Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Some collectors assume everything on the stack is a potential pointer (like Boehm GC). This turns out to be not as bad as one might expect, but is clearly suboptimal. More often in managed languages, some extra tagging information is left with the stack to help the collector figure out where the pointers are. </p> <p>Remember that in most compiled languages, the layout of a stack frame is the same every time you enter a function, therefore it is not that hard to ensure that you tag your data in the right way.</p> <p>The "bitmap" approach is one way of doing this. Each bit of the bitmap corresponds to one word on the stack. If the bit is a 1 then the location on the stack is a pointer, and if it is a 0 then the location is just a number from the point of view of the collector (or something along those lines). The exceptionally well written GHC runtime and calling conventions use a one word layout for most functions, such that a few bits communicate the size of the stack frame, with the rest serving as the bitmap. Larger stack frames need a multi word structure, but the idea is the same.</p> <p>The point is that the overhead is low, since the layout information is computed at compile time, and then included in the stack every time a function is called. </p> <p>An even simpler approach is "pointer first", where all the pointers are located at the beginning of the stack. You only need to include a length prior to the pointers, or a special "end" word after them, to tell which words are pointers given this layout. </p> <p>Interestingly, trying to get this management information on to the stack produces a host of problem related to interop with C. For example, it is sub optimal to compile high level languages to C, since even though C is portable, it is hard to carry this kind of information. Optimizing compilers designed for C like languages (GCC,LLVM) may restructure the stack frame, producing problems, so the GHC LLVM backend uses its own "stack" rather than the LLVM stack which costs it some optimizations. Similarly, the boundary between C code, and "managed" code needs to be constructed carefully to keep from confusing the GC. </p> <p>For this reason, when you create a new thread on the JVM you actually create two stacks (one for Java, one for C).</p>
    singulars
    1. This table or related slice is empty.
    1. POHow can a garbage collector find out about object references done from the stack?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USPhilip JF
    1. USPhilip JF
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COFor some platforms (e.g. IA-64, x86-64) and operating systems (such as Linux), you can use libunwind (http://www.nongnu.org/libunwind/) to unwind the stack and have access to the C stack frames. This takes out some of the complexity dealing with the C stack (but aggressive compiler optimizations may still cause problems).
      singulars
      1. PO
      1. USgfour
    2. COWhy is Haskell using the bitmap-approach? Wouldn't it be better to use the pointers-first approach?
      singulars
      1. PO
      1. USfuz
    3. COIt seems that LLVM actually does have [support for garbage collection](http://goo.gl/AzLsr).
      singulars
      1. PO
      1. USfuz
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload