Re: PR88751: Backport to GCC 8 and 9 branches?

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

On Fri, Sep 6, 2019 at 10:11 AM Andreas Krebbel <krebbel@linux.ibm.com> wrote:
>
> Hi,
>
> since this caused a critical performance regression in the OpenJ9 byte code interpreter after
> migrating from GCC 4.8 to GCC 7 I would like to backport this patch also to GCC 8 and 9 branch.
>
> Ok - after bootstrap and regression test went fine?

Looks reasonable to me.  But what about GCC 7?  I assume you also verified the
actual performance regression is gone.

Richard.

>
> Andreas
>
>
> commit d3dc20418aad41af83fe45ccba527deb0b334983
> Author: krebbel <krebbel@138bc75d-0d04-0410-961f-82ee72b054a4>
> Date:   Thu Jun 6 11:35:04 2019 +0000
>
>     Fix PR88751
>
>     This patch implements a small improvement for the heuristic in lra
>     which decides when it has to activate the simpler register allocation
>     algorithm.
>
>     gcc/ChangeLog:
>
>     2019-06-06  Andreas Krebbel  <krebbel@linux.ibm.com>
>
>             PR rtl-optimization/88751
>             * ira.c (ira): Use the number of the actually referenced registers
>             when calculating the threshold.
>
>
>
>     git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@271996 138bc75d-0d04-0410-961f-82ee72b054a4
>
>
> diff --git a/gcc/ira.c b/gcc/ira.c
> index 4a14fb31583..725636d8dc5 100644
> --- a/gcc/ira.c
> +++ b/gcc/ira.c
> @@ -5198,6 +5198,8 @@ ira (FILE *f)
>    int ira_max_point_before_emit;
>    bool saved_flag_caller_saves = flag_caller_saves;
>    enum ira_region saved_flag_ira_region = flag_ira_region;
> +  unsigned int i;
> +  int num_used_regs = 0;
>
>    clear_bb_flags ();
>
> @@ -5213,12 +5215,17 @@ ira (FILE *f)
>
>    ira_conflicts_p = optimize > 0;
>
> +  /* Determine the number of pseudos actually requiring coloring.  */
> +  for (i = FIRST_PSEUDO_REGISTER; i < DF_REG_SIZE (df); i++)
> +    num_used_regs += !!(DF_REG_USE_COUNT (i) + DF_REG_DEF_COUNT (i));
> +
>    /* If there are too many pseudos and/or basic blocks (e.g. 10K
>       pseudos and 10K blocks or 100K pseudos and 1K blocks), we will
>       use simplified and faster algorithms in LRA.  */
>    lra_simple_p
>      = (ira_use_lra_p
> -       && max_reg_num () >= (1 << 26) / last_basic_block_for_fn (cfun));
> +       && num_used_regs >= (1 << 26) / last_basic_block_for_fn (cfun));
> +
>    if (lra_simple_p)
>      {
>        /* It permits to skip live range splitting in LRA.  */
>