RE: [Patch,rtl Optimization]: Better register pressure estimate for Loop Invariant Code Motion.

[Ajit Kumar Agarwal <ajit dot kumar dot agarwal at xilinx dot com>]

-----Original Message-----
From: Richard Biener [mailto:richard.guenther@gmail.com] 
Sent: Wednesday, December 09, 2015 4:06 PM
To: Ajit Kumar Agarwal
Cc: Jeff Law; GCC Patches; Vinod Kathail; Shail Aditya Gupta; Vidhumouli Hunsigida; Nagaraju Mekala
Subject: Re: [Patch,rtl Optimization]: Better register pressure estimate for Loop Invariant Code Motion.

On Tue, Dec 8, 2015 at 11:24 PM, Ajit Kumar Agarwal <ajit.kumar.agarwal@xilinx.com> wrote:
> Based on the comments on RFC patch this patch incorporates all the comments from Jeff. Thanks Jeff for the valuable feedback.
>
> This patch enables the better register pressure estimate for Loop 
> Invariant code motion. This patch Calculate the Loop Liveness used for regs_used  used to calculate the register pressure  used in the cost estimation.
>
> Loop Liveness is based on the following properties.  we only require 
> to calculate the set of objects that are live at the birth or the 
> header of the loop.  We don't need to calculate the live through the Loop considering Live in and Live out of all the basic blocks of the Loop. This is because the set of objects. That are live-in at the birth or header of the loop will be live-in at every node in the Loop.
>
> If a v live out at the header of the loop then the variable is live-in 
> at every node in the Loop. To prove this, Consider a Loop L with header h such that The variable v defined at d is live-in at h. Since v is live at h, d is not part of L. This follows from the dominance property, i.e. h is strictly dominated by d.
> Furthermore, there exists a path from h to a use of v which does not 
> go through d. For every node of the loop, p, since the loop is strongly connected Component of the CFG, there exists a path, consisting only of nodes of L from p to h. Concatenating those two paths prove that v is live-in and live-out Of p.
>
> Also Calculate the Live-Out and Live-In for the exit edge of the loop. This considers liveness for not only the Loop latch but also the liveness outside the Loops.
>
> Bootstrapped and Regtested for x86_64 and microblaze target.
>
> There is an extra failure for the testcase gcc.dg/loop-invariant.c  with the change that looks correct to me.
>
> This is because the available_regs = 6 and the regs_needed = 1 and 
> new_regs = 0 and the regs_used = 10.  As the reg_used that are based 
> on the Liveness given above is greater than the available_regs, then it's candidate of spill and the estimate_register_pressure calculates the spill cost. This spill cost is greater than inv_cost and gain comes to be negative. The disables the loop invariant for the above testcase.
>
> Disabling of the Loop invariant for the testcase loop-invariant.c with 
> this patch  looks correct to me considering the calculation of available_regs in cfgloopanal.c is correct.

>>You keep a lot of data (bitmaps) live just to use the number of bits set in the end.

>>I'll note that this also increases the complexity of the pass which is enabled at -O1+ where
>>-O1 should limit itself to O(n*log n) algorithms but live is quadratic.

>.So I don't think doing this unconditionally is a good idea (and we have -fira-loop-pressure after all).

>>Please watch not making -O1 worse again after we spent so much time making it suitable for all the weird autogenerated code.

I can also implement without keeping the bitmaps live data  and would not require to set the bits in the end.  I am interested in the
Liveness in the loop header and the loop exit which I can calculate where I am setting the regs_used for the curr_loop.

This will not require to set and keep the bitmaps for liveness and also make it available for the curr_loop that are candidates of loop
Invariant. 

Thanks & Regards
Ajit

Richard.

