Re: [PATCH 2/2] backwards threader: handle SSA names occurring in abnormal PHIs

> Am 13.08.2026 um 19:52 schrieb Aldy Hernandez <[email protected]>:
> 
> The backward threader refuses to look at any SSA name occurring in an
> abnormal PHI, but DOM's threader does, at least for PHIs where the
> path taken does not include an abnormal edge.  For example:
> 
>    x_5(ab) = PHI <45(5), y_8(ab)>
>    ...
>    if (x_5 == 45)
> 
> If we're threading a path coming in from BB5, it's perfectly valid to
> thread this path, even if x_5 itself is AB.
> 
> With gimple_range_ssa_p no longer rejecting these names, the path
> solver handles them as is: a path never includes abnormal edges, so an
> abnormal PHI still resolves to its argument on the path's incoming
> edge.  Drop the last discovery restriction in the backward threader.
> Only normal predecessor edges are ever walked, so no abnormal edge is
> duplicated.
> 
> This also enables threading computed gotos whose destination occurs
> in an abnormal PHI, which happens when the goto block is itself one
> of the goto's targets.
> 
> Tested on ppc64le Linux.
> 
> OK?

Ok as well

Richard 

> gcc/ChangeLog:
> 
>    PR tree-optimization/126103
>    * tree-ssa-threadbackward.cc (back_threader::find_paths_to_names):
>    Follow abnormal PHIs.
> 
> gcc/testsuite/ChangeLog:
> 
>    PR tree-optimization/126103
>    * gcc.dg/tree-ssa/ssa-thread-abnormal-phi-1.c: New test.
>    * gcc.dg/tree-ssa/ssa-thread-abnormal-phi-2.c: New test.
>    * gcc.dg/tree-ssa/backthread-computed-goto-2.c: New test.
> ---
> .../tree-ssa/backthread-computed-goto-2.c     | 22 +++++++++
> .../tree-ssa/ssa-thread-abnormal-phi-1.c      | 46 +++++++++++++++++++
> .../tree-ssa/ssa-thread-abnormal-phi-2.c      | 45 ++++++++++++++++++
> gcc/tree-ssa-threadbackward.cc                |  6 +--
> 4 files changed, 114 insertions(+), 5 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/backthread-computed-goto-2.c
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-abnormal-phi-1.c
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-abnormal-phi-2.c
> 
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/backthread-computed-goto-2.c b/gcc/testsuite/gcc.dg/tree-ssa/backthread-computed-goto-2.c
> new file mode 100644
> index 00000000000..86d0f688210
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/backthread-computed-goto-2.c
> @@ -0,0 +1,22 @@
> +/* PR tree-optimization/126103 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdisable-tree-ethread -fdump-tree-threadfull1-details" } */
> +
> +int g;
> +
> +void
> +h (int a)
> +{
> +  void *q;
> +  if (a)
> +    q = &&L0;
> +  else
> +    q = &&L2;
> +L2:
> +  g++;
> +  goto *q;
> +L0:
> +  return;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "Registering jump thread" 2 "threadfull1" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-abnormal-phi-1.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-abnormal-phi-1.c
> new file mode 100644
> index 00000000000..7fe315fdea8
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-abnormal-phi-1.c
> @@ -0,0 +1,46 @@
> +/* PR tree-optimization/126103 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdisable-tree-ethread -fdump-tree-threadfull1-details" } */
> +
> +/* The conditional at "join" tests the result of an abnormal PHI:
> +   t = PHI <k(ab), 0, 0>.  Both normal predecessors feed constant 0,
> +   so the backward threader must thread them past the conditional;
> +   the abnormal predecessor keeps the original block.  The two normal
> +   predecessors of the computed goto also know its destination, so
> +   they are threaded as well.  */
> +
> +void foo (void);
> +void bar (void);
> +
> +void
> +f (int k)
> +{
> +  void *p = (k & 1) ? &&yes : &&join;
> +  int t;
> +
> +  if (k > 0)
> +    {
> +      t = k;
> +      goto *p;
> +    }
> +
> +  if (k < -5)
> +    {
> +      foo ();
> +      t = 0;
> +    }
> +  else
> +    {
> +      bar ();
> +      t = 0;
> +    }
> +
> +join:
> +  if (t != 0)
> +    {
> +yes:
> +      foo ();
> +    }
> +}
> +
> +/* { dg-final { scan-tree-dump-times "Registering jump thread" 4 "threadfull1" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-abnormal-phi-2.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-abnormal-phi-2.c
> new file mode 100644
> index 00000000000..8f74084ee2c
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-abnormal-phi-2.c
> @@ -0,0 +1,45 @@
> +/* PR tree-optimization/126103 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdisable-tree-ethread -fdump-tree-threadfull1-details" } */
> +
> +/* Like ssa-thread-abnormal-phi-1.c, but the conditional tests a value
> +   derived from the abnormal PHI.  */
> +
> +void foo (void);
> +void bar (void);
> +int g;
> +
> +void
> +f (int k)
> +{
> +  void *p = (k & 1) ? &&yes : &&join;
> +  int t;
> +
> +  if (k > 0)
> +    {
> +      t = k;
> +      goto *p;
> +    }
> +
> +  if (k < -5)
> +    {
> +      foo ();
> +      t = 1;
> +    }
> +  else
> +    {
> +      bar ();
> +      t = 0;
> +    }
> +
> +join:
> +  if (g)
> +    bar ();
> +  if ((t & 2) == 0)
> +    {
> +yes:
> +      foo ();
> +    }
> +}
> +
> +/* { dg-final { scan-tree-dump-times "Registering jump thread" 6 "threadfull1" } } */
> diff --git a/gcc/tree-ssa-threadbackward.cc b/gcc/tree-ssa-threadbackward.cc
> index 77726ba66cd..ca1af87bb40 100644
> --- a/gcc/tree-ssa-threadbackward.cc
> +++ b/gcc/tree-ssa-threadbackward.cc
> @@ -450,11 +450,7 @@ back_threader::find_paths_to_names (basic_block bb, bitmap interesting,
>        }
>          /* Local PHIs participate in renaming below.  */
>          if (gphi *phi = dyn_cast<gphi *> (def_stmt))
> -        {
> -          tree res = gimple_phi_result (phi);
> -          if (!SSA_NAME_OCCURS_IN_ABNORMAL_PHI (res))
> -            interesting_phis.safe_push (phi);
> -        }
> +        interesting_phis.safe_push (phi);
>          /* For other local defs process their uses, amending
>         imports on the way.  */
>          else
> --
> 2.47.3
>