[gcc r17-3299] Add new relation before starting search

https://gcc.gnu.org/g:62214130b61f48cf7cdb37c69bb14c22aea6c65a

commit r17-3299-g62214130b61f48cf7cdb37c69bb14c22aea6c65a
Author: Andrew MacLeod <[email protected]>
Date:   Thu Aug 13 13:44:09 2026 -0400

    Add new relation before starting search
    
    Create a new relation immediately. Then Calculate any known range and
    intersect the result.
    
            PR tree-optimization/126845
            gcc/
            * value-relation.cc (dom_oracle::search_and_merge_relation): Create
            relation immediately and update it later.
    
            gcc/testsuite/
            * gcc.dg/pr126845.c: New.

Diff:
---
 gcc/testsuite/gcc.dg/pr126845.c | 19 +++++++++++++++++++
 gcc/value-relation.cc           | 27 ++++++++++++++++-----------
 2 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/pr126845.c b/gcc/testsuite/gcc.dg/pr126845.c
new file mode 100644
index 000000000000..0ae91d9a7731
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr126845.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fno-move-loop-stores" } */
+
+int a, c, d, e, **f, *g, h;
+static int i() {}
+void j() {
+  do {
+    f = &g;
+    c = 1 + (1 >> a);
+    if (2 + c >= c - a)
+      goto k;
+    g = &c;
+  } while (i());
+  c = a + c + c;
+k:
+  e = **f;
+  for (int b = 0; b < 3; b++)
+    h = d = d ^ e;
+}
diff --git a/gcc/value-relation.cc b/gcc/value-relation.cc
index 04e9cc198437..cc9e8e56c67b 100644
--- a/gcc/value-relation.cc
+++ b/gcc/value-relation.cc
@@ -1241,22 +1241,22 @@ dom_oracle::search_and_merge_relation (basic_block bb, relation_kind k,
 {
   gcc_checking_assert (k != VREL_VARYING && k != VREL_EQ);
 
-  int bbi = bb->index;
-
-  relation_kind curr;
   relation_chain *ptr;
-  curr = find_relation_block (bbi, op1, op2, &ptr);
-  // There is an existing relation in this block, just intersect with it.
+  relation_kind curr = find_relation_block (bb->index, op1, op2, &ptr);
+
+  // If there is an existing relation in this block, just intersect with it.
   if (curr != VREL_VARYING)
     {
-      // Check into whether we can simply replace the relation rather than
-      // intersecting it.  This may help with some optimistic iterative
-      // updating algorithms.  If there was no change, return no record..
+      // If there was no change, return no record.
       value_relation vr (k, op1, op2);
       if (!ptr->intersect (vr))
 	return NULL;
+      return ptr;
     }
-  else
+
+  // Create the relation in this block.
+  ptr = create_relation_in_bb (bb, k, op1, op2);
+  if (ptr)
     {
       // Check for an existing relation further up the DOM chain.
       // By including dominating relations, The first one found in any search
@@ -1264,8 +1264,13 @@ dom_oracle::search_and_merge_relation (basic_block bb, relation_kind k,
       curr = find_relation_dom (get_immediate_dominator (CDI_DOMINATORS, bb),
 				op1, op2);
       if (curr != VREL_VARYING)
-	k = relation_intersect (curr, k);
-      ptr = create_relation_in_bb (bb, k, op1, op2);
+	{
+	  curr = relation_intersect (curr, k);
+	  // Intersect the new relation with the existing one, unless the
+	  // result is UNDEFINED.  Then just leave it.
+	  if (curr != k && curr != VREL_UNDEFINED)
+	    ptr->set_relation (curr, op1, op2);
+	}
     }
   return ptr;
 }