Re-add Minification Mapping Input Suggestion

[tommywilkinson]

Looks like this ability used to be in the compiler, but was removed long ago. A piece of the functionality was re-added for #433, but not the piece allowing for an input mapping to use as a starting point.

It's a very common practice for website to use hashed file names to allow browsers to cache unchanged portions of scripts, especially as a part of a webpack bundle, which allows only certain portions of the bundle to need to get re-downloaded by users in the new version.

However, if you're using Closure Compiler as a minifier (perhaps via the webpack plugin), the instability of the minification will mean that all chunk contents will likely change every build, even if there were no real changes to the contents of some of the chunks.

In an ideal world, we'd be able to give Closure the minification map of the previous build, which it could then use as a "suggestion" for the current mapping. If there's a big enough upside, it can choose to re-map any or all of the identifiers. This means that for cases where there's no upside to re-mapping, Closure could choose stability, perhaps allowing more of those chunks to have unchanged contents.

The main pitch here is to make minification slightly less "chaotic" (in the sense that small changes anywhere within the scope can lead to big output differences everywhere) for cases where there is little-to-no upside.

[L3P3]

Also good for any diffing use, including differential updates.

[nhelfman]

@tommywilkinson To expand on the use case described - if one wishes to use Compress Dictionary Transport (CDT)(https://github.com/WICG/compression-dictionary-transport) this is likely a must. The CDT feature enables diff download of JS resources using compression dictionaries. Without it, the typical diff between builds will be too large to be useful with CDT.

Experimenting locally I found that sometimes even small actual difference in origin code results in a size diff of minified code of over 40% due to renaming which is inconsistent between builds.

For example, the following is the same code with different renames:

< var rk = "function" == typeof Object.defineProperties ? Object.defineProperty : function(C, y, a) {
<     if (C == Array.prototype || C == Object.prototype)
<         return C;
<     C[y] = a.value;
<     return C
---
> var Ho = "function" == typeof Object.defineProperties ? Object.defineProperty : function(z, y, a) {
>     if (z == Array.prototype || z == Object.prototype)
>         return z;
>     z[y] = a.value;
>     return z

When using compression dictionary (e.g. brotli) it will result with as a large diff.

What would it take to get this prioritized?

[niloc132]

It looks like the feature might still be present, just not plumbed to the command line? https://github.com/google/closure-compiler/blob/558e96f386c378ff8ef9cbdc36a3ba266d008517/src/com/google/javascript/jscomp/AbstractCommandLineRunner.java#L2759-L2769

Have you tried generating the prop/variable maps and then reading them using this to confirm it doesn't work? I recognize that the commit linked at the top claims that it isn't intended for this purpose, but the tests seem to confirm that it works, and there are compiler passes that check for the presence of these input maps and make use of them to keep names consistent: https://github.com/google/closure-compiler/blob/558e96f386c378ff8ef9cbdc36a3ba266d008517/src/com/google/javascript/jscomp/RenameProperties.java#L220-L243 etc

[nhelfman]

This is what I was thinking when I looked at this code earlier.

I plan to try it and will report results here.

[nhelfman]

Update on my testing - when enabling input maps on a local build of the compiler I was able to create a brolti diff of 10% between 2 production versions of our code. This is compared to the 40% diff without it. This means that this feature is very important and will play a significant part in our efforts to roll out Compress Dictionary Transport (CDT)(https://github.com/WICG/compression-dictionary-transport)

I'm working on submitting a PR with the changes.