[gcc r17-3265] backwards threader: handle SSA names occurring in abnormal PHIs

https://gcc.gnu.org/g:3e43b55eb8fb520b6ed161697249fcef6699d11b

commit r17-3265-g3e43b55eb8fb520b6ed161697249fcef6699d11b
Author: Aldy Hernandez <[email protected]>
Date:   Thu Aug 13 10:28:51 2026 +0000

    backwards threader: handle SSA names occurring in abnormal PHIs
    
    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.
    
    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.

Diff:
---
 .../gcc.dg/tree-ssa/backthread-computed-goto-2.c   | 22 +++++++++++
 .../gcc.dg/tree-ssa/ssa-thread-abnormal-phi-1.c    | 46 ++++++++++++++++++++++
 .../gcc.dg/tree-ssa/ssa-thread-abnormal-phi-2.c    | 45 +++++++++++++++++++++
 gcc/tree-ssa-threadbackward.cc                     |  6 +--
 4 files changed, 114 insertions(+), 5 deletions(-)

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 000000000000..86d0f6882103
--- /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 000000000000..7fe315fdea84
--- /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 000000000000..8f74084ee2c3
--- /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 77726ba66cd9..ca1af87bb401 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