+
+\f
+/* When LISP_FEATURE_C_STACK_IS_CONTROL_STACK, we cannot safely scrub
+ * the control stack from C, largely due to not knowing where the
+ * active stack frame ends. On such platforms, we reimplement the
+ * core scrubbing logic in assembly, in this case here:
+ */
+ .text
+ .align align_16byte,0x90
+ .globl GNAME(arch_scrub_control_stack)
+ TYPE(GNAME(arch_scrub_control_stack))
+GNAME(arch_scrub_control_stack):
+ /* We are passed three parameters:
+ * A (struct thread *) at [ESP+4],
+ * the address of the guard page at [ESP+8], and
+ * the address of the hard guard page at [ESP+12].
+ * We may trash EAX, ECX, and EDX with impunity.
+ * [ESP] is our return address, [ESP-4] is the first
+ * stack slot to scrub. */
+
+ /* We start by setting up our scrub pointer in EAX, our
+ * guard page upper bound in ECX, and our hard guard
+ * page upper bound in EDX. */
+ lea -4(%esp), %eax
+ mov GNAME(os_vm_page_size),%edx
+ mov %edx, %ecx
+ add 8(%esp), %ecx
+ add 12(%esp), %edx
+
+ /* We need to do a memory operation relative to the
+ * thread pointer, so put it in %ecx and our guard
+ * page upper bound in 4(%esp). */
+ xchg 4(%esp), %ecx
+
+ /* Now we begin our main scrub loop. */
+ascs_outer_loop:
+
+ /* If we're about to scrub the hard guard page, exit. */
+ cmp %edx, %eax
+ jae ascs_check_guard_page
+ cmp 12(%esp), %eax
+ ja ascs_finished
+
+ascs_check_guard_page:
+ /* If we're about to scrub the guard page, and the guard
+ * page is protected, exit. */
+ cmp 4(%esp), %eax
+ jae ascs_clear_loop
+ cmp 8(%esp), %eax
+ jbe ascs_clear_loop
+ cmpl $(NIL), THREAD_CONTROL_STACK_GUARD_PAGE_PROTECTED_OFFSET(%ecx)
+ jne ascs_finished
+
+ /* Clear memory backwards to the start of the (4KiB) page */
+ascs_clear_loop:
+ movl $0, (%eax)
+ test $0xfff, %eax
+ lea -4(%eax), %eax
+ jnz ascs_clear_loop
+
+ /* If we're about to hit the hard guard page, exit. */
+ cmp %edx, %eax
+ jae ascs_finished
+
+ /* If the next (previous?) 4KiB page contains a non-zero
+ * word, continue scrubbing. */
+ascs_check_loop:
+ testl $-1, (%eax)
+ jnz ascs_outer_loop
+ test $0xfff, %eax
+ lea -4(%eax), %eax
+ jnz ascs_check_loop
+
+ascs_finished:
+ ret
+ SIZE(GNAME(arch_scrub_control_stack))
+\f