data-layout

Fixnum:
...data|0|

Non-fixnum:
...data|ttt1|
t are tag bits

All non-cons objects have a 1-word header at the start.

...data|tttttttt|00|
Data is specific to the object type, t are type tag bits, the low two bits must be zero during normal operation and are used by the GC.
Having bit 0 always be 0 allows the header to be treated as a fixnum.

1D simple arrays treat data as the number of elements in the array and store the array data immediately after the header.
Certain 1D simple arrays insert a word of padding after the header for alignment:
single-float, double-float, long-float, complex-single-float, complex-double-float, complex-long-float, xmm-vector

Symbols are exactly 6 words long (including the header).
Header data layout:
...0|SSSSSSSSSSSSSSSS|tttttttt|00|

S is the symbol's TLS offset. 0 when no offset has been specified.

Symbol layout:
-1 header
+0 Name (always a string)
+1 Package (may be anything, but usually either NIL or a package)
+2 Value (may be anything, including unbound-value)
+3 Function (either NIL or a function reference) (could be weak ref?)
+4 Plist (must be a list)


Function layout:
-1 header
+0 primary entry point address, unboxed machine integer
+1 start of machine code
+n end of machine code & start of gc info
+n+m end of gc info & start of constant pool
+n+m+p end of constant pool

Function header -1:
GGGGGGGGGGGGGGGG|PPPPPPPPPPPPPPPP|MMMMMMMMMMMMMMMM|00000000|tttttttt|00|

M is machine code size in double-word units (128 bits/16 bytes)
P is constant pool size in double-word units (128 bits/16 bytes)
G is GC info length in bytes



Struct layout:
-1 header
+0 structure definition or 0 (uninitialized)
+1 slots

struct header data holds the number of slots plus one (the definition slot).



Complex array layout:
This layout is used for all non-simple-1d and non-memory arrays.

-1 header
+0 storage
+1 fill-pointer
+2 displacement (fixnum index if displaced, nil otherwise)
+3... Dimensions as fixnums

header data contains the array rank.


Memory array layout

-1 header
+0 storage (fixnum memory address)
+1 fill-pointer
+2 element-type (as what?)
+3... Dimensions as fixnums

header data contains the array rank.



Function reference

-1 header
+0 name
+1 function
+2 entry-point (unbound functions have entry-point point to undefined-function thunk)

Calls via function reference must have fref in r13 for the undefined-function thunk.