Tuesday, October 04, 2011

Delphi XE2 compiler performance

Delphi XE2 introduced namespaces across the runtime library. This stressed unit name lookup inside the compiler, and led to some severe performance regressions in certain cases. So during the runup to the XE2 release, I fired up a profiler and started investigating. It turns out there were numerous situations where lookups were being performed repeatedly with the same arguments, and logically the results should have been consistent across these repeated calls. A relatively cheap and easy fix seemed to be memoization. So I added a caching infrastructure to the compiler and used the profiler to guide me where to inject it.

For the most part - particularly for full builds - the cache works really well. But I've had some reports of bugs that I suspected were caused by the cache hanging on to function results slightly too long, and upon investigation, this turned out to be true. The problem with caches is usually in invalidation; if you don't invalidate the cache soon enough and in all required situations, you end up serving stale results. So there are a few bugs lurking in the Delphi compiler here, which I'm seeking out and squashing.

Some good news, however; I had anticipated that this might be the case, so I added a super secret switch that enables diagnosing a probable cache failure: caches can be selectively disabled, and if a problem goes away with the cache disabled, it's probably because of stale results.

Caches can be disabled by setting an environment variable:


The above environment variable setting disables all the compiler's caches. By including fewer than the four separate cache names, the problem can iteratively be narrowed down to a specific cache.

I've just been fixing one bug caused by the cache that brought home how needed it is. The project is large; almost 2 million lines. An initial build with the cache enabled takes about a minute on my machine; the bug exhibits itself in later incremental compiles when modifying the source code and pressing F9, producing spurious errors. However, once I disabled the cache (or rather, I recompiled the compiler with cache sanity checking enabled, which still filled out the cache, but also invoked the underlying logic, and simply compared the results to verify the cache), the build time took nearly 3 hours!

Note: invalidation is most likely to be a problem on incremental compiles, rather than full rebuilds, especially from within the IDE. The reason is that the compiler may have lots of stale data for one half of an incremental compile that it later decides is out of date (e.g. a dependency changed); this can leave a bunch of stale entries in the cache for all the memoized function calls that occurred in the first half of the compile. The cache is strictly per-compile; it keeps no data across multiple compilations, even partial compilations.