Re: [PATCH] Bump up precision size to 16 bits.

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Feb 2, 2023 at 6:39 PM Michael Meissner via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> The new __dmr type that is being added as a possible future PowerPC instruction

"is being added" means this feature is already in GCC 13?

> set bumps into a structure field size issue.  The size of the __dmr type is 1024 bits.
> The precision field in tree_type_common is currently 10 bits, so if you store
> 1,024 into field, you get a 0 back.  When you get 0 in the precision field, the
> ccp pass passes this 0 to sext_hwi in hwint.h.  That function in turn generates
> a shift that is equal to the host wide int bit size, which is undefined as
> machine dependent for shifting in C/C++.
>
>       int shift = HOST_BITS_PER_WIDE_INT - prec;
>       return ((HOST_WIDE_INT) ((unsigned HOST_WIDE_INT) src << shift)) >> shift;
>
> It turns out the x86_64 where I first did my tests returns the original input
> before the two shifts, while the PowerPC always returns 0.  In the ccp pass, the
> original input is -1, and so it worked.  When I did the runs on the PowerPC, the
> result was 0, which ultimately led to the failure.
>
> In addition, once the precision field is larger, it will help PR C/102989 (C2x
> _BigInt) as well as the implementation of the SET_TYPE_VECTOR_SUBPARTS macro.
>
> I bootstraped various PowerPC compilers (power10 LE, power9 LE, power8 BE)
> along with an x86_64 build.  There were no regressions.  My proposed patches
> for the __dmr type now run fine.  Can I install this into the master branch for
> GCC 13?

... because since we're in stage4 this should fix a regression or at least a
bug that's like ice-on-valid or wrong-code?

Definitely OK for stage1.

Thanks,
Richard.

> 2023-02-02   Richard Biener  <rguenther@suse.de>
>              Michael Meissner  <meissner@linux.ibm.com>
>
> gcc/
>
>         PR middle-end/108623
>         * hwint.h (sext_hwi): Add assertion against precision 0.
>         * tree-core.h (tree_type_common): Bump up precision field to 16 bits.
>         Align bit fields > 1 bit to at least an 8-bit boundary.
> ---
>  gcc/hwint.h     |  1 +
>  gcc/tree-core.h | 24 ++++++++++++------------
>  2 files changed, 13 insertions(+), 12 deletions(-)
>
> diff --git a/gcc/hwint.h b/gcc/hwint.h
> index e31aa006fa4..ba92efbfc25 100644
> --- a/gcc/hwint.h
> +++ b/gcc/hwint.h
> @@ -277,6 +277,7 @@ ctz_or_zero (unsigned HOST_WIDE_INT x)
>  static inline HOST_WIDE_INT
>  sext_hwi (HOST_WIDE_INT src, unsigned int prec)
>  {
> +  gcc_checking_assert (prec != 0);
>    if (prec == HOST_BITS_PER_WIDE_INT)
>      return src;
>    else
> diff --git a/gcc/tree-core.h b/gcc/tree-core.h
> index 8124a1328d4..b71748c6c02 100644
> --- a/gcc/tree-core.h
> +++ b/gcc/tree-core.h
> @@ -1686,18 +1686,8 @@ struct GTY(()) tree_type_common {
>    tree attributes;
>    unsigned int uid;
>
> -  unsigned int precision : 10;
> -  unsigned no_force_blk_flag : 1;
> -  unsigned needs_constructing_flag : 1;
> -  unsigned transparent_aggr_flag : 1;
> -  unsigned restrict_flag : 1;
> -  unsigned contains_placeholder_bits : 2;
> -
> +  unsigned int precision : 16;
>    ENUM_BITFIELD(machine_mode) mode : 8;
> -
> -  /* TYPE_STRING_FLAG for INTEGER_TYPE and ARRAY_TYPE.
> -     TYPE_CXX_ODR_P for RECORD_TYPE and UNION_TYPE.  */
> -  unsigned string_flag : 1;
>    unsigned lang_flag_0 : 1;
>    unsigned lang_flag_1 : 1;
>    unsigned lang_flag_2 : 1;
> @@ -1713,12 +1703,22 @@ struct GTY(()) tree_type_common {
>       so we need to store the value 32 (not 31, as we need the zero
>       as well), hence six bits.  */
>    unsigned align : 6;
> +  /* TYPE_STRING_FLAG for INTEGER_TYPE and ARRAY_TYPE.
> +     TYPE_CXX_ODR_P for RECORD_TYPE and UNION_TYPE.  */
> +  unsigned string_flag : 1;
> +  unsigned no_force_blk_flag : 1;
> +
>    unsigned warn_if_not_align : 6;
> +  unsigned needs_constructing_flag : 1;
> +  unsigned transparent_aggr_flag : 1;
> +
> +  unsigned contains_placeholder_bits : 2;
> +  unsigned restrict_flag : 1;
>    unsigned typeless_storage : 1;
>    unsigned empty_flag : 1;
>    unsigned indivisible_p : 1;
>    unsigned no_named_args_stdarg_p : 1;
> -  unsigned spare : 15;
> +  unsigned spare : 9;
>
>    alias_set_type alias_set;
>    tree pointer_to;
> --
> 2.39.1
>
>
> --
> Michael Meissner, IBM
> PO Box 98, Ayer, Massachusetts, USA, 01432
> email: meissner@linux.ibm.com