[clang] c6f0f88 - Workaround for MSVC ARM64 build performance regression (#65215)

[via cfe-commits]

Author: Alexander Smarus
Date: 2023-10-20T22:47:12+05:30
New Revision: c6f0f8810db5ce4de4bd326950b644e846a1497e

URL: https://github.com/llvm/llvm-project/commit/c6f0f8810db5ce4de4bd326950b644e846a1497e
DIFF: https://github.com/llvm/llvm-project/commit/c6f0f8810db5ce4de4bd326950b644e846a1497e.diff

LOG: Workaround for MSVC ARM64 build performance regression (#65215)

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).

Added: 
    

Modified: 
    clang/lib/CodeGen/CMakeLists.txt
    clang/lib/Tooling/Inclusions/Stdlib/CMakeLists.txt

Removed: 
    


################################################################################
diff  --git a/clang/lib/CodeGen/CMakeLists.txt b/clang/lib/CodeGen/CMakeLists.txt
index 9fab15abe6404c6..d67ce982d78acf3 100644
--- a/clang/lib/CodeGen/CMakeLists.txt
+++ b/clang/lib/CodeGen/CMakeLists.txt
@@ -31,6 +31,26 @@ 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