fix: avoid string copy when processing input source-map

[JLHwung]

Q	A
Fixed Issues?	Fixes #10875
Patch: Bug Fix?	Yes
Tests Added + Pass?	No test is added, it is covered by current test suite
License	MIT

This PR fixes a performance regression introduced in #10623.

babel/packages/babel-core/src/transformation/normalize-file.js

Lines 68 to 69 in c35ba3d

	// fromMapFileComment requires the whole comment block
	`//${lastComment}`,

The lastComment was copied into a new string, prefixed by // and later unwrapped by convert-source-map.
https://github.com/thlorenz/convert-source-map/blob/9e9a7a9b652c30878c9c9aa591d861a9fdf61a7e/index.js#L31

This approach introduced significant memory footprint when there are many files with input source-map, i.e. the test repo mentioned in #10875. Here we use convertSourceMap.fromJSON to avoid unnecessary string copy.

@artisb45 Thank you for the reproduction repo, it helps me a lot! You can test flight this fix by downloading and overwrite node_modules/@babel/core/lib/transformation/normalize-file.js with this file. We are expected to land it in a patch release if review is good.

[JLHwung]

Dogfooding named-capturing-regex here.

The file size is bloated by helpers (6678 B -> 11276 B) but I think it is okay since named-capturing-regex is supported on Node.js 10+, which means we can get rid of the helpers in v8.

I have changed my mind.

[nicolo-ribaudo]

I agree with @jridgewell suggestion. Apart from that, ✔️

[existentialism]

Derp, missed the flow errors... will push a PR fix in the morning if no one gets to it first!

[abonckus]

Hello, thanks for such a quick response to this issue @JLHwung ! However I tried to replace the file you provided and it did not seem to do anything. Not sure if I'm doing it wrong, but I'm just supposed to replace the file at the path right? I'm using the same repo I linked in the issue. Also tried rolling back before ejection and then replacing - same result.

[nicolo-ribaudo]

#10885 (comment)

Ops, I didn't see your comment 😅

[JLHwung]

@artisb45 Oops, you may need clear the babel-loader cache first because it is an ad-hoc approach.

# Clear babel-loader cache
rm -rf node_modules/.cache/babel-loader
cp normalize-file.js node_modules/@babel/core/lib/transformation/normalize-file.js
npm run build

It went well on my local environment. You can also wait for the upcoming 7.7.7 release.

Update:

#10875 is not fixed: #10875 (comment)