[clang] Workaround for MSVC ARM64 build performance regression (PR #65215)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

<details>
<summary>Changes</summary>

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).

---
Full diff: https://github.com/llvm/llvm-project/pull/65215.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CMakeLists.txt (+20) 
- (modified) clang/lib/Tooling/Inclusions/Stdlib/CMakeLists.txt (+20) 


``````````diff
diff --git a/clang/lib/CodeGen/CMakeLists.txt b/clang/lib/CodeGen/CMakeLists.txt
index 1debeb6d9cce9e0..2dd93a57f4e22d2 100644
--- a/clang/lib/CodeGen/CMakeLists.txt
+++ b/clang/lib/CodeGen/CMakeLists.txt
@@ -30,6 +30,26 @@ set(LLVM_LINK_COMPONENTS
   TransformUtils
   )
 
+# Workaround for MSVC ARM64 performance regression:
+# https://developercommunity.visualstudio.com/t/Compiling-a-specific-code-for-ARM64-with/10444970
+# Since /O1 and /O2 represent a set of optimizations,
+# our goal is to disable the /Og flag while retaining the other optimizations from the /O1|/O2 set
+if(MSVC AND NOT CMAKE_CXX_COMPILER_ID MATCHES Clang
+    AND MSVC_VERSION VERSION_GREATER_EQUAL 1932
+    AND CMAKE_SYSTEM_PROCESSOR MATCHES "ARM64")
+
+  string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
+  string(REGEX MATCHALL "/[Oo][12]" opt_flags "${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE}}")
+  if (opt_flags)
+    if(opt_flags MATCHES "1$")
+      set(opt_flags "/Od;/Os;/Oy;/Ob2;/GF;/Gy")
+    elseif (opt_flags MATCHES "2$")
+      set(opt_flags "/Od;/Oi;/Ot;/Oy;/Ob2;/GF;/Gy")
+    endif()
+    set_source_files_properties(CGBuiltin.cpp PROPERTIES COMPILE_OPTIONS "${opt_flags}")
+  endif()
+endif()
+
 add_clang_library(clangCodeGen
   ABIInfo.cpp
   ABIInfoImpl.cpp
diff --git a/clang/lib/Tooling/Inclusions/Stdlib/CMakeLists.txt b/clang/lib/Tooling/Inclusions/Stdlib/CMakeLists.txt
index 0f52c3590ac9af1..ed323ab3528b104 100644
--- a/clang/lib/Tooling/Inclusions/Stdlib/CMakeLists.txt
+++ b/clang/lib/Tooling/Inclusions/Stdlib/CMakeLists.txt
@@ -1,3 +1,23 @@
+# Workaround for MSVC ARM64 performance regression:
+# https://developercommunity.visualstudio.com/t/Compiling-a-specific-code-for-ARM64-with/10444970
+# Since /O1 and /O2 represent a set of optimizations,
+# our goal is to disable the /Og flag while retaining the other optimizations from the /O1|/O2 set
+if(MSVC AND NOT CMAKE_CXX_COMPILER_ID MATCHES Clang
+    AND MSVC_VERSION VERSION_GREATER_EQUAL 1932
+    AND CMAKE_SYSTEM_PROCESSOR MATCHES "ARM64")
+
+  string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
+  string(REGEX MATCHALL "/[Oo][12]" opt_flags "${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE}}")
+  if (opt_flags)
+    if(opt_flags MATCHES "1$")
+      set(opt_flags "/Od;/Os;/Oy;/Ob2;/GF;/Gy")
+    elseif (opt_flags MATCHES "2$")
+      set(opt_flags "/Od;/Oi;/Ot;/Oy;/Ob2;/GF;/Gy")
+    endif()
+    set_source_files_properties(StandardLibrary.cpp PROPERTIES COMPILE_OPTIONS "${opt_flags}")
+  endif()
+endif()
+
 add_clang_library(clangToolingInclusionsStdlib
   StandardLibrary.cpp
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/65215