[PATCH] aarch64: don't emit bti j after NOTE_INSN_DELETED_LABEL [PR94748]

[Szabolcs Nagy]

It was previously discussed that indirect branches cannot go to
NOTE_INSN_DELETED_LABEL so inserting a landing pad is unnecessary.
See https://gcc.gnu.org/pipermail/gcc-patches/2019-May/522625.html

Before the patch a bti j was inserted after the label in

  __attribute__((target("branch-protection=bti")))
  int foo (void)
  {
  label:
    return 0;
  }

This is not necessary and weakens the security protection.

gcc/ChangeLog:

2020-04-28  Szabolcs Nagy  <szabolcs.n...@arm.com>

        PR target/94748
        * config/aarch64/aarch64-bti-insert.c (rest_of_insert_bti): Remove
        the check for NOTE_INSN_DELETED_LABEL.

gcc/testsuite/ChangeLog:

2020-04-28  Szabolcs Nagy  <szabolcs.n...@arm.com>

        PR target/94748
        * gcc.target/aarch64/pr94748.c: New test.
---
 gcc/config/aarch64/aarch64-bti-insert.c    |  8 ++------
 gcc/testsuite/gcc.target/aarch64/pr94748.c | 10 ++++++++++
 2 files changed, 12 insertions(+), 6 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/aarch64/pr94748.c

diff --git a/gcc/config/aarch64/aarch64-bti-insert.c 
b/gcc/config/aarch64/aarch64-bti-insert.c
index aa091c308f6..57663ee23b4 100644
--- a/gcc/config/aarch64/aarch64-bti-insert.c
+++ b/gcc/config/aarch64/aarch64-bti-insert.c
@@ -139,14 +139,10 @@ rest_of_insert_bti (void)
           insn = NEXT_INSN (insn))
        {
          /* If a label is marked to be preserved or can be a non-local goto
-            target, it must be protected with a BTI J.  The same applies to
-            NOTE_INSN_DELETED_LABEL since they are basically labels that might
-            be referenced via variables or constant pool.  */
-         if ((LABEL_P (insn)
+            target, it must be protected with a BTI J.  */
+         if (LABEL_P (insn)
               && (LABEL_PRESERVE_P (insn)
                   || bb->flags & BB_NON_LOCAL_GOTO_TARGET))
-             || (NOTE_P (insn)
-                 && NOTE_KIND (insn) == NOTE_INSN_DELETED_LABEL))
            {
              bti_insn = gen_bti_j ();
              emit_insn_after (bti_insn, insn);
diff --git a/gcc/testsuite/gcc.target/aarch64/pr94748.c 
b/gcc/testsuite/gcc.target/aarch64/pr94748.c
new file mode 100644
index 00000000000..2a2850d2fac
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr94748.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+
+__attribute__ ((target("branch-protection=bti")))
+int foo ()
+{
+label:
+  return 0;
+}
+
+/* { dg-final { scan-assembler-not {hint (36|38) // bti (j|jc)} } } */
-- 
2.17.1