Re: [PATCH] rtl-ssa: Fix splitting of clobber groups [PR108508]

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

On Thu, Feb 2, 2023 at 10:49 AM Richard Sandiford via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Since rtl-ssa isn't a real/native SSA representation, it has
> to honour the constraints of the underlying rtl representation.
> Part of this involves maintaining an rpo list of definitions
> for each rtl register, backed by a splay tree where necessary
> for quick lookup/insertion.
>
> However, clobbers of a register don't act as barriers to
> other clobbers of a register.  E.g. it's possible to move one
> flag-clobbering instruction across an arbitrary number of other
> flag-clobbering instructions.  In order to allow passes to do
> that without quadratic complexity, the splay tree groups all
> consecutive clobbers into groups, with only the group being
> entered into the splay tree.  These groups in turn have an
> internal splay tree of clobbers where necessary.
>
> This means that, if we insert a new definition and use into
> the middle of a sea of clobbers, we need to split the clobber
> group into two groups.  This was quite a difficult condition
> to trigger during development, and the PR shows that the code
> to handle it had (at least) two bugs.
>
> First, the process involves searching the clobber tree for
> the split point.  This search can give either the previous
> clobber (which will belong to the first of the split groups)
> or the next clobber (which will belong to the second of the
> split groups).  The code for the former case handled the
> split correctly but the code for the latter case didn't.
>
> Second, I'd forgotten to add the second clobber group to the
> main splay tree. :-(
>
> Tested on aarch64-linux-gnu & x86_64-linux-gnu.  OK for trunk
> & GCC 12?  Although the testcase is "only" a regression from GCC 12,
> I think the rtl-ssa patch should be backported to GCC 11 too.

OK.

Thanks,
Richard.

> Richard
>
>
> gcc/
>         PR rtl-optimization/108508
>         * rtl-ssa/accesses.cc (function_info::split_clobber_group): When
>         the splay tree search gives the first clobber in the second group,
>         make sure that the root of the first clobber group is updated
>         correctly.  Enter the new clobber group into the definition splay
>         tree.
>
> gcc/testsuite/
>         PR rtl-optimization/108508
>         * gcc.target/aarch64/pr108508.c: New test.
> ---
>  gcc/rtl-ssa/accesses.cc                     | 14 ++++++++---
>  gcc/testsuite/gcc.target/aarch64/pr108508.c | 28 +++++++++++++++++++++
>  2 files changed, 38 insertions(+), 4 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/aarch64/pr108508.c
>
> diff --git a/gcc/rtl-ssa/accesses.cc b/gcc/rtl-ssa/accesses.cc
> index 03b9a475d3b..f12b5f4dd77 100644
> --- a/gcc/rtl-ssa/accesses.cc
> +++ b/gcc/rtl-ssa/accesses.cc
> @@ -794,23 +794,26 @@ function_info::merge_clobber_groups (clobber_info *clobber1,
>  // GROUP spans INSN, and INSN now sets the resource that GROUP clobbers.
>  // Split GROUP around INSN and return the clobber that comes immediately
>  // before INSN.
> +//
> +// The resource that GROUP clobbers is known to have an associated
> +// splay tree.
>  clobber_info *
>  function_info::split_clobber_group (clobber_group *group, insn_info *insn)
>  {
>    // Search for either the previous or next clobber in the group.
>    // The result is less than zero if CLOBBER should come before NEIGHBOR
>    // or greater than zero if CLOBBER should come after NEIGHBOR.
> -  int comparison = lookup_clobber (group->m_clobber_tree, insn);
> +  clobber_tree &tree1 = group->m_clobber_tree;
> +  int comparison = lookup_clobber (tree1, insn);
>    gcc_checking_assert (comparison != 0);
> -  clobber_info *neighbor = group->m_clobber_tree.root ();
> +  clobber_info *neighbor = tree1.root ();
>
> -  clobber_tree tree1, tree2;
> +  clobber_tree tree2;
>    clobber_info *prev;
>    clobber_info *next;
>    if (comparison > 0)
>      {
>        // NEIGHBOR is the last clobber in what will become the first group.
> -      tree1 = neighbor;
>        tree2 = tree1.split_after_root ();
>        prev = neighbor;
>        next = as_a<clobber_info *> (prev->next_def ());
> @@ -843,6 +846,9 @@ function_info::split_clobber_group (clobber_group *group, insn_info *insn)
>    tree2->set_group (group2);
>    last_clobber->set_group (group2);
>
> +  // Insert GROUP2 into the splay tree as an immediate successor of GROUP1.
> +  def_splay_tree::insert_child (group1, 1, group2);
> +
>    return prev;
>  }
>
> diff --git a/gcc/testsuite/gcc.target/aarch64/pr108508.c b/gcc/testsuite/gcc.target/aarch64/pr108508.c
> new file mode 100644
> index 00000000000..e97896b6a1b
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/pr108508.c
> @@ -0,0 +1,28 @@
> +/* { dg-options "-O3 -fharden-conditional-branches -fno-dce -fno-guess-branch-probability" } */
> +
> +#include <arm_neon.h>
> +
> +int
> +test_vld3q_lane_f64 (void)
> +{
> +  float64x2x3_t vectors;
> +  float64_t temp[2];
> +  int i, j;
> +
> +  for (i = 0; i < 3; i++)
> +  {
> +    vst1q_f64 (temp, vectors.val[i]);
> +    for (j = 0; j < 2; j++)
> +      if (temp[j])
> +        return 1;
> +  }
> +
> +  return 0;
> +}
> +
> +void
> +foo (void)
> +{
> +  if (test_vld3q_lane_f64 () || test_vld3q_lane_f64 ())
> +    __builtin_abort ();
> +}
> --
> 2.25.1
>