[PATCH] D61103: [clang] Add tryToAttachCommentsToDecls method to ASTContext

[Dmitri Gribenko via Phabricator via cfe-commits]

gribozavr added inline comments.


================
Comment at: clang/lib/AST/ASTContext.cpp:584
+    // terminating early.
+    for (auto CIt = RawComments.begin(); CIt != RawComments.end(); ++CIt) {
+      RawComment *C = *CIt;
----------------
jkorous wrote:
> gribozavr wrote:
> > Scanning all comments for every decl?  Isn't that O(n^2)?
> > 
> > Also logic duplication below.  I was expecting a call to `getRawCommentForDeclNoCache`, with an appropriate flag to disable loading external comments (it is a low-level API, users generally don't call it).
> The important thing is that the expensive operation is source location decomposition. That's why I cache everything - `GetCachedCommentBegin` etc.
> 
> So while you're right that iteration-wise it's O(iteration*c*d) it`s actually O(decompose*c + decompose*d) because of the caching. The current code (which is sorting all the comments) is at least O(decompose*c*ln(c)) once you have more comments than `sqrt(300)` (==`MagicCacheSize` in `SourceManager::getInBeforeInTUCache()`).
> 
> That being said - you're right that just not-loading external comments in `getRawCommentForDeclNoCache` definitely has it's appeal. I'm running a test now get some idea about performance of both approaches.
> 
> BTW in theory we could also do one of these:
> 1. Allow clients to transparently set `MagicCacheSize` in `SourceManager::getInBeforeInTUCache()` which is used for SourceLocation sorting (`BeforeThanCompare<RawComment>`) is currently hard-coded to 300 while we are comparing ~100k x ~100k locations.
> 2. Change caching strategies in `SourceManager::getFileID` and `SourceManager::getLineNumber`.
> So while you're right that iteration-wise it's O(iteration*c*d) it`s actually O(decompose*c + decompose*d) because of the caching. 

The cost of decomposing is non-trivial, but the cost of each iteration is still at least a hash table lookup.

> That being said - you're right that just not-loading external comments in getRawCommentForDeclNoCache definitely has it's appeal. I'm running a test now get some idea about performance of both approaches.

Reopening, waiting for the results.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D61103/new/

https://reviews.llvm.org/D61103