Parser speed optimization zaach#341: when both --no-try-catch and `…

…--no-default-action` are specified, all safeties are moved, including protection in the kernel against incorrect use of `$$`/`$0` in grammar rule action code blocks. Example examples/test-unused-rules-reporting.jison is augmented to showcase the (**technically correct but functionally buggy!!!) behaviour.

examples/test-unused-rules-reporting.jison

@@ -3,6 +3,10 @@
 // rule, i.e. will be *unused* in the generated parser/grammar!
 //
 
+// build options:
+%options no-try-catch no-default-action main
+
+
 %lex
 
 %%
@@ -69,9 +73,42 @@ var assert = require("assert");
 
 parser.main = function () {
     var rv = parser.parse("a");
+
+    //
+    // BIG FAT WARNING:
+    //
+    // Due to `%options no-try-catch no-default-action`, combined with the
+    // *lack* of action code for the `e: WORD` grammar rule, *that* rule's
+    // result `$$` will be UNDETERMINED, i.e. up for grabs, random, chaos.
+    //
+    // In *this particular example and input being parsed* this 
+    // translates to the *previous* `$$` being picked up, which happens to
+    // be known as it comes out of the epsilon rule: `"ε"`, hence
+    // the 'expected output' IS NOT `"εεaε"` but `"εεεε"`! 
+    //
+    // Yes, *functionally* that's an ERROR/BUG for certain, but the
+    // technicalities mentioned above make this the 'correct' output
+    // anyway. The ACTUAL BUG in this grammar example is the `e: WORD` rule
+    // **not having any action code assigning a value to `$$`** since that
+    // is the only sane activity *everywhere* when you choose to compile
+    // with `%options no-default-action`.
+    //
+    // Aside: note that `%options no-try-catch no-default-action` does
+    // remove additional safeguards in the parser kernel code: this option
+    // combo also causes code to be removed with sets `$0` and `$$` to
+    // `undefined` *before* any rule's action code is executed so any
+    // protection against grammar-coder mistakes such as this action code
+    // chunk is removed in exchange for parser execution speed:
+    //
+    //     %{ $$ = Math.sin($$ /* BUG! NOT SANE TO USE `$$` or `$0` AS INPUT! */); %}
+    //
+    // *Caveat Emptor* i.e. when ou choose Speed, you are expected to
+    // be fully aware that Safety is Not An Option Any More(tm)!
+    //
+
     console.log("(reduce epsilon rules as early as possible)\n\n  a ==> ", rv);
 
-    assert.equal(rv, "εεaε");
+    assert.equal(rv, "εεεε");
 
     // if you get past the assert(), you're good.
     console.log("tested OK");

lib/jison.js

@@ -3267,6 +3267,19 @@ function removeUnusedKernelFeatures(parseFn, info) {
         .replace(/^[^\n]+\b__reentrant_call_depth\b[^\n]+$/gm, '\n');
     }
 
+    if (info.options.noDefaultAction && info.options.noTryCatch) {
+        /*
+         * This is a very performance-oriented setting and does not care if the 
+         * userland code for the grammar rules is flaky. 
+         * Kill this protection code:
+         *
+         *     // Do this to prevent ...
+         *     vstack[sp] = undefined;
+         */
+        parseFn = parseFn
+        .replace(/\s+\/\/ Do this to prevent nasty action block codes[^\n]+\n\s+vstack\[sp\] = undefined;[^\n]+\n/g, '\n\n');
+    }
+
     info.performAction = actionFn;
 
     return parseFn;