Changes between Version 1 and Version 2 of Commentary/Rts/Stack

Sep 7, 2006 10:52:25 AM (9 years ago)



  • Commentary/Rts/Stack

    v1 v2  
    1717== Info tables for stack frames ==
    19 The info table for a stack frame has a couple of extra fields in addition to the [wiki:Commentary/Rts/HeapObjects#InfoTables basic info table layout].
     19The info table for a stack frame has a couple of extra fields in addition to the [wiki:Commentary/Rts/HeapObjects#InfoTables basic info table layout]:
     23The ''SRT'' field points to the SRT table for this stack frame (see [wiki: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 [wiki:Commentary/Rts/HaskellExecution#VectoredReturns vectored returns] for more details.
     25== Layout of the payload ==
     27Unlike 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.
     29The 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'':
     31=== Small bitmaps ===
     33A small bitmap fits into a single word (the layout word of the info table), and looks like this:
     35|| Size (bits 0-4) || Bitmap (bits 5-31) ||
     37(for a 64-bit word size, the size is given 6 bits instead of 5). 
     39The 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.
     41The macros {{{MK_BITMAP}}}, {{{BITMAP_SIZE}}}, and {{{BITMAP_BITS}}} in [[GhcFile(includes/InfoTables.h)]] provide ways to conveniently operate on small bitmaps.
     43=== Large bitmaps ===
     45If 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 [[GhcFile(includes/InfoTables.h)]].
    2147== Stack Frames ==