mozilla/rust — pull request #4931 by thestinger (2013-02-14 20:02:10+00:00)
Title: stop applying noinline to glue code

Using `noinline` causes a 3-10% hit in performance for most compiled Rust code. For the TreeMap it's ~15% and that's where I first noticed it.

Removing the noinline attribute doesn't slow down unoptimized builds, but it does significantly increase the time spent in LLVM passes for optimized builds. The improved speed of the compiler actually improves compile-times when optimization is off.

However, the reason for the increase is because more optimization is being done - I'm sure it would speed up compiles to mark *everything* with noinline, but it wouldn't be a good idea.

LLVM is clever enough with the inlining heuristics that this doesn't cause a notable increase in code size - some code becomes a bit bigger, some becomes a bit smaller. There are some cases where it's able to strip out a ton of code thanks to inlining.

I tried out `optsize` for glue code instead but it caused the same hit for LLVM passes in the compile time and the compiled code was a bit slower than just trusting LLVM to make the decisions.

* [TIME_PASSES=1 benchmarks](http://ompldr.org/vaGdxaA) (showing the performance increase in `rustc` and also the time lost in LLVM passes for more optimization)