[llvm-dev] Making LLD PDB generation faster

[Alexandre Ganea via llvm-dev]

Leonardo, to answer to your questions, yes to all of them  You can take a
look at this prototype/proposal: https://reviews.llvm.org/D55585

Overall, computing ghashes in parallel at link-time and merging Types with them
is less costly that the current approach to merging. The 35sec you’re seeing
for merging should go down to about 15sec. The patch doesn’t parallelize
(yet) the Type merging itself, but we have an alternate multithread-suitable
implementation of DenseHash which already supports lockless, wait-free,
insert/fetch/resize.

The prototype allows for testing different hashing algorithms, and indeed
xxHash seems to be the best general-purpose choice. I’ve also added support
for more specialized hardware-based hashes, like Casey Muratori’s Meow Hash
(uses hardware AES SSE 4.2 instructions), which brings the figures down a bit.

Future changes could write back the computed ghash stream back to OBJs if
/INCREMENTAL is specified (just an idea). Incrementally linking will be faster
that way when working with MSVC OBJs.

As for creating PDBs for independent projects, that would help most likely.
However the ghash stream would need to be stored in the PDB in that case
(currently, ghashes are dropped after merging). That could help when using
rarely compiled projects, used along with network caches.

I will start sending smaller patches to converge towards the functionally of
the prototype above.

Best,
Alex.

From: Zachary Turner <zturner at google.com>
Sent: Sunday, February 24, 2019 1:20 AM
To: Leonardo Santagada <santagada at gmail.com>
Cc: Alexandre Ganea <alexandre.ganea at ubisoft.com>; Reid Kleckner <rnk at google.com>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Making LLD PDB generation faster

+Reid and Alexandre, who have been doing work in this area recently

On Sat, Feb 23, 2019 at 4:07 AM Leonardo Santagada via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi,

Is anyone working on making the PDB generation on LLD faster? Looking
of a trace for linking one of our binaries (it takes 1min6s-1min20s) I
see two things:

1) LookupBucketFor(Val, ConstFoundBucket); takes 35s so almost half of
the time of linking, mostly finding duplicates
2) There is no parallelization inside of addObjectsToPDB

Is anyone working on those? Also has anyone thought about merging .obj
files to deduplicate type infomation so we can do the linking on
projects to generate something like a lib file, but deduplicated debug
information (as far as I know actual .lib just put all pdbs or /Z7
debug info inside a file without dedup).

Just looking at the code it seems it is much more mature and also the
choice of SHA1_8 seems interesting (still don't know why not use
xxHash64).

ps: My code to add ghashes to msvc compiled .obj files is almost ready
to be pushed as an option for llvm-objcopy.

--

Leonardo Santagada
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