ipa: Disallow signature changes in fun->has_musttail functions [PR121023]

As the following testcase shows e.g. on ia32, letting IPA opts change
signature of functions which have [[{gnu,clang}::musttail]] calls
can turn programs that would be compiled normally into something
that is rejected because the caller has fewer argument stack slots
than the function being tail called.

The following patch prevents signature changes for such functions.
It is perhaps too big hammer in some cases, but it might be hard
to try to figure out what signature changes are still acceptable and which
are not at IPA time.

2025-07-11  Jakub Jelinek  <jakub@redhat.com>
	    Martin Jambor  <mjambor@suse.cz>

	PR ipa/121023
	* ipa-fnsummary.cc (compute_fn_summary): Disallow signature changes
	on cfun->has_musttail functions.

	* c-c++-common/musttail32.c: New test.

(cherry picked from commit 89b9372d61ccd45cb6c71518d62215917e3aaebc)

diff --git a/gcc/ipa-fnsummary.cc b/gcc/ipa-fnsummary.cc
index 4c062fe8a0e286b07f98817d84d1b4d9671a34c9..0fb4f6cf29f0bd16d297c729c4f7222cb8e6ef61 100644 (file)
--- a/gcc/ipa-fnsummary.cc
+++ b/gcc/ipa-fnsummary.cc
@@ -3421,6 +3421,21 @@ compute_fn_summary (struct cgraph_node *node, bool early)
         info->inlinable = tree_inlinable_function_p (node->decl);
 
        bool no_signature = false;
+
+       /* Don't allow signature changes for functions which have
+         [[gnu::musttail]] or [[clang::musttail]] calls.  Sometimes
+         (more often on targets which pass everything on the stack)
+         signature changes can result in tail calls being impossible
+         even when without the signature changes they would be ok.
+         See PR121023.  */
+       if (cfun->has_musttail)
+        {
+          if (dump_file)
+           fprintf (dump_file, "No signature change:"
+                    " function has calls with musttail attribute.\n");
+          no_signature = true;
+        }
+
        /* Type attributes can use parameter indices to describe them.
          Special case fn spec since we can safely preserve them in
          modref summaries.  */

diff --git a/gcc/testsuite/c-c++-common/musttail32.c b/gcc/testsuite/c-c++-common/musttail32.c
new file mode 100644 (file)
index 0000000..f1b7052
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/musttail32.c
@@ -0,0 +1,23 @@
+/* PR ipa/121023 */
+/* { dg-do compile { target musttail } } */
+/* { dg-options "-O2" } */
+
+struct S { int a, b; };
+
+[[gnu::noipa]] int
+foo (struct S x, int y, int z)
+{
+  return x.a + y + z;
+}
+
+[[gnu::noinline]] static int
+bar (struct S x, int y, int z)
+{
+  [[gnu::musttail]] return foo ((struct S) { x.a, 0 }, y, 1);
+}
+
+int
+baz (int x)
+{
+  return bar ((struct S) { 1, 2 }, x, 2) + bar ((struct S) { 2, 3 }, x + 1, 2);
+}