Workaround for MSVC ARM64 build performance regression #8149
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MSVC has a major performance regression observed when targeting ARM64 since v19.32 (VS 17.2.0). `cl.exe` spends a lot of time on compiling `StandardLibrary.cpp` and `CGBuiltin.cpp`, and total build duration rises extremely. This makes builds stagnate even on a real hardware, but VM-based builds (like building on cloud agents from GitHub Actions and Azure Pipelines) are experiencing most damage as they also performance- and time-limited. The issue appears to be related to some optimizations applied in `/O2` mode. It is reported on [Developer Community](https://developercommunity.visualstudio.com/t/Compiling-a-specific-code-for-ARM64-with/10444970). While the investigation is in progress, we could apply a workaround to improve build time. `/O2` actually enables a set of optimizations, and only one of them does all slowdown. The idea is to disable optimizations, and then apply all but one back, effectively excluding the problematic option from the set. This patch alters the CMake configuration for aforementioned files. Changes are limited to: - non-debug builds - MSVC of the specific version - target arch (ARM64). This cherrypicks upstream commit llvm@c6f0f88
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@swift-ci Please test |
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CC @compnerd |
compnerd
approved these changes
Feb 8, 2024
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@swift-ci please test Windows platform |
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MSVC has a major performance regression observed when targeting ARM64 since v19.32 (VS 17.2.0).
cl.exespends a lot of time on compilingStandardLibrary.cppandCGBuiltin.cpp, and total build duration rises extremely. This makes builds stagnate even on a real hardware, but VM-based builds (like building on cloud agents from GitHub Actions and Azure Pipelines) are experiencing most damage as they also performance- and time-limited.The issue appears to be related to some optimizations applied in
/O2mode. It is reported on DeveloperCommunity. While the investigation is in progress, we could apply a workaround to improve build time.
/O2actually enables a set of optimizations, and only one of them does all slowdown. The idea is to disable optimizations, and then apply all but one back, effectively excluding the problematic option from the set.This patch alters the CMake configuration for aforementioned files. Changes are limited to:
This cherrypicks upstream commit llvm@c6f0f88