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

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

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 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?

> 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/technical/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
>