Wednesday, June 16, 2010

Becoming a console programmer

Recently on a games industry forum, a friend who teaches over in the Netherlands (not Denmark :p) was asking us (programmers) what we would ensure a PC programmer moving to Console Development should know, before considering themselves a "console programmer"

None of these particularly apply to console programming however i've found over the years that programmers who come form a pure PC background often are lacking in this area so i personally consider them important.

Programmers new to console should strive to achieve:
  • an awareness of memory consumption - PC programmers tend to be somewhat liberal with memory allocation and stack usage. On Consoles we're often faced with very limited memory resources that shrink with time (due to the game getting larger) rather than the opposite on PC where virtual memory allows incredibly large memory footprints.
  • knowledge of alignment - internal to structures and how it pertains to their size AND how not paying attention can massively increase cache misses
  • appreciation of the cost of type conversions both in terms of float <=> integer, float <=> simd and the assumed simpler integer to integer conversions thar can cost significantly more than is assumed.
  • full understanding of what a Load Hits Store (LHS) is, how it occurs, how to avoid it and how to spot it in everyday code.
  • full understanding of the implications of a cache miss, what it is, how to avoid them and how to fill the time around the unavoidable ones.
  • full understanding of what a branch is, how it affects the cpu, how to avoid using them and what patterns should be avoided to minimize them in general purpose code
  • understand what select functions are (fsel, vsel family) and how/when to use them.
SIMD is important however there are MANY companies that write a math lib once and never look back, i've been guilty of this myself however now i know i was wrong back then and no doubt in many ways what i've written most recently is still wrong.

writing it once implies that when you wrote the lib you knew everything there was to know then and nothing changed in the years afterwards.... yes many games will be fine with an "ok" math lib however a "good" math lib can change the performance of a game and an entire team.

re-write it as often as you can afford learning from the mistakes each time... so much is based upon it that this is one of the few systems that i'd recommend this behavior on.

so a final point
  • learn simd math in all its forms, learn the extra instructions that most don't use and what they provide, learn how to use #.INF to your advantage and most of all use instructions such as vsel / shuffle/ permute to their fullest advantage.
I will be posting expansions on the above points over the next few weeks depending on demand.

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