[llvm] Enable LLVM_LINK_LLVM_DYLIB by default on non-Windows platforms #138187

rnk · 2025-05-01T19:30:27Z

As discussed in the RFC thread [1], this build configuration is a better
default because it uses less RAM and disk to build LLVM, which makes it
more accessible to beginners. The downside to the libLLVM build is that
it increases tool binary startup time, which can have major impacts on
configuration script times and the LLVM test suite itself. However, it
is better for new users configuring LLVM builds for the first time.

Long-time developers with beefy machines who prefer the finer grained
dependencies of static linking can still use the old model by opting out
with -DLLVM_LINK_LLVM_DYLIB=OFF. The fine-grained shared lib build is
also still supported via -DBUILD_SHARED_LIBS=ON.

MLIR's dylib build is configured to follow the LLVM dylib build, so this
changes the default for MLIR as well.

[1] https://discourse.llvm.org/t/rfc-llvm-link-llvm-dylib-should-default-to-on-on-posix-platforms/85908/1

As discussed in the RFC thread [1], this build configuration is a better default because it uses less RAM and disk to build LLVM, which makes it more accessible to beginners. The downside to the libLLVM build is that it increases tool binary startup time, which can have major impacts on configuration script times and the LLVM test suite itself. However, it is better for new users configuring LLVM builds for the first time. Long-time developers with beefy machines who prefer the finer grained dependencies of static linking can still use the old model by opting out with -DLLVM_LINK_LLVM_DYLIB=OFF. The fine-grained shared lib build is also still supported via -DBUILD_SHARED_LIBS=ON. MLIR's dylib build is configured to follow the LLVM dylib build, so this changes the default for MLIR as well. [1] https://discourse.llvm.org/t/rfc-llvm-link-llvm-dylib-should-default-to-on-on-posix-platforms/85908/1

rnk · 2025-06-25T16:02:17Z

The premerge check failures seem related, and they seem related to bolt:

llvm-lit: /usr/local/google/home/rnk/llvm-project/llvm/utils/lit/lit/formats/googletest.py:39: warning: unable to discover google-tests in '/usr/local/google/home/rnk/llvm-project/build_cmake/tools/bolt/unittests/Core/./CoreTests': Command '['/usr/local/google/home/rnk/llvm-project/build_cmake/tools/bolt/unittests/Core/./CoreTests', '--gtest_list_tests', '--gtest_filter=-*DISABLED_*']' died with <Signals.SIGABRT: 6>.. Process output: b''
LLVM ERROR: Option 'si' already exists!
llvm-lit: /usr/local/google/home/rnk/llvm-project/llvm/utils/lit/lit/formats/googletest.py:39: warning: unable to discover google-tests in '/usr/local/google/home/rnk/llvm-project/build_cmake/tools/bolt/unittests/Profile/./ProfileTests': Command '['/usr/local/google/home/rnk/llvm-project/build_cmake/tools/bolt/unittests/Profile/./ProfileTests', '--gtest_list_tests', '--gtest_filter=-*DISABLED_*']' died with <Signals.SIGABRT: 6>.. Process output: b''
FAIL: BOLT-Unit :: Profile/./ProfileTests/failed_to_discover_tests_from_gtest (1 of 2)
******************** TEST 'BOLT-Unit :: Profile/./ProfileTests/failed_to_discover_tests_from_gtest' FAILED ********************

********************
FAIL: BOLT-Unit :: Core/./CoreTests/failed_to_discover_tests_from_gtest (2 of 2)
******************** TEST 'BOLT-Unit :: Core/./CoreTests/failed_to_discover_tests_from_gtest' FAILED ********************

********************
********************
Failed Tests (2):
  BOLT-Unit :: Core/./CoreTests/failed_to_discover_tests_from_gtest
  BOLT-Unit :: Profile/./ProfileTests/failed_to_discover_tests_from_gtest


Testing Time: 0.02s

Total Discovered Tests: 2
  Failed: 2 (100.00%)

It was pointed out in the RFC that Bolt links LLVM statically instead of using the dylib because it directly refers to symbols in Target libraries that are hidden visibility. I think resolving that may become a blocker.

compnerd

I'm not a big fan of the naming as LLVM_LINK_LLVM_DYLIB and LLVM_BUILD_LLVM_DYLIB are easy to mix up in a quick read. However, it also makes sense to name them similarly for the configuration behaviour. I'm not sure if there is a great solution here, but pointing this out just incase someone happens to have some brilliant inishgt.

llvmbot · 2025-06-26T04:02:05Z

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-bolt

Author: Reid Kleckner (rnk)

Changes

As discussed in the RFC thread [1], this build configuration is a better
default because it uses less RAM and disk to build LLVM, which makes it
more accessible to beginners. The downside to the libLLVM build is that
it increases tool binary startup time, which can have major impacts on
configuration script times and the LLVM test suite itself. However, it
is better for new users configuring LLVM builds for the first time.

