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C language => Beginner questions => Topic started by: Grincheux on June 15, 2017, 05:38:59 PM

Title: #pragma density(0.0)
Post by: Grincheux on June 15, 2017, 05:38:59 PM
I have not really understood the documentation about this pragma.
By default it is 0.5.
How to use it (0.0 ... 1.0)
What is the result on the optimization?
thx
Title: Re: #pragma density(0.0)
Post by: frankie on June 16, 2017, 11:50:29 AM
Compilers use a mix of jump tables and binary searches to execute the correct case code in a switch instruction.
I.e.
Code: [Select]
switch a
{
  case 0:
    ...
    break;
  case 1:
    ...
    break;
  case 2:
    ...
    break;
  case 5:
    ...
    break;
  case 6:
    ...
    break;
  case 9:
    ...
    break;
  case 10:
    ...
    break;
}
The code generated will use a binary search to approach the three sequences available: 0-2, 5-6, 9-10. Then will use a (pseudo)jump table to access in the range. I said pseudo jump table because it is not really a table, but a sequence of offsetting addresses and then jump that emulates a jump table mechanism. The problem is that actual CPU's that make intense use of instruction flow caching must reset the caches when jumping. For this reason happens that a binary search, with predictable jumps, works faster than a pure jump table.
When you have very large ranges, i.e. cases 0-999, instead of creating a very large jump table you can decide to reduce it to a fraction and reach inside cases using a binary search.
I.e. crate a table of 10 entries: 0-99, 100-199, 200-299, ..., 900-999.
Then search the subrange using a binary search and last access with a reduced jump table. In this case we used a density  of 0.1.
Also the reverse is possible, using binary search first and table jump after.

Result on optimization is on the quantity of used memory and/or execution speed (memory for tables, speed for binary search code).
Anyway using optimization reset the density to the default that is 0.5.

For a more indeep explanation you can read article from Vlad Lazarenko (http://lazarenko.me/switch/).
Title: Re: #pragma density(0.0)
Post by: jj2007 on June 16, 2017, 12:39:32 PM
Pinnnnnngback! (http://masm32.com/board/index.php?topic=5305.msg67803#msg67803)