RE: [PATCH 0/3] [APX CFCMOV] Support APX CFCMOV

["Liu, Hongtao" <hongtao.liu@intel.com>]

> -----Original Message-----
> From: Richard Biener <richard.guenther@gmail.com>
> Sent: Friday, June 14, 2024 2:52 PM
> To: Kong, Lingling <lingling.kong@intel.com>
> Cc: gcc-patches@gcc.gnu.org; Liu, Hongtao <hongtao.liu@intel.com>; Uros
> Bizjak <ubizjak@gmail.com>
> Subject: Re: [PATCH 0/3] [APX CFCMOV] Support APX CFCMOV
> 
> On Fri, Jun 14, 2024 at 5:12 AM Kong, Lingling <lingling.kong@intel.com>
> wrote:
> >
> > APX CFCMOV[1] feature implements conditionally faulting which means
> > that all memory faults are suppressed when the condition code
> > evaluates to false and load or store a memory operand. Now we could load
> or store a memory operand may trap or fault for conditional move.
> >
> > In middle-end, now we don't support a conditional move if we knew that a
> load from A or B could trap or fault.
> 
> What's the cost of suppressing a fault?  ISTR that for example fault suppression
> for vector masked load/store is quite expensive, so when this is for example
Yes, avx512 masked load/store, the cost is expensive when memory is invalid.
> done in a loop where there's always a fault that's suppressed you can see
> 1000-fold slowdown.  I would suspect this is similar for cfcmov?  So how is this
> reflected in the decision to if-convert?
But for APXF, we were told the cost of invalid memory is as cheap as valid ones.
(Why else would this instructions be designed?)
> 
> > To enable CFCMOV, we add a target HOOK
> > TARGET_HAVE_CONDITIONAL_MOVE_MEM_NOTRAP
> > in if-conversion pass to allow convert to cmov.
> >
> > All the changes passed bootstrap & regtest x86-64-pc-linux-gnu.
> > We also tested spec with SDE and passed the runtime test.
> >
> > Ok for trunk?
> >
> > [1].https://www.intel.com/content/www/us/en/developer/articles/technic
> > al/advanced-performance-extensions-apx.html
> >
> > Lingling Kong (3):
> >   [APX CFCMOV] Add a new target hook:
> TARGET_HAVE_CONDITIONAL_MOVE_MEM_NOTRAP
> >   [APX CFCMOV] Support APX CFCMOV in if_convert pass
> >   [APX CFCMOV] Support APX CFCMOV in backend
> >
> >  gcc/config/i386/i386-expand.cc               |  63 +++++
> >  gcc/config/i386/i386-opts.h                  |   4 +-
> >  gcc/config/i386/i386.cc                      |  33 ++-
> >  gcc/config/i386/i386.h                       |   1 +
> >  gcc/config/i386/i386.md                      |  53 +++-
> >  gcc/config/i386/i386.opt                     |   3 +
> >  gcc/config/i386/predicates.md                |   7 +
> >  gcc/doc/tm.texi                              |   6 +
> >  gcc/doc/tm.texi.in                           |   2 +
> >  gcc/ifcvt.cc                                 | 247 ++++++++++++++++++-
> >  gcc/target.def                               |  11 +
> >  gcc/targhooks.cc                             |   8 +
> >  gcc/targhooks.h                              |   1 +
> >  gcc/testsuite/gcc.target/i386/apx-cfcmov-1.c |  73 ++++++
> > gcc/testsuite/gcc.target/i386/apx-cfcmov-2.c |  40 +++
> >  15 files changed, 539 insertions(+), 13 deletions(-)  create mode
> > 100644 gcc/testsuite/gcc.target/i386/apx-cfcmov-1.c
> >  create mode 100644 gcc/testsuite/gcc.target/i386/apx-cfcmov-2.c
> >
> > --
> > 2.31.1
> >