[PATCH] D150874: [JITLink][NFC] Store external symbols in a StringMap

[Job Noorman via Phabricator via llvm-commits]

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External symbols used to be stored in a `DenseSet`. An `assert` in
`addExternalSymbol` ensures that names of external symbols are unique.
However, for objects containing a huge number of external symbols, this
`assert` can be a performance bottleneck.

This patch proposes to store external symbols in a `StringMap` instead
making it significantly cheaper to check if a certain symbol name
already exists.

This issue came up while porting BOLT to JITLink (D147544 <https://reviews.llvm.org/D147544>): linking a
large binary using the JITLink port turned out to be about 4x slower
than the current version of BOLT that uses RuntimeDyld. This slowdown
was caused entirely by the `assert` in `addExternalSymbol`. Using this
patch, the JITLink port is slightly faster than RuntimeDyld.

Note that I fully understand that adding complexity to improve the
performance of an `assert` might not be acceptable (since the checks
shouldn't exist in release builds). This patch has another advantage,
however: since external symbol names really have to be unique, it could
make sense to make this clear at the type level.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D150874

Files:
  llvm/include/llvm/ExecutionEngine/JITLink/JITLink.h

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