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ConstantFolding.md

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Constant folding

Table of contents

Overview

This transformation aims to evaluate as much numerical/boolean constants as possible.

Working cases

Fully evaluating constant integral/boolean expressions in any context

For example:

  • 1 + 2 * 3 - 8 // 3 + 7 ^ 3 | 11 is replaced with 15
  • True and 1 + 1 == 2 is replaced with True

Note: The power operator ** is special in that the size of its result can be much larger than the size of its operands. Because of this, ** is only evaluated when both its operands and its result fit into the JVM Long type (64-bits signed integer).

Maximal (by inclusion) evaluating subexpressions of non-constant expressions

For example:

  • 1 + 2 + x is replaced with 3 + x

Partially evaluating boolean and and or operators

For example:

  • [x, y] or 42 is replaced with [x, y], since the left part is truthy regardless of what specific values x and y have.

Partially evaluating commutative operators

Arguments of known commutative operators are partially evaluated regardless of the order of arguments given. All of the constant arguments do not have to form a distinct subtree in this case.

For example:

  • x + 1 + 2 is replaced with x + 3 when it is possible to deduce that x is of type int (to ensure the commutativity of +)

Concatenating strings and lists

For example:

  • "Hello, " + "world!" is replaced with "Hello, world!"
  • [x] + [y] is replaced with [x, y]

Algorithm

The transformation works on any expression by recursively simplifying all of its subexpressions and then evaluating the top-level node using the results.