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

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

On Fri, Feb 3, 2023 at 8:34 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> 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.

I have pushed this now after bootstrapping / testing on
x86_64-unknown-linux-gnu.

Richard.

> 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