[gcc r16-9269] tree-optimization/125668 - missed alignment updating from forwprop

https://gcc.gnu.org/g:9c4272c58571ddcae364d1fc7d01a30f213dce10

commit r16-9269-g9c4272c58571ddcae364d1fc7d01a30f213dce10
Author: Richard Biener <[email protected]>
Date:   Mon Jun 8 14:02:38 2026 +0200

    tree-optimization/125668 - missed alignment updating from forwprop
    
    With the PR125296 fix I forgot to handle the LHS propagation case.
    
            PR tree-optimization/125668
            * tree-ssa-forwprop.cc (forward_propagate_addr_expr_1):
            Preserve alignment of the original access also for propagating
            into the LHS.
    
            * gcc.dg/pr125668.c: New testcase.
    
    (cherry picked from commit 878b5b0030d4b42b94b7df38375d6bf5f187dc33)

Diff:
---
 gcc/testsuite/gcc.dg/pr125668.c | 20 ++++++++++++++++++++
 gcc/tree-ssa-forwprop.cc        |  6 ++++--
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/pr125668.c b/gcc/testsuite/gcc.dg/pr125668.c
new file mode 100644
index 000000000000..4cacc4bd2350
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr125668.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fno-strict-aliasing -fdump-tree-forwprop1-gimple" } */
+
+struct S {
+  long x;
+  int y;
+};
+
+struct S2 {
+  struct S bufs[];
+};
+
+void foo(struct S2 *p, int i)
+{
+  struct S *buf = &p->bufs[i];
+  if (*((volatile int *)&buf->y) < 0)
+    *((volatile int *)&buf->y) = 0;
+}
+
+/* { dg-final { scan-tree-dump-times ", 32>" 2 "forwprop1" { target lp64 } } } */
diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
index 47f897a9e1ad..6b745382fa0e 100644
--- a/gcc/tree-ssa-forwprop.cc
+++ b/gcc/tree-ssa-forwprop.cc
@@ -770,8 +770,10 @@ forward_propagate_addr_expr_1 (tree name, tree def_rhs,
 	      new_base = build_fold_addr_expr (*def_rhs_basep);
 	      new_offset = TREE_OPERAND (lhs, 1);
 	    }
-	  *def_rhs_basep = build2 (MEM_REF, TREE_TYPE (*def_rhs_basep),
-				   new_base, new_offset);
+	  tree atype = TREE_TYPE (*def_rhs_basep);
+	  if (TYPE_ALIGN (TREE_TYPE (lhs)) < TYPE_ALIGN (atype))
+	    atype = build_aligned_type (atype, TYPE_ALIGN (TREE_TYPE (lhs)));
+	  *def_rhs_basep = build2 (MEM_REF, atype, new_base, new_offset);
 	  TREE_THIS_VOLATILE (*def_rhs_basep) = TREE_THIS_VOLATILE (lhs);
 	  TREE_SIDE_EFFECTS (*def_rhs_basep) = TREE_SIDE_EFFECTS (lhs);
 	  TREE_THIS_NOTRAP (*def_rhs_basep) = TREE_THIS_NOTRAP (lhs);