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Optimize source mapping into external source map sources #40055

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Aug 19, 2020
Merged

Optimize source mapping into external source map sources #40055

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6 changes: 6 additions & 0 deletions src/compiler/sourcemap.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@ namespace ts {
const rawSources: string[] = [];
const sources: string[] = [];
const sourceToSourceIndexMap = new Map<string, number>();
const fileNameToSourceIndexMap = new Map<string, number>();
let sourcesContent: (string | null)[] | undefined;

const names: string[] = [];
Expand Down Expand Up @@ -51,6 +52,10 @@ namespace ts {

function addSource(fileName: string) {
enter();
const sourceIndexByFileName = fileNameToSourceIndexMap.get(fileName);
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Implementation note: this could alternatively be done without a full cache, using just a single MRU fileName/sourceIndex. That does however require that the second call to setSourceMapSource in emitSourcePos has to take an additional parameter to not even call addSource (which is safe as reverting to the prior source map source is known to have been added) as otherwise 2 MRU items are needed:

TypeScript/src/compiler/emitter.ts

Lines 5290 to 5300 in a7d8011

function emitSourcePos(source: SourceMapSource, pos: number) {
if (source !== sourceMapSource) {
const savedSourceMapSource = sourceMapSource;
setSourceMapSource(source);
emitPos(pos);
setSourceMapSource(savedSourceMapSource);
}
else {
emitPos(pos);
}
}

This alternative would also mitigate the issue (I verified that) as it's typical for a single external source map source to be applied on nodes without interference from nodes that have a different external source map source, so a single MRU cache entry is sufficient.

if (sourceIndexByFileName !== undefined) {
return sourceIndexByFileName;
}
const source = getRelativePathToDirectoryOrUrl(sourcesDirectoryPath,
fileName,
host.getCurrentDirectory(),
Expand All @@ -64,6 +69,7 @@ namespace ts {
rawSources.push(fileName);
sourceToSourceIndexMap.set(source, sourceIndex);
}
fileNameToSourceIndexMap.set(fileName, sourceIndex);
exit();
return sourceIndex;
}
Expand Down