robin
>
> Subject: [PATCH v2] gimple-match: Do not try UNCOND optimization with
>  COND_LEN.
>
> On riscv we mis-optimize conditional (length) operations into
> unconditional operations e.g. in slp-reduc-7.c and
> gcc.dg/pr92301.c.
>
> This patch prevents optimizing e.g.
>  COND_LEN_ADD ({-1, ... }, a, 0, c, len, bias)
> unconditionally into just "a".
>
> Currently, we assume that COND_LEN operations can be optimized similarly
> to COND operations.  As the length is part of the mask (and usually not
> compile-time constant), we must not perform any optimization that relies
> on just the mask being "true".  This patch ensures that we still have a
> COND_LEN pattern after optimization.
>
> gcc/ChangeLog:
>
> 	PR target/111311
> 	* gimple-match-exports.cc (maybe_resimplify_conditional_op):
> 	Check for length masking.
> 	(try_conditional_simplification): Check that the result is still
> 	length masked.
> ---
>  gcc/gimple-match-exports.cc | 38 ++++++++++++++++++++++++++++++-------
>  gcc/gimple-match.h          |  3 ++-
>  2 files changed, 33 insertions(+), 8 deletions(-)
>
> diff --git a/gcc/gimple-match-exports.cc b/gcc/gimple-match-exports.cc
> index b36027b0bad..d41de98a3d3 100644
> --- a/gcc/gimple-match-exports.cc
> +++ b/gcc/gimple-match-exports.cc
> @@ -262,7 +262,8 @@ maybe_resimplify_conditional_op (gimple_seq *seq, gimple_match_op *res_op,
>    if (!res_op->cond.cond)
>      return false;
>  
> -  if (!res_op->cond.else_value
> +  if (!res_op->cond.len
> +      && !res_op->cond.else_value
>        && res_op->code.is_tree_code ())
>      {
>        /* The "else" value doesn't matter.  If the "then" value is a

Why are the contents of this if statement wrong for COND_LEN?
If the "else" value doesn't matter, then the masked form can use
the "then" value for all elements.  I would have expected the same
thing to be true of COND_LEN.

> @@ -301,9 +302,12 @@ maybe_resimplify_conditional_op (gimple_seq *seq, gimple_match_op *res_op,
>  
>    /* If the "then" value is a gimple value and the "else" value matters,
>       create a VEC_COND_EXPR between them, then see if it can be further
> -     simplified.  */
> +     simplified.
> +     Don't do this if we have a COND_LEN_ as that would make us lose the
> +     length masking.  */
>    gimple_match_op new_op;
> -  if (res_op->cond.else_value
> +  if (!res_op->cond.len
> +      && res_op->cond.else_value
>        && VECTOR_TYPE_P (res_op->type)
>        && gimple_simplified_result_is_gimple_val (res_op))
>      {

The change LGTM, but it would be nice to phrase the comment to avoid
the "Do A.  Don't do A if B" pattern.  Maybe:

  /* If the condition represents MASK ? THEN : ELSE, where THEN is a gimple
     value and ELSE matters, create a VEC_COND_EXPR between them, then see
     if it can be further simplified.  */

> @@ -314,7 +318,7 @@ maybe_resimplify_conditional_op (gimple_seq *seq, gimple_match_op *res_op,
>        return gimple_resimplify3 (seq, res_op, valueize);
>      }
>  
> -  /* Otherwise try rewriting the operation as an IFN_COND_* call.
> +  /* Otherwise try rewriting the operation as an IFN_COND_(LEN_)* call.
>       Again, this isn't a simplification in itself, since it's what
>       RES_OP already described.  */
>    if (convert_conditional_op (res_op, &new_op))
> @@ -386,9 +390,29 @@ try_conditional_simplification (internal_fn ifn, gimple_match_op *res_op,
>      default:
>        gcc_unreachable ();
>      }
> -  *res_op = cond_op;
> -  maybe_resimplify_conditional_op (seq, res_op, valueize);
> -  return true;
> +
> +  if (len)
> +    {
> +      /* If we had a COND_LEN before we need to ensure that it stays that
> +	 way.  */
> +      gimple_match_op old_op = *res_op;
> +      *res_op = cond_op;
> +      maybe_resimplify_conditional_op (seq, res_op, valueize);
> +
> +      auto cfn = combined_fn (res_op->code);
> +      if (internal_fn_p (cfn)
> +	  && internal_fn_len_index (as_internal_fn (cfn)) != -1)
> +	return true;

Why isn't it enough to check the result of maybe_resimplify_conditional_op?

Thanks,
Richard

> +
> +      *res_op = old_op;
> +      return false;
> +    }
> +  else
> +    {
> +      *res_op = cond_op;
> +      maybe_resimplify_conditional_op (seq, res_op, valueize);
> +      return true;
> +    }
>  }
>  
>  /* Helper for the autogenerated code, valueize OP.  */
> diff --git a/gcc/gimple-match.h b/gcc/gimple-match.h
> index bec3ff42e3e..d192b7dae3e 100644
> --- a/gcc/gimple-match.h
> +++ b/gcc/gimple-match.h
> @@ -56,7 +56,8 @@ public:
>  
>  inline
>  gimple_match_cond::gimple_match_cond (tree cond_in, tree else_value_in)
> -  : cond (cond_in), else_value (else_value_in)
> +  : cond (cond_in), else_value (else_value_in), len (NULL_TREE),
> +    bias (NULL_TREE)
>  {
>  }