Re: [RFC] [Patch] Relax tree-if-conv.c trap assumptions.

[Andrew Pinski <pinskia at gmail dot com>]

On Fri, Oct 30, 2015 at 6:06 PM, Kumar, Venkataramanan
<Venkataramanan.Kumar@amd.com> wrote:
> Hi Richard,
>
> I am trying to "if covert the store" in the below test case and later help it to get vectorized under -Ofast -ftree-loop-if-convert-stores -fno-common
>
> #define LEN 4096
>  __attribute__((aligned(32))) float array[LEN]; void test() { for (int i = 0; i < LEN; i++) {
>    if (array[i] > (float)0.)
>                 array[i] =3 ;
>
> }
> }
>
> Currently in GCC 5.2  does not vectorize it.
> https://goo.gl/9nS6Pd
>
> However ICC seems to vectorize it
> https://goo.gl/y1yGHx
>
> As discussed with you  earlier, to allow "if convert store"  I am checking the following:
>
> (1) We already  read the reference "array[i]"  unconditionally once .
> (2) I am now checking  if we are conditionally writing to memory which is defined as read and write and is bound to the definition we are seeing.


I don't think this is thread safe ....

Thanks,
Andrew

>
> With this change, I get able to if convert and the vectorize the case also.
>
> /build/gcc/xgcc -B ./build/gcc/  ifconv.c -Ofast -fopt-info-vec  -S -ftree-loop-if-convert-stores -fno-common
> ifconv.c:2:63: note: loop vectorized
>
> Patch
> ------
> diff --git a/gcc/tree-if-conv.c b/gcc/tree-if-conv.c
> index f201ab5..6475cc0 100644
> --- a/gcc/tree-if-conv.c
> +++ b/gcc/tree-if-conv.c
> @@ -727,6 +727,34 @@ write_memrefs_written_at_least_once (gimple *stmt,
>    return true;
> }
>
> +static bool
> +write_memrefs_safe_to_access_unconditionally (gimple *stmt,
> +                                                                                   vec<data_reference_p> drs)
> +{
> +  int i;
> +  data_reference_p a;
> +  bool found = false;
> +
> +  for (i = 0; drs.iterate (i, &a); i++)
> +    {
> +      if (DR_STMT (a) == stmt
> +               && DR_IS_WRITE (a)
> +               && (DR_WRITTEN_AT_LEAST_ONCE (a) == 0)
> +               && (DR_RW_UNCONDITIONALLY (a) == 1))
> +             {
> +               tree base = get_base_address (DR_REF (a));
> +               found = false;
> +               if (DECL_P (base)
> +                   && decl_binds_to_current_def_p (base)
> +                   && !TREE_READONLY (base))
> +                 {
> +                   found = true;
> +                 }
> +             }
> +    }
> +  return found;
> +}
> +
> /* Return true when the memory references of STMT won't trap in the
>     if-converted code.  There are two things that we have to check for:
>
> @@ -748,8 +776,20 @@ write_memrefs_written_at_least_once (gimple *stmt,
> static bool
> ifcvt_memrefs_wont_trap (gimple *stmt, vec<data_reference_p> refs)
> {
> -  return write_memrefs_written_at_least_once (stmt, refs)
> -    && memrefs_read_or_written_unconditionally (stmt, refs);
> +  bool memrefs_written_once, memrefs_read_written_unconditionally;
> +  bool memrefs_safe_to_access;
> +
> +  memrefs_written_once
> +             = write_memrefs_written_at_least_once (stmt, refs);
> +
> +  memrefs_read_written_unconditionally
> +             =  memrefs_read_or_written_unconditionally (stmt, refs);
> +
> +  memrefs_safe_to_access
> +             = write_memrefs_safe_to_access_unconditionally (stmt, refs);
> +
> +  return ((memrefs_written_once || memrefs_safe_to_access)
> +                && memrefs_read_written_unconditionally);
> }
>
>  /* Wrapper around gimple_could_trap_p refined for the needs of the
>
>
> do I need this function write_memrefs_written_at_least_once anymore?
> Please suggest if there a better way to do this.
>
> Bootstrapped and regression  tested on x86_64.
> I am not  adding change log and comments now, as I  wanted to check  approach first.
>
> Regards,
> Venkat.
>
>