Version 2 (modified by simonmar, 11 years ago) (diff)


[ Up: Commentary/Rts ]

Layout of the stack

Every TSO object contains a stack. The stack of a TSO grows downwards, with the topmost (most recently pushed) word pointed to by tso->sp, and the bottom of the stack given by tso->stack + tso->stack_size.

The stack consists of a sequence of stack frames (also sometimes called activation records) where each frame has the same layout as a heap object:

Header Payload...

There are several kinds of stack frame, but the most common types are those pushed when evaluating a case expression:

  case e0 of p1 -> e1; ...; pn -> en 

The code for evaluating a case pushes a new stack frame representing the alternatives of the case, and continues by evaluating e0. When e0 completes, it returns to the stack frame pushed earlier, which inspects the value and selects the appropriate branch of the case. The stack frame for a case includes the values of all the free variables in the case alternatives.

Info tables for stack frames

The info table for a stack frame has a couple of extra fields in addition to the basic info table layout:

No image "ret-itbl.png" attached to Commentary/Rts/Stack

The SRT field points to the SRT table for this stack frame (see Commentary/Rts/CAFs for details of SRTs). The return vector gives a vector of return addresses in the case of the RET_VEC_SMALL and RET_VEC_BIG types of return addresses; see vectored returns for more details.

Layout of the payload

Unlike heap objects which mainly have "pointers first" layout, in a stack frame the pointers and non-pointers are intermingled. This is so that we can support "stack stubbing" whereby a live variable stored on the stack can be later marked as dead simply by pushing a new stack frame that identifies that slot as containing a non-pointer, so the GC will not follow it.

The stack frame describes the pointerhood of each word in the payload by means of a bitmap. There are two kinds of bitmap: small and large:

Small bitmaps

A small bitmap fits into a single word (the layout word of the info table), and looks like this:

Size (bits 0-4) Bitmap (bits 5-31)

(for a 64-bit word size, the size is given 6 bits instead of 5).

The size field gives the size of the payload, and each bit of the bitmap is 1 if the corresponding word of payload contains a pointer to a live object.

The macros MK_BITMAP, BITMAP_SIZE, and BITMAP_BITS in includes/InfoTables.h provide ways to conveniently operate on small bitmaps.

Large bitmaps

If the size of the stack frame is larger than the 27 words that a small bitmap can describe, then the fallback mechanism is the large bitmap. A large bitmap is a separate structure, containing a single word size and a multi-word bitmap: see StgLargeBitmap in includes/InfoTables.h.

Stack Frames