Reduce memory usage of slices

[tinygo-org]

Reduce memory usage of slices

[aykevl]

I don't think it is actually possible to calculate cap() on demand, because it can be set to an arbitrary value: consider slice[:5:7], which sets len to 5 and cap to 7 no matter what they were before.

In general, I think the compiler will be able to optimize cap away in trivial cases where it is never used (no append or cap call). This will probably improve once I get around to splitting the slice struct into independent variables in function calls etc. which LLVM is better at optimizing away.

[deadprogram]

Is this issue still relevant given all of the internal changes to TinyGo, or should it be closed?

[aykevl]

I don't think there is anything I can do here. I don't see a way to perform this optimization while still adhering to the Go language specification. Additionally, slices in local variables and function parameters are easily optimized by LLVM: function parameters are split up in #20 and local variables are split up by LLVM. Unnecessary copies are usually avoided by dead code elimination passes and the deadargelim pass. This means that the cap part of a slice is often already eliminated.

If there is something I'm missing I'd like to know about it. For now, I'm going to close the issue.