https://github.com/akkartik/mu/blob/master/041jump_target.cc

      1 //: Support jumps to a specific label (the 'jump target') in the same recipe.
      2 //: Jump targets must be unique and unambiguous within any recipe.
      3 //:
      4 //: The 'break' and 'loop' pseudo instructions can also take jump targets.
      5 //: Which instruction you use is just documentation about intent; use 'break'
      6 //: to indicate you're exiting one or more loop nests, and 'loop' to indicate
      7 //: you're skipping to the next iteration of some containing loop nest.
      8
      9 //: Since they have to be unique in a recipe, not all labels can be jump
     10 //: targets.
     11 bool is_jump_target(const string& label) {
     12   if (label == "{" || label == "}") return false;
     13   // End is_jump_target Special-cases
     14   return is_label_word(label);
     15 }
     16
     17 :(scenario jump_to_label)
     18 def main [
     19   jump +target:label
     20   1:num <- copy 0
     21   +target
     22 ]
     23 -mem: storing 0 in location 1
     24
     25 :(before "End Mu Types Initialization")
     26 put(Type_ordinal, "label", 0);
     27
     28 :(before "End Instruction Modifying Transforms")
     29 Transform.push_back(transform_labels);  // idempotent
     30
     31 :(code)
     32 void transform_labels(const recipe_ordinal r) {
     33   map<string, int> offset;
     34   for (int i = 0;  i < SIZE(get(Recipe, r).steps);  ++i) {
     35     const instruction& inst = get(Recipe, r).steps.at(i);
     36     if (!inst.is_label) continue;
     37     if (is_jump_target(inst.label)) {
     38       if (!contains_key(offset, inst.label)) {
     39         put(offset, inst.label, i);
     40       }
     41       else {
     42         raise << maybe(get(Recipe, r).name) << "duplicate label '" << inst.label << "'" << end();
     43         // have all jumps skip some random but noticeable and deterministic amount of code
     44         put(offset, inst.label, 9999);
     45       }
     46     }
     47   }
     48   for (int i = 0;  i < SIZE(get(Recipe, r).steps);  ++i) {
     49     instruction& inst = get(Recipe, r).steps.at(i);
     50     if (inst.name == "jump") {
     51       if (inst.ingredients.empty()) {
     52         raise << maybe(get(Recipe, r).name) << "'" << to_original_string(inst) << "' expects an ingredient but got 0\n" << end();
     53         return;
     54       }
     55       replace_offset(inst.ingredients.at(0), offset, i, r);
     56     }
     57     if (inst.name == "jump-if" || inst.name == "jump-unless") {
     58       if (SIZE(inst.ingredients) < 2) {
     59         raise << maybe(get(Recipe, r).name) << "'" << to_original_string(inst) << "' expects 2 ingredients but got " << SIZE(inst.ingredients) << '\n' << end();
     60         return;
     61       }
     62       replace_offset(inst.ingredients.at(1), offset, i, r);
     63     }
     64     if ((inst.name == "loop" || inst.name == "break")
     65         && SIZE(inst.ingredients) >= 1) {
     66       replace_offset(inst.ingredients.at(0), offset, i, r);
     67     }
     68     if ((inst.name == "loop-if" || inst.name == "loop-unless"
     69             || inst.name == "break-if" || inst.name == "break-unless")
     70         && SIZE(inst.ingredients) >= 2) {
     71       replace_offset(inst.ingredients.at(1), offset, i, r);
     72     }
     73   }
     74 }
     75
     76 void replace_offset(reagent& x, /*const*/ map<string, int>& offset, const int current_offset, const recipe_ordinal r) {
     77   if (!is_literal(x)) {
     78     raise << maybe(get(Recipe, r).name) << "jump target must be offset or label but is '" << x.original_string << "'\n" << end();
     79     x.set_value(0);  // no jump by default
     80     return;
     81   }
     82   if (x.initialized) return;
     83   if (is_integer(x.name)) return;  // non-labels will be handled like other number operands
     84   if (!is_jump_target(x.name)) {
     85     raise << maybe(get(Recipe, r).name) << "can't jump to label '" << x.name << "'\n" << end();
     86     x.set_value(0);  // no jump by default
     87     return;
     88   }
     89   if (!contains_key(offset, x.name)) {
     90     raise << maybe(get(Recipe, r).name) << "can't find label '" << x.name << "'\n" << end();
     91     x.set_value(0);  // no jump by default
     92     return;
     93   }
     94   x.set_value(get(offset, x.name) - current_offset);
     95 }
     96
     97 :(scenario break_to_label)
     98 def main [
     99   {
    100     {
    101       break +target:label
    102       1:num <- copy 0
    103     }
    104   }
    105   +target
    106 ]
    107 -mem: storing 0 in location 1
    108
    109 :(scenario jump_if_to_label)
    110 def main [
    111   {
    112     {
    113       jump-if 1, +target:label
    114       1:num <- copy 0
    115     }
    116   }
    117   +target
    118 ]
    119 -mem: storing 0 in location 1
    120
    121 :(scenario loop_unless_to_label)
    122 def main [
    123   {
    124     {
    125       loop-unless 0, +target:label  # loop/break with a label don't care about braces
    126       1:num <- copy 0
    127     }
    128   }
    129   +target
    130 ]
    131 -mem: storing 0 in location 1
    132
    133 :(scenario jump_runs_code_after_label)
    134 def main [
    135   # first a few lines of padding to exercise the offset computation
    136   1:num <- copy 0
    137   2:num <- copy 0
    138   3:num <- copy 0
    139   jump +target:label
    140   4:num <- copy 0
    141   +target
    142   5:num <- copy 0
    143 ]
    144 +mem: storing 0 in location 5
    145 -mem: storing 0 in location 4
    146
    147 :(scenario jump_fails_without_target)
    148 % Hide_errors = true;
    149 def main [
    150   jump
    151 ]
    152 +error: main: 'jump' expects an ingredient but got 0
    153
    154 :(scenario jump_fails_without_target_2)
    155 % Hide_errors = true;
    156 def main [
    157   jump-if true
    158 ]
    159 +error: main: 'jump-if true' expects 2 ingredients but got 1
    160
    161 :(scenario recipe_fails_on_duplicate_jump_target)
    162 % Hide_errors = true;
    163 def main [
    164   +label
    165   1:num <- copy 0
    166   +label
    167   2:num <- copy 0
    168 ]
    169 +error: main: duplicate label '+label'
    170
    171 :(scenario jump_ignores_nontarget_label)
    172 % Hide_errors = true;
    173 def main [
    174   # first a few lines of padding to exercise the offset computation
    175   1:num <- copy 0
    176   2:num <- copy 0
    177   3:num <- copy 0
    178   jump $target:label
    179   4:num <- copy 0
    180   $target
    181   5:num <- copy 0
    182 ]
    183 +error: main: can't jump to label '$target'