ffi: couple of tests; assertion.

* ffi.c (make_ffi_type_struct): Add check for impossible condition.
The bits_alloc variable could only exceed bits_type (and thus
cause the room variable to have a nonsensical, large value)
if the bitfield allocation tried to continue allocating bits into
an aligned unit, whose alignment exceeds the size of the underlying
type. But in that case, tft->aligned would have to be true, and
so the offset would have been aligned prior to this code, rendering
bits_alloc zero.

* tests/017/bitfields.tl: New tests.

1 files changed, 2 insertions, 0 deletions

diff --git a/ffi.c b/ffi.c
index 1c44b442..db7a4c38 100644
--- a/ffi.c
+++ b/ffi.c
@@ -3817,6 +3817,8 @@ static val make_ffi_type_struct(val syntax, val lisp_type,
       ucnum bits_alloc = 8 * (offs - unit_offs) + bit_offs;
       ucnum room = bits_type - bits_alloc;
 
+      bug_unless (bits_type >= bits_alloc);
+
       if (bits == 0) {
         ucnum szmask = size - 1;
         ucnum unit_offs = offs & ~szmask;