https://github.com/akkartik/mu/blob/master/036abandon.cc

      1 //: Reclaiming memory when it's no longer used.
      2
      3 :(scenario new_reclaim)
      4 def main [
      5   10:&:num <- new number:type
      6   20:num <- deaddress 10:&:num
      7   abandon 10:&:num
      8   30:&:num <- new number:type  # must be same size as abandoned memory to reuse
      9   40:num <- deaddress 30:&:num
     10   50:bool <- equal 20:num, 40:num
     11 ]
     12 # both allocations should have returned the same address
     13 +mem: storing 1 in location 50
     14
     15 //: When abandoning addresses we'll save them to a 'free list', segregated by size.
     16
     17 //: Before, suppose variable V contains address A which points to payload P:
     18 //:   location V contains an alloc-id N
     19 //:   location V+1 contains A
     20 //:   location A contains alloc-id N
     21 //:   location A+1 onwards contains P
     22 //: Additionally, suppose the head of the free list is initially F.
     23 //: After abandoning:
     24 //:   location V contains invalid alloc-id -1
     25 //:   location V+1 contains 0
     26 //:   location A contains invalid alloc-id N
     27 //:   location A+1 contains the previous head of free-list F
     28
     29 :(before "End routine Fields")
     30 map<int, int> free_list;
     31
     32 :(before "End Primitive Recipe Declarations")
     33 ABANDON,
     34 :(before "End Primitive Recipe Numbers")
     35 put(Recipe_ordinal, "abandon", ABANDON);
     36 :(before "End Primitive Recipe Checks")
     37 case ABANDON: {
     38   if (!inst.products.empty()) {
     39     raise << maybe(get(Recipe, r).name) << "'abandon' shouldn't write to any products in '" << to_original_string(inst) << "'\n" << end();
     40     break;
     41   }
     42   for (int i = 0;  i < SIZE(inst.ingredients);  ++i) {
     43     if (!is_mu_address(inst.ingredients.at(i)))
     44       raise << maybe(get(Recipe, r).name) << "ingredients of 'abandon' should be addresses, but ingredient " << i << " is '" << to_string(inst.ingredients.at(i)) << '\n' << end();
     45     break;
     46   }
     47   break;
     48 }
     49 :(before "End Primitive Recipe Implementations")
     50 case ABANDON: {
     51   for (int i = 0;  i < SIZE(current_instruction().ingredients);  ++i) {
     52     reagent/*copy*/ ingredient = current_instruction().ingredients.at(i);
     53     canonize(ingredient);
     54     abandon(get_or_insert(Memory, ingredient.value+/*skip alloc id*/1), payload_size(ingredient));
     55 //?     cerr << "clear after abandon: " << ingredient.value << '\n';
     56     put(Memory, /*alloc id*/ingredient.value, /*invalid*/-1);
     57     put(Memory, /*address*/ingredient.value+1, 0);
     58   }
     59   break;
     60 }
     61
     62 :(code)
     63 void abandon(int address, int payload_size) {
     64   put(Memory, address, /*invalid alloc-id*/-1);
     65 //?   cerr << "abandon: " << address << '\n';
     66   // clear rest of payload
     67   for (int curr = address+1;  curr < address+payload_size;  ++curr)
     68     put(Memory, curr, 0);
     69   // append existing free list to address
     70   trace("abandon") << "saving " << address << " in free-list of size " << payload_size << end();
     71   put(Memory, address+/*skip invalid alloc-id*/1, get_or_insert(Current_routine->free_list, payload_size));
     72   put(Current_routine->free_list, payload_size, address);
     73 }
     74
     75 int payload_size(reagent/*copy*/ x) {
     76   x.properties.push_back(pair<string, string_tree*>("lookup", NULL));
     77   lookup_memory_core(x, /*check_for_null*/false);
     78   return size_of(x)+/*alloc id*/1;
     79 }
     80
     81 :(after "Allocate Special-cases")
     82 if (get_or_insert(Current_routine->free_list, size)) {
     83   trace("abandon") << "picking up space from free-list of size " << size << end();
     84   int result = get_or_insert(Current_routine->free_list, size);
     85   trace("mem") << "new alloc from free list: " << result << end();
     86   put(Current_routine->free_list, size, get_or_insert(Memory, result+/*skip alloc id*/1));
     87   // clear 'deleted' tag
     88   put(Memory, result, 0);
     89   // clear next pointer
     90   put(Memory, result+/*skip alloc id*/1, 0);
     91   for (int curr = result;  curr < result+size;  ++curr) {
     92     if (get_or_insert(Memory, curr) != 0) {
     93       raise << maybe(current_recipe_name()) << "memory in free list was not zeroed out: " << curr << '/' << result << "; somebody wrote to us after free!!!\n" << end();
     94       break;  // always fatal
     95     }
     96   }
     97   return result;
     98 }
     99
    100 :(scenario new_differing_size_no_reclaim)
    101 def main [
    102   1:&:num <- new number:type
    103   2:num <- deaddress 1:&:num
    104   abandon 1:&:num
    105   3:&:@:num <- new number:type, 2  # different size
    106   4:num <- deaddress 3:&:@:num
    107   5:bool <- equal 2:num, 4:num
    108 ]
    109 # no reuse
    110 +mem: storing 0 in location 5
    111
    112 :(scenario new_reclaim_array)
    113 def main [
    114   10:&:@:num <- new number:type, 2
    115   20:num <- deaddress 10:&:@:num
    116   abandon 10:&:@:num
    117   30:&:@:num <- new number:type, 2  # same size
    118   40:num <- deaddress 30:&:@:num
    119   50:bool <- equal 20:num, 40:num
    120 ]
    121 # both calls to new returned identical addresses
    122 +mem: storing 1 in location 50
    123
    124 :(scenario lookup_of_abandoned_address_raises_error)
    125 % Hide_errors = true;
    126 def main [
    127   1:&:num <- new num:type
    128   3:&:num <- copy 1:&:num
    129   abandon 1:&:num
    130   5:num/raw <- copy *3:&:num
    131 ]
    132 +error: main: address is already abandoned in '5:num/raw <- copy *3:&:num'