[PATCH] aarch64: Avoid duplicating bti j insns for jump tables [PR99988]

[Alex Coplan via Gcc-patches]

Hi all,

This patch fixes PR99988 which shows us generating large (> 250)
sequences of back-to-back bti j instructions.

The fix is simply to avoid inserting bti j instructions at the target of
a jump table if we've already inserted one for a given label.

Testing:
 * Bootstrapped and regtested on aarch64-linux-gnu (with and without
 -mbranch-protection=standard), no regressions.

Presumably this is stage 1 material since the bug isn't a regression? If
so, OK for GCC 12 stage 1?

Thanks,
Alex

gcc/ChangeLog:

        PR target/99988
        * config/aarch64/aarch64-bti-insert.c (aarch64_bti_j_insn_p): New.
        (rest_of_insert_bti): Avoid inserting duplicate bti j insns for
        jump table targets.

gcc/testsuite/ChangeLog:

        PR target/99988
        * gcc.target/aarch64/pr99988.c: New test.

diff --git a/gcc/config/aarch64/aarch64-bti-insert.c 
b/gcc/config/aarch64/aarch64-bti-insert.c
index 936649769c7..943fa3c1097 100644
--- a/gcc/config/aarch64/aarch64-bti-insert.c
+++ b/gcc/config/aarch64/aarch64-bti-insert.c
@@ -120,6 +120,13 @@ aarch64_pac_insn_p (rtx x)
   return false;
 }
 
+static bool
+aarch64_bti_j_insn_p (rtx_insn *insn)
+{
+  rtx pat = PATTERN (insn);
+  return GET_CODE (pat) == UNSPEC_VOLATILE && XINT (pat, 1) == UNSPECV_BTI_J;
+}
+
 /* Insert the BTI instruction.  */
 /* This is implemented as a late RTL pass that runs before branch
    shortening and does the following.  */
@@ -165,6 +172,9 @@ rest_of_insert_bti (void)
                  for (j = GET_NUM_ELEM (vec) - 1; j >= 0; --j)
                    {
                      label = as_a <rtx_insn *> (XEXP (RTVEC_ELT (vec, j), 0));
+                     if (aarch64_bti_j_insn_p (next_nonnote_insn (label)))
+                       continue;
+
                      bti_insn = gen_bti_j ();
                      emit_insn_after (bti_insn, label);
                    }
diff --git a/gcc/testsuite/gcc.target/aarch64/pr99988.c 
b/gcc/testsuite/gcc.target/aarch64/pr99988.c
new file mode 100644
index 00000000000..2d87f41a717
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr99988.c
@@ -0,0 +1,66 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -mbranch-protection=standard" } */
+/* { dg-final { scan-assembler-times {bti j} 13 } } */
+int a;
+int c();
+int d();
+int e();
+int f();
+int g();
+void h() {
+  switch (a) {
+  case 0:
+  case 56:
+  case 57:
+    break;
+  case 58:
+  case 59:
+  case 61:
+  case 62:
+    c();
+  case 64:
+  case 63:
+    d();
+  case 66:
+  case 65:
+    d();
+  case 68:
+  case 67:
+    d();
+  case 69:
+  case 70:
+    d();
+  case 71:
+  case 72:
+  case 88:
+  case 87:
+    d();
+  case 90:
+  case 89:
+    d();
+  case 92:
+  case 1:
+    d();
+  case 93:
+  case 73:
+  case 4:
+    e();
+  case 76:
+  case 5:
+    f();
+  case 7:
+  case 8:
+  case 84:
+  case 85:
+    break;
+  case 6:
+  case 299:
+  case 9:
+  case 80:
+  case 2:
+  case 3:
+    e();
+  default:
+    g();
+  }
+}