[gcc r16-6918] middle-end/123697 - fix .MASK_LOAD_LANES folding

[Richard Biener]

https://gcc.gnu.org/g:46a3355c7f165661a048151ce36cf6e08a4e5d9d

commit r16-6918-g46a3355c7f165661a048151ce36cf6e08a4e5d9d
Author: Richard Biener <rguenther@suse.de>
Date:   Tue Jan 20 10:57:34 2026 +0100

    middle-end/123697 - fix .MASK_LOAD_LANES folding
    
    .MASK_LOAD_LANES has an aggregate (array of vectors) return value
    which is not compatible with the else value used when trying to
    fold this with all lanes inactive.  Instead use an empty CTOR if
    the else value is zero and otherwise do not simplify.
    
            PR middle-end/123697
            * gimple-fold.cc (gimple_fold_partial_load_store): Use an
            empty CTOR for a zero else value in .MASK_LOAD_LANES.
    
            * gcc.dg/vect/vect-pr123697.c: New testcase.

Diff:
---
 gcc/gimple-fold.cc                        |  6 ++++++
 gcc/testsuite/gcc.dg/vect/vect-pr123697.c | 13 +++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
index b92bf450aba3..4766187e0b9d 100644
--- a/gcc/gimple-fold.cc
+++ b/gcc/gimple-fold.cc
@@ -5899,6 +5899,12 @@ gimple_fold_partial_load_store (gimple_stmt_iterator *gsi, gcall *call)
 	  /* Replace load with else value.  */
 	  int else_index = internal_fn_else_index (ifn);
 	  tree else_value = gimple_call_arg (call, else_index);
+	  if (!is_gimple_reg (lhs))
+	    {
+	      if (!zerop (else_value))
+		return false;
+	      else_value = build_constructor (TREE_TYPE (lhs), NULL);
+	    }
 	  gassign *new_stmt = gimple_build_assign (lhs, else_value);
 	  gimple_set_location (new_stmt, gimple_location (call));
 	  gsi_replace (gsi, new_stmt, false);
diff --git a/gcc/testsuite/gcc.dg/vect/vect-pr123697.c b/gcc/testsuite/gcc.dg/vect/vect-pr123697.c
new file mode 100644
index 000000000000..e1717012e0a3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/vect-pr123697.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-O3" } */
+
+struct a {
+  int c[2];
+};
+struct a d[3];
+double f;
+void g()
+{
+  for (int e = 0; e < 3; ++e)
+    f += d[e].c[1];
+}