Caching D compiler - preview version

[Dmitry Olshansky]

What is dcache?

It's a patch for dmd that enables a *persistent* shared-memory 
hash-map, protected by a spin-lock from races. Dmd processes with 
-cache flag would detect the following pattern:

enum/static variable = func(args..);

And if mangle of func indicates it is from std.* we use a cache 
to store D source code form of a result of function call (a 
literal) produced by CTFE.

In action:

https://github.com/dlang/dmd/pull/7239

(Watch as 2.8s - 4.4s to compile various ctRegex programs becomes 
constant ~1.0s.)

Caching is done per expression so it stays active even after you 
change various parts of your files.

Broadening the scope to 3rd party libraries is planned but cache 
invalidation is going to be tricky. Likewise there is a trove of 
things aside from CTFE that can be easily cached and shared 
across both parallel and sequental compiler invocations.


Why caching compiler?

It became apparent that CTFE computations could be quite 
time-consuming and memory intensive. The fact that each CTFE 
invocation depends on a set of constant arguments, makes it a 
perfect candidate for caching.

Motivating example is ctRegex, patterns are hardly ever change 
and std.library changes only on compiler upgrade,  yet each 
change to a file causes complete re-evaluation of all patterns in 
a module.

With presistent per-expression cache we can precompile all of 
CTFE evluations for regexes, so we get to use ctRegex and 
maintain sane compile-times.

----

How to use

Pass new option to dmd:

-cache=mmap

This enables persistent cache using memory-mapped file.
Future backends would take the form of e.g.:

-cache=memcache:memcached.my.network:11211

----

Implementation

Caveats emptor: this is alpha version, use at your own risk!

https://github.com/DmitryOlshansky/dmd/tree/dcache

Keeping things simple - it's a patch of around 200 SLOCs.
I envision it becoming a hundred lines more if we get to do 
things cleanly.

Instead of going with strangely popular idea of compilation 
servers I opted for simple distributed cache, as it doesn't 
require changing any of the build systems.

Shared memory mapping split in 3 sections: Metadata (spinlock) + 
ToC (hash-table index) + Data (chunks)

For now it's an immutable cache w/o eviction.

A ToC entry is as follows:

hash(64-bit), data index, data size, last_recent_use

Indexes point to Data section of memory map.

Data itself is a linked list of blocks, where a header contains:

(isFree, next, 0-terminated key, padding to 16 bytes)

last_recent_use is a ts of the start of the respective 
compilation.  last_recent < now - 24h is considered unutilized 
and may be reused.

In theory we can cache result of any compilation step with a 
proper key and invalidation strategy.

1. Lexing - key is compiler-version + abs path + timestamp, store 
as is. Lexing from cache is simply taking slices of memory.

2. Parsing to Ast - key is compiler-version + abs path + 
timestamp + version/debug flags

3. CTFE invocations - key is tricky, for now only enabled for 
std.* as follows:

enum/static varname = func(args...);

Use compiler-version + compiler-flags + mangleof + stringof args.