Better Scripts, Better Games

Blizzard.j file. This function takes a rectangle and loops through all of the military units that appear in that rectangle; in that loop, it uses the function NudgeUnitsInRectEnum to push units apart so that there is a minimum distance between pairs of units.

All the operations in this script are external functions provided by the software engineers. The scripting language is not aware that these functions implement the equivalent of a for-each loop (a loop over a fixed set of objects); otherwise, the compiler would be able to perform loop optimizations on it. Given the number of times this pattern appears in the Warcraft III scripts, this could result in significant performance improvements.

CONCURRENC Y PAT TERNS

Iteration is not the only case in which developers could benefit from alternative control structures. Many games execute scripts in parallel, which requires scriptwriters to be cognizant of concurrency issues. As an example, consider inventory management in online games, a notoriously problematic scenario, with consistency violations resulting in lost or duplicated objects. Consider the following simple script written to put an item in a container such as a sack or a backpack:

// Test a container, and insert an object if okay success = TestPutItem(me, container, item) if (!success):

Bail() else:

PutItemInContainer(item, container)

This script tests if a container has the capacity to hold an item, then adds the item if there is space. Nothing in the script says that this action must be executed atomically, so in a distributed or concurrent setting, the container could fill up between the time it is tested and the time the item is added to the container. Obviously, this could be eliminated by the addition of locks or synchronization primitives to the scripting language. Locks can be expensive and error-prone, however, so game developers like to avoid them if at all possible. They are particularly dangerous in the hands of designers.

24 November/December 2008 ACM QUEUE

Additionally, lock-based synchronization is incompatible with the state-effect pattern. In the state-effect pattern, the state of the container consists of the contents at the end of the last iteration of the simulation loop, while an effect attribute is used to gather the items being added to the container. Effect variables cannot be read, even with locks, so the script cannot test for conflicting items being added simultaneously.

Instead of trying to solve this problem with traditional concurrency approaches, it is best to step back and understand what the programmer is trying to do in this pattern. The programmer wants to update an object, but under some conditions this update may result in an inconsistent state. The function TestPutItem defines which states are consistent. If the language knew this was the consistency function for PutItemInContainer, it could delay the check to ensure consistency without a lock. The language could first gather all of the items to be added to the container and then use the consistency check to place as many as the container can hold. In some cases, the language could even place multiple objects with a single consistency check.

Of course, this approach does not solve arbitrary problems with parallel execution, but game companies use languages with almost no concurrency support, and they rely on coding conventions to limit consistency errors. Adding features that provide concurrency guarantees for the more common design patterns in games would allow the game developers to trust their scriptwriters with a wider variety of scripts, increasing their artistic freedom.

GAME-AWARE RUNTIMES

Language features provide the runtime with clues on how best to execute the code, but some games have properties outside of the scripting language that the runtime can also leverage. For example, the right optimization strategy for a set of scripts depends on the current state of the game. If the game is controlling a large army marching toward an enemy, then the game should optimize movement of soldiers; on the other hand, if the army is guarding against an attack, the game should optimize individual perception. Games often have a small number of these high-level states, and changes between them happen relatively slowly. If the runtime can recognize which state the game is in, it can switch to an optimized execution plan and improve performance.

To some degree, game developers already take advantage of this fact in their performance tuning. Currently they log runs of the game during play-testing, and later

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