Long-time developers with beefy machines who prefer the finer grained
dependencies of static linking can still use the old model by opting out
with -DLLVM_LINK_LLVM_DYLIB=OFF. The fine-grained shared lib build is
also still supported via -DBUILD_SHARED_LIBS=ON.

MLIR's dylib build is configured to follow the LLVM dylib build, so this
changes the default for MLIR as well.

[1] https://discourse.llvm.org/t/rfc-llvm-link-llvm-dylib-should-default-to-on-on-posix-platforms/85908/1

Full diff: https://github.com/llvm/llvm-project/pull/138187.diff

4 Files Affected:

(modified) bolt/unittests/Profile/CMakeLists.txt (-2)
(modified) llvm/CMakeLists.txt (+10-2)
(modified) llvm/docs/ReleaseNotes.md (+5)
(modified) llvm/lib/Testing/Support/CMakeLists.txt (+2)

diff --git a/bolt/unittests/Profile/CMakeLists.txt b/bolt/unittests/Profile/CMakeLists.txt
index ce01c6c4b949e..4d468372f27fc 100644
--- a/bolt/unittests/Profile/CMakeLists.txt
+++ b/bolt/unittests/Profile/CMakeLists.txt
@@ -15,8 +15,6 @@ target_link_libraries(ProfileTests
   PRIVATE
   LLVMBOLTCore
   LLVMBOLTProfile
-  LLVMTargetParser
-  LLVMTestingSupport
   )
 
 foreach (tgt ${BOLT_TARGETS_TO_BUILD})
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 4b9e70fad2152..cd2adf1cddace 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -919,14 +919,22 @@ if(NOT MSVC OR LLVM_BUILD_LLVM_DYLIB_VIS)
   set(CAN_BUILD_LLVM_DYLIB ON)
 endif()
 
-cmake_dependent_option(LLVM_LINK_LLVM_DYLIB "Link tools against the libllvm dynamic library" OFF
+# Link the tools against the libllvm DSO by default.
+set(LLVM_LINK_LLVM_DYLIB_default ON)
+if (BUILD_SHARED_LIBS OR WIN32)
+  set(LLVM_LINK_LLVM_DYLIB_default OFF)
+endif()
+
+cmake_dependent_option(LLVM_LINK_LLVM_DYLIB "Link tools against the libllvm dynamic library"
+                       "${LLVM_LINK_LLVM_DYLIB_default}"
                        "CAN_BUILD_LLVM_DYLIB" OFF)
 
 set(LLVM_BUILD_LLVM_DYLIB_default OFF)
 if(LLVM_LINK_LLVM_DYLIB OR LLVM_BUILD_LLVM_C_DYLIB)
   set(LLVM_BUILD_LLVM_DYLIB_default ON)
 endif()
-cmake_dependent_option(LLVM_BUILD_LLVM_DYLIB "Build libllvm dynamic library" ${LLVM_BUILD_LLVM_DYLIB_default}
+cmake_dependent_option(LLVM_BUILD_LLVM_DYLIB "Build libllvm dynamic library"
+                       "${LLVM_BUILD_LLVM_DYLIB_default}"
                        "CAN_BUILD_LLVM_DYLIB" OFF)
 
 cmake_dependent_option(LLVM_DYLIB_EXPORT_INLINES "Force inline members of classes to be DLL exported when
diff --git a/llvm/docs/ReleaseNotes.md b/llvm/docs/ReleaseNotes.md
index 4a1005667692d..d4b1e353499f5 100644
--- a/llvm/docs/ReleaseNotes.md
+++ b/llvm/docs/ReleaseNotes.md
@@ -90,6 +90,11 @@ Changes to LLVM infrastructure
 Changes to building LLVM
 ------------------------
 
+* On non-Windows platforms, LLVM now builds as a large shared library, libLLVM,
+  by default. To revert to the old behavior of producing and linking static
+  libraries, pass ``-DLLVM_LINK_LLVM_DYLIB=OFF`` to CMake when configuring your
+  build.
+
 Changes to TableGen
 -------------------
 
diff --git a/llvm/lib/Testing/Support/CMakeLists.txt b/llvm/lib/Testing/Support/CMakeLists.txt
index 6955271239ca6..e5f3af23a6a13 100644
--- a/llvm/lib/Testing/Support/CMakeLists.txt
+++ b/llvm/lib/Testing/Support/CMakeLists.txt
@@ -14,6 +14,8 @@ add_llvm_library(LLVMTestingSupport
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Testing/Support
 
+  DISABLE_LLVM_LINK_LLVM_DYLIB
+
   LINK_COMPONENTS
   Support
   )

llvmbot · 2025-06-26T04:02:06Z

@llvm/pr-subscribers-llvm-support

Author: Reid Kleckner (rnk)

Changes

As discussed in the RFC thread [1], this build configuration is a better
default because it uses less RAM and disk to build LLVM, which makes it
more accessible to beginners. The downside to the libLLVM build is that
it increases tool binary startup time, which can have major impacts on
configuration script times and the LLVM test suite itself. However, it
is better for new users configuring LLVM builds for the first time.

