> Memory usage building Firefox with debug enabled was reduced from 15GB to 3.5GB; link time from 1700 seconds to 350 seconds.
This is huge- you can now do this on a laptop
This is huge- you can now do this on a laptop
Although it seems obvious that this might be a good idea, why would it
1) use exorbitant amounts of memory; and
2) be "pretty awesome" instead of, say, mildly useful?
edit: And both questions satisfactorily answered in the time it took me to peruse the preamble of https://en.wikipedia.org/wiki/Interprocedural_optimization
Thank you!
It uses a lot of memory because it requires keeping a representation of more or less the entire program in memory at one time, in a format which is amenable to analysis. It is not out of the ordinary for an optimizer's internal representation to be on the order of 1000x the size of the source code.
Functions can be inlined across module boundaries, even when they're not declared inline. You can turn virtual functions into regular functions, if you know that the virtual function is never overridden, or if you can derive the exact type. You can change calling conventions for functions. You can do better escape and aliasing analysis. If a function is only called once, then you can probably optimize it a lot better because you know exactly how it will be called.
As with all optimizations, not all programs will see any significant benefit. Programs with heavy inner loops like physics simulators and graphics processors will not see much benefit, since the local optimizer works well enough. Programs like compilers, interpreters, and web browsers will see larger benefits. However, the benefits can be high—30% improvements in running time are not unheard of.
In short, LTO is a high-cost, high-benefit optimization.
> Early removal of virtual methods reduces the size of object files and improves link-time memory usage and compile time.
> Function bodies are now loaded on-demand and released early improving overall memory usage at link time.