refactor bitfields

Use 2 level strategy to access packed bitfields cleanly:

1) Allow to override the original declaration type with
   an auxilary "access type".  This solves cases such as

    struct {
        ...
        unsigned f1:1;
    };

    by using VT_BYTE to access f1.

2) Allow byte-wise split accesses using two new functions
   load/store_packed_bf. This solves any cases, also ones
   such as

    struct __attribute((packed)) _s {
        unsigned x : 12;
        unsigned char y : 7;
        unsigned z : 28;
        unsigned a: 3;
        unsigned b: 3;
        unsigned c: 3;
    };

    where for field 'z':
    - VT_INT access from offset 2 would be unaligned
    - VT_LLONG from offset 0 would go past the total
      struct size (7)

    and for field 'a' because it is in two bytes and
    aligned access with VT_SHORT/INT is not possible.

Also, static bitfield initializers are stored byte-wise always.
Also, cleanup the struct_layout function a bit.

1 files changed, 1 insertions, 0 deletions

diff --git a/tcc.h b/tcc.h
index 3e81e03..47c3c5f 100644
--- a/tcc.h
+++ b/tcc.h
@@ -450,6 +450,7 @@ typedef struct Sym {
                 int sym_scope; /* scope level for locals */
                 int jnext; /* next jump label */
                 struct FuncAttr f; /* function attributes */
+                int auxtype; /* bitfield access type */
             };
         };
         long long enum_val; /* enum constant if IS_ENUM_VAL */