On Thu, 23 Oct 2025, Artemiy Volkov wrote:
> This patch adds a match.pd transformation to strip unnecessary
> view_converts of BIT_FIELD_REFs. This should work for all combinations of
> types except where loss of precision is involved, i.e. in narrowing
> conversions between integer types; this is checked for in the if clause of
> the new pattern.
>
> The change survives bootstrap + regtest on aarch64 and x86_64, and one
> additional test case has been added to gcc.dg/tree-ssa.
>
> gcc/ChangeLog:
>
> * match.pd: Add pattern to simplify view_convert (BIT_FIELD_REF).
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/tree-ssa/forwprop-42.c: New test.
> ---
> gcc/match.pd | 8 ++++++++
> gcc/testsuite/gcc.dg/tree-ssa/forwprop-42.c | 12 ++++++++++++
> 2 files changed, 20 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/forwprop-42.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index a4248a521cf..660f4ecb3cb 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -5624,6 +5624,14 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> (view_convert (view_convert @0))
> (view_convert @0))
>
> +/* Squash view_converts of BFRs if no precision is lost. */
> +(simplify
> + (view_convert (BIT_FIELD_REF@0 @1 @2 @3))
> + (if (!INTEGRAL_TYPE_P (type)
> + || !INTEGRAL_TYPE_P (TREE_TYPE (@0))
> + || TYPE_PRECISION (type) == TYPE_PRECISION (TREE_TYPE (@0)))
> + (BIT_FIELD_REF:type @1 @2 @3)))
I think you want to ensure that you do not fold in size changes.
See tree-cfg.cc:verify_types_in_gimple_reference. The view_convert
could convert to a variably-sized type.
I'm concerned there are many corner cases you can't think of. So
instead of a negative list (aka !INTEGRAL_TYPE_P) I'm looking for
a positive-list we are sure will work. I'd suggest to use
is_gimple_reg_type. Also require type_has_mode_precision_p
on integral types.
Richard.
> +
> /* For integral conversions with the same precision or pointer
> conversions use a NOP_EXPR instead. */
> (simplify
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/forwprop-42.c
> b/gcc/testsuite/gcc.dg/tree-ssa/forwprop-42.c
> new file mode 100644
> index 00000000000..5785e51c089
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/forwprop-42.c
> @@ -0,0 +1,12 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-forwprop1" } */
> +
> +typedef unsigned int vec2 __attribute__ ((vector_size (2 * sizeof (unsigned
> int))));
> +typedef unsigned int vec1 __attribute__ ((vector_size (sizeof (unsigned
> int))));
> +
> +vec1 foo (vec2 x)
> +{
> + return (vec1) x[1];
> +}
> +
> +/* { dg-final { scan-tree-dump-not "VIEW_CONVERT_EXPR" "forwprop1" } } */
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
