Ant - a scripting engine

Ant is an experiment to build an ultra fast and ultra small embedded scripting engine with a close-to-native performance, small size and embedding simplicity. Ant is implemented as a single C/C++ standard-compliant header file that works on any platform, starting from 8-bit AVRs to 64-bit servers.

Performance

The ant.h header file implements 3 engines: an infix engine (ant), a postfix engine (ant2) and a bytecode engine (ant3). All engines where tested on the Arduino XIAO board (SAMD21 Cortex-M0+ processor) by calculating the following routine (see Ant.ino):

static long exec_c(void) {
  long res = 0;
  for (long i = 0; i < 1000; i++) res += i + i / 3;
  return res;
}

Here is the result for all 3 engines and native C implementation. Note that antx is the same as ant3 but using computed goto feature.

 ant, result: 665667, microseconds: 115434
ant2, result: 665667, microseconds: 41908
ant3, result: 665667, microseconds: 14548
antx, result: 665667, microseconds: 14542
   c, result: 665667, microseconds: 2020

This result shows that the 7x slowness of the bytecode implementation can be considered close-to-native, but it also suggests that the implementation should use compilation step to convert source code into bytecode.

Infix Syntax

• Infix notation
• Arithmetics: +, -, *, /
• Single-letter variables from a to z
• Assignments a = 0; b = 2;
• Increments and decrements a += 2; b += a * (c + d);
• Labels for jumps: #
• Jumps: @f expr jumps forward, @b expr jumps backward if expr is true. Jumps are performed to the nearest # label
• Loops and conditionals are implemented using labels and jumps, e.g. if (a > 0) i++; ... -> @tf a < 0 i += 1 # ...

Notes

Below are major places where a scripting engine slows down in comparison with the native code:

1. Expression parsing. To alleviate this slowness, either
   • a postfix notation should be used, e.g. 1 2 + 3 * instead of (1 + 2) * 3
   • an extremely fast infix expression parser
2. Variable lookup: a = 123 or a = b + c. Compiled code assigns some memory locations for each variable and references that memory directly. A scripting engine performs a variable lookup every time a variable gets referenced. To speed up variable lookups, the following tactics can be used:
   • using only single-letter variable names, for example from a to z. This way, ant reserves 26 variables in total, and a variable name gives a direct index: vars[*pc - 'a']
   • using explicit variable indices, e.g. 17 v which gives vars[17]. For example, JavaScript's let foo = 123 can become ant's 123 0 v = if a postfix notation is used, which in turn executes vars[0] = 123
3. Marshalling arguments to FFI calls. To alleviate this, a scripting engine can push arguments to machine's stack directly and therefore avoid any extra marshalling layer, e.g. "world" "hello, %s\n" P where P references stdlib's printf, and strings "world" and "hello, %s" push respective pointers to the machine's stack. This is not portable however, since some architectures might not use stack for arguments
4. Floating-point versus integer math
5. Conditionals. Consider JavaScript's if (cond) { body }, where cond is false. Then, we should jump over the body whilst not executing it. If there is no compilation step, we don't know where body ends and therefore we should "execute" the body without evaluating it. The solution is to use labels and jump commands, just like in the assembly code.