ValueMapper: Delete unused initializers of replaced appending globals.
A full LTO link time performance and memory regression was introduced
by #137081 in cases where the modules contain large quantities of llvm.used
globals. This was unnoticed because it was not expected that this would
be a typical case, but this is exactly what coverage collection does,
and when this feature is enabled together with full LTO we end up with
quadratic memory consumption (from the unused constants) and quadratic
complexity in the function Verifier::visitGlobalValue (which visits all
the unused constants in the use list of each global value). This is a
targeted fix that avoids reintroducing the quadratic complexity from
before #137081, by having ValueMapper delete the old initializer of an
appending global if it is unused, instead of visiting every global in
the context after every link.
The repro-cfi-64 reproducer from #167037 before and after this change:
```
Elapsed time Max RSS (KB)
Before 12:05.11 52537184
After 3:27.68 7520696
```
Fixes #167037.
Reviewers: nikic, teresajohnson
Reviewed By: teresajohnson
Pull Request: https://github.com/llvm/llvm-project/pull/167629
Welcome to the LLVM project!
This repository contains the source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and run-time environments.
The LLVM project has multiple components. The core of the project is itself called “LLVM”. This contains all of the tools, libraries, and header files needed to process intermediate representations and convert them into object files. Tools include an assembler, disassembler, bitcode analyzer, and bitcode optimizer.
C-like languages use the Clang frontend. This component compiles C, C++, Objective-C, and Objective-C++ code into LLVM bitcode -- and from there into object files, using LLVM.
Other components include: the libc++ C++ standard library, the LLD linker, and more.
Consult the Getting Started with LLVM page for information on building and running LLVM.
For information on how to contribute to the LLVM project, please take a look at the Contributing to LLVM guide.
Join the LLVM Discourse forums, Discord chat, LLVM Office Hours or Regular sync-ups.
The LLVM project has adopted a code of conduct for participants to all modes of communication within the project.