RE: [PATCH] Support -m[no-]gather -m[no-]scatter to enable/disable vectorization for all gather/scatter instructions.

[Liu, Hongtao via Gcc-patches]

> -----Original Message-----
> From: Xi Ruoyao <xry...@xry111.site>
> Sent: Thursday, August 10, 2023 9:48 AM
> To: Liu, Hongtao <hongtao....@intel.com>; gcc-patches@gcc.gnu.org
> Cc: richard.guent...@gmail.com; ubiz...@gmail.com; hubi...@ucw.cz
> Subject: Re: [PATCH] Support -m[no-]gather -m[no-]scatter to enable/disable
> vectorization for all gather/scatter instructions.
> 
> On Thu, 2023-08-10 at 09:11 +0800, liuhongt via Gcc-patches wrote:
> > Currently we have 3 different independent tunes for gather
> > "use_gather,use_gather_2parts,use_gather_4parts",
> > similar for scatter, there're
> > "use_scatter,use_scatter_2parts,use_scatter_4parts"
> >
> > The patch support 2 standardizing options to enable/disable
> > vectorization for all gather/scatter instructions. The options is
> > interpreted by driver to 3 tunes.
> >
> > bootstrapped and regtested on x86_64-pc-linux-gnu.
> > Ok for trunk?
> 
> And should we set -mno-gather as the default for GDS affected processors?
> We'll likely apply the ucode update for them, and then the gathering
> instructions will be much slower.
Assume you're talking about 
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/gather-data-sampling.html
Yes, there will be an separate patch for microarchitecture tuning.
> 
> > gcc/ChangeLog:
> >
> >         * config/i386/i386.h (DRIVER_SELF_SPECS): Add
> >         GATHER_SCATTER_DRIVER_SELF_SPECS.
> >         (GATHER_SCATTER_DRIVER_SELF_SPECS): New macro.
> >         * config/i386/i386.opt (mgather): New option.
> >         (mscatter): Ditto.
> > ---
> >  gcc/config/i386/i386.h   | 12 +++++++++++-
> >  gcc/config/i386/i386.opt |  8 ++++++++
> >  2 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/gcc/config/i386/i386.h b/gcc/config/i386/i386.h index
> > ef342fcee9b..d9ac2c29bde 100644
> > --- a/gcc/config/i386/i386.h
> > +++ b/gcc/config/i386/i386.h
> > @@ -565,7 +565,17 @@ extern GTY(()) tree x86_mfence;
> >  # define SUBTARGET_DRIVER_SELF_SPECS ""
> >  #endif
> >
> > -#define DRIVER_SELF_SPECS SUBTARGET_DRIVER_SELF_SPECS
> > +#ifndef GATHER_SCATTER_DRIVER_SELF_SPECS # define
> > +GATHER_SCATTER_DRIVER_SELF_SPECS \
> > +  "%{mno-gather:-mtune-
> > ctrl=^use_gather_2parts,^use_gather_4parts,^use_gather} \
> > +   %{mgather:-mtune-
> > ctrl=use_gather_2parts,use_gather_4parts,use_gather} \
> > +   %{mno-scatter:-mtune-
> > ctrl=^use_scatter_2parts,^use_scatter_4parts,^use_scatter} \
> > +   %{mscatter:-mtune-
> > ctrl=use_scatter_2parts,use_scatter_4parts,use_scatter}"
> > +#endif
> > +
> > +#define DRIVER_SELF_SPECS \
> > +  SUBTARGET_DRIVER_SELF_SPECS " " \
> > +  GATHER_SCATTER_DRIVER_SELF_SPECS
> >
> >  /* -march=native handling only makes sense with compiler running on
> >     an x86 or x86_64 chip.  If changing this condition, also change
> > diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt index
> > ddb7f110aa2..99948644a8d 100644
> > --- a/gcc/config/i386/i386.opt
> > +++ b/gcc/config/i386/i386.opt
> > @@ -424,6 +424,14 @@ mdaz-ftz
> >  Target
> >  Set the FTZ and DAZ Flags.
> >
> > +mgather
> > +Target
> > +Enable vectorization for gather instruction.
> > +
> > +mscatter
> > +Target
> > +Enable vectorization for scatter instruction.
> > +
> >  mpreferred-stack-boundary=
> >  Target RejectNegative Joined UInteger
> > Var(ix86_preferred_stack_boundary_arg)
> >  Attempt to keep stack aligned to this power of 2.
> 
> --
> Xi Ruoyao <xry...@xry111.site>
> School of Aerospace Science and Technology, Xidian University