>  ChangeLog:
>  2015-12-09  Ajit Agarwal  <ajitkum@xilinx.com>
>
>         * loop-invariant.c
>         (calculate_loop_liveness): New.
>         (move_loop_invariants): Use of calculate_loop_liveness.
>
>     Signed-off-by:Ajit Agarwal ajitkum@xilinx.com
>
> ---
>  gcc/loop-invariant.c |   77 ++++++++++++++++++++++++++++++++++++++++++--------
>  1 files changed, 65 insertions(+), 12 deletions(-)
>
> diff --git a/gcc/loop-invariant.c b/gcc/loop-invariant.c index 
> 53d1377..ac08594 100644
> --- a/gcc/loop-invariant.c
> +++ b/gcc/loop-invariant.c
> @@ -1464,18 +1464,7 @@ find_invariants_to_move (bool speed, bool call_p)
>         registers used; we put some initial bound here to stand for
>         induction variables etc.  that we do not detect.  */
>      {
> -      unsigned int n_regs = DF_REG_SIZE (df);
> -
> -      regs_used = 2;
> -
> -      for (i = 0; i < n_regs; i++)
> -       {
> -         if (!DF_REGNO_FIRST_DEF (i) && DF_REGNO_LAST_USE (i))
> -           {
> -             /* This is a value that is used but not changed inside loop.  */
> -             regs_used++;
> -           }
> -       }
> +      regs_used = bitmap_count_bits (&LOOP_DATA 
> + (curr_loop)->regs_live) + 2;
>      }
>
>    if (! flag_ira_loop_pressure)
> @@ -1966,7 +1955,63 @@ mark_ref_regs (rtx x)
>        }
>  }
>
> +/* Calculate the Loop Liveness used for regs_used used in
> +   heuristics to calculate the register pressure.  */
> +
> +static void
> +calculate_loop_liveness (void)
> +{
> +  struct loop *loop;
> +
> +  FOR_EACH_LOOP (loop, 0)
> +    if (loop->aux == NULL)
> +      {
> +        loop->aux = xcalloc (1, sizeof (struct loop_data));
> +        bitmap_initialize (&LOOP_DATA (loop)->regs_live, &reg_obstack);
> +     }
> +
> +  FOR_EACH_LOOP (loop, LI_FROM_INNERMOST)  {
> +    int  i;
> +    edge e;
> +    vec<edge> edges;
> +    edges = get_loop_exit_edges (loop);
> +
> +    /* Loop Liveness is based on the following proprties.
> +       we only require to calculate the set of objects that are live at
> +       the birth or the header of the loop.
> +       We don't need to calculate the live through the Loop considering
> +       Live in and Live out of all the basic blocks of the Loop. This is
> +       because the set of objects. That are live-in at the birth or header
> +       of the loop will be live-in at every node in the Loop.
> +
> +       If a v live out at the header of the loop then the variable is live-in
> +       at every node in the Loop. To prove this, Consider a Loop L with header
> +       h such that The variable v defined at d is live-in at h. Since v is live
> +       at h, d is not part of L. This follows from the dominance property, i.e.
> +       h is strictly dominated by d. Furthermore, there exists a path from h to
> +       a use of v which does not go through d. For every node of the loop, p,
> +       since the loop is strongly connected Component of the CFG, there exists
> +       a path, consisting only of nodes of L from p to h. Concatenating those
> +       two paths prove that v is live-in and live-out Of p.  */
> +
> +    bitmap_ior_into (&LOOP_DATA (loop)->regs_live, DF_LR_IN (loop->header));
> +    bitmap_ior_into (&LOOP_DATA (loop)->regs_live, DF_LR_OUT 
> + (loop->header));
> +
> +    /* Calculate the Live-Out and Live-In for the exit edge of the loop.
> +       This considers liveness for not only the Loop latch but also the
> +       liveness outside the Loops.  */
> +
> +    FOR_EACH_VEC_ELT (edges, i, e)
> +    {
> +      bitmap_ior_into (&LOOP_DATA (loop)->regs_live, DF_LR_OUT (e->src));
> +      bitmap_ior_into (&LOOP_DATA (loop)->regs_live, DF_LR_IN (e->dest));
> +    }
> +  }
> +}
> +
>  /* Calculate register pressure in the loops.  */
> +
>  static void
>  calculate_loop_reg_pressure (void)
>  {
> @@ -2103,6 +2148,14 @@ move_loop_invariants (void)
>        calculate_loop_reg_pressure ();
>        regstat_free_n_sets_and_refs ();
>      }
> +  else
> +    {
> +      df_analyze();
> +      regstat_init_n_sets_and_refs ();
> +      calculate_loop_liveness();
> +      regstat_free_n_sets_and_refs ();
> +    }
> +
>    df_set_flags (DF_EQ_NOTES + DF_DEFER_INSN_RESCAN);
>    /* Process the loops, innermost first.  */
>    FOR_EACH_LOOP (loop, LI_FROM_INNERMOST)
> --
> 1.7.1
>
> Thanks & Regards
> Ajit
>
>