Long-time developers with beefy machines who prefer the finer grained
dependencies of static linking can still use the old model by opting out
with -DLLVM_LINK_LLVM_DYLIB=OFF. The fine-grained shared lib build is
also still supported via -DBUILD_SHARED_LIBS=ON.

MLIR's dylib build is configured to follow the LLVM dylib build, so this
changes the default for MLIR as well.

[1] https://discourse.llvm.org/t/rfc-llvm-link-llvm-dylib-should-default-to-on-on-posix-platforms/85908/1

Full diff: https://github.com/llvm/llvm-project/pull/138187.diff

4 Files Affected:

(modified) bolt/unittests/Profile/CMakeLists.txt (-2)
(modified) llvm/CMakeLists.txt (+10-2)
(modified) llvm/docs/ReleaseNotes.md (+5)
(modified) llvm/lib/Testing/Support/CMakeLists.txt (+2)

diff --git a/bolt/unittests/Profile/CMakeLists.txt b/bolt/unittests/Profile/CMakeLists.txt
index ce01c6c4b949e..4d468372f27fc 100644
--- a/bolt/unittests/Profile/CMakeLists.txt
+++ b/bolt/unittests/Profile/CMakeLists.txt
@@ -15,8 +15,6 @@ target_link_libraries(ProfileTests
   PRIVATE
   LLVMBOLTCore
   LLVMBOLTProfile
-  LLVMTargetParser
-  LLVMTestingSupport
   )
 
 foreach (tgt ${BOLT_TARGETS_TO_BUILD})
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 4b9e70fad2152..cd2adf1cddace 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -919,14 +919,22 @@ if(NOT MSVC OR LLVM_BUILD_LLVM_DYLIB_VIS)
   set(CAN_BUILD_LLVM_DYLIB ON)
 endif()
 
-cmake_dependent_option(LLVM_LINK_LLVM_DYLIB "Link tools against the libllvm dynamic library" OFF
+# Link the tools against the libllvm DSO by default.
+set(LLVM_LINK_LLVM_DYLIB_default ON)
+if (BUILD_SHARED_LIBS OR WIN32)
+  set(LLVM_LINK_LLVM_DYLIB_default OFF)
+endif()
+
+cmake_dependent_option(LLVM_LINK_LLVM_DYLIB "Link tools against the libllvm dynamic library"
+                       "${LLVM_LINK_LLVM_DYLIB_default}"
                        "CAN_BUILD_LLVM_DYLIB" OFF)
 
 set(LLVM_BUILD_LLVM_DYLIB_default OFF)
 if(LLVM_LINK_LLVM_DYLIB OR LLVM_BUILD_LLVM_C_DYLIB)
   set(LLVM_BUILD_LLVM_DYLIB_default ON)
 endif()
-cmake_dependent_option(LLVM_BUILD_LLVM_DYLIB "Build libllvm dynamic library" ${LLVM_BUILD_LLVM_DYLIB_default}
+cmake_dependent_option(LLVM_BUILD_LLVM_DYLIB "Build libllvm dynamic library"
+                       "${LLVM_BUILD_LLVM_DYLIB_default}"
                        "CAN_BUILD_LLVM_DYLIB" OFF)
 
 cmake_dependent_option(LLVM_DYLIB_EXPORT_INLINES "Force inline members of classes to be DLL exported when
diff --git a/llvm/docs/ReleaseNotes.md b/llvm/docs/ReleaseNotes.md
index 4a1005667692d..d4b1e353499f5 100644
--- a/llvm/docs/ReleaseNotes.md
+++ b/llvm/docs/ReleaseNotes.md
@@ -90,6 +90,11 @@ Changes to LLVM infrastructure
 Changes to building LLVM
 ------------------------
 
+* On non-Windows platforms, LLVM now builds as a large shared library, libLLVM,
+  by default. To revert to the old behavior of producing and linking static
+  libraries, pass ``-DLLVM_LINK_LLVM_DYLIB=OFF`` to CMake when configuring your
+  build.
+
 Changes to TableGen
 -------------------
 
diff --git a/llvm/lib/Testing/Support/CMakeLists.txt b/llvm/lib/Testing/Support/CMakeLists.txt
index 6955271239ca6..e5f3af23a6a13 100644
--- a/llvm/lib/Testing/Support/CMakeLists.txt
+++ b/llvm/lib/Testing/Support/CMakeLists.txt
@@ -14,6 +14,8 @@ add_llvm_library(LLVMTestingSupport
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Testing/Support
 
+  DISABLE_LLVM_LINK_LLVM_DYLIB
+
   LINK_COMPONENTS
   Support
   )

rnk · 2025-06-26T13:23:59Z

bolt/unittests/Core/CMakeLists.txt

@@ -20,7 +20,6 @@ target_link_libraries(CoreTests
  LLVMBOLTRewrite
  LLVMBOLTProfile
  