load: fix vm gc issue.

This issue manifested itself as a segfault on Mac OS
while compiling a library file using the compiled
implementation (hash.tl -> hash.tlo2).

The problem is premature reclamation of a virtual machine
description.

While the asm.tlo module is loaded, suddenly a function table
goes corrupt.

When we execute machine using vm_execute_toplevel, the vm's
stack doesn't actually contain a reference to the description
object anywhere. It's passed in as a parameter, and right away
converted to the (struct vm_desc *) handle which is planted in
the struct vm representing the running instance, which lives
on the stack. That pointer isn't a reference traversed by the
garbage collector. If the original Lisp object is optimized
away and overwritten both in vm_execute_toplevel and in that
function's caller, then it can be reclaimed even as it
executes.

* parser.c (read_file_common): Add a gc_hint(desc) after the
vm_execute_toplevel so that the compiler thinks the value of
the variable is still needed after the call. That keeps it
around on the stack or in a register so that the garbage
collector can see it as a root pointer.

1 files changed, 1 insertions, 0 deletions

diff --git a/parser.c b/parser.c
index 237860c8..5d310b5b 100644
--- a/parser.c
+++ b/parser.c
@@ -641,6 +641,7 @@ static val read_file_common(val stream, val error_stream, val compiled)
         val funvec = car(item);
         val desc = vm_make_desc(nlevels, nregs, bytecode, datavec, funvec);
         (void) vm_execute_toplevel(desc);
+        gc_hint(desc);
       }
     } else {
       (void) eval_intrinsic(form, nil);