phiopt: Fix up DEBUG_EXPR_DECL creation in spaceship_replacement [PR122394]

The following testcase ICEs in gcc 15 (and is at least latent in 12-14 too),
because the DEBUG_EXPR_DECL has incorrect mode.  It has
TREE_TYPE (orig_use_lhs) type, but TYPE_MODE (type) rather than
TYPE_MODE (TREE_TYPE (orig_use_lhs)) where the two types are sometimes
the same, but sometimes different (same if !has_cast_debug_uses, different
otherwise).

Though, there wouldn't be the this issue if it used the proper API to create
the DEBUG_EXPR_DECL which takes care of everything.  This is the sole
spot that doesn't use that API.

Doesn't affect the trunk because the code has been removed and replaced with
different stuff after the libstdc++ ABI change in r16-3474.
Before r15-5557 the mode has been always wrong because this was done only
for has_cast_debug_uses.  And the bug has been introduced with r12-5490.

Enough archeology, while it could be fixed by changing the second
SET_DECL_MODE argument, I think it is better to use build_debug_expr_decl.

On the trunk only the testcase is being added.

2025-10-27  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/122394
	* g++.dg/opt/pr122394.C: New test.

diff --git a/gcc/testsuite/g++.dg/opt/pr122394.C b/gcc/testsuite/g++.dg/opt/pr122394.C
new file mode 100644 (file)
index 0000000..1f84beb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr122394.C
@@ -0,0 +1,20 @@
+// PR tree-optimization/122394
+// { dg-do compile { target c++23 } }
+// { dg-options "-O1 -g" }
+
+#include <compare>
+
+struct A {
+  friend auto operator<=> (A, A) = default;
+  double a;
+};
+void foo ();
+A b, c;
+
+void
+bar ()
+{
+  bool d = c >= b;
+  if (d)
+    foo ();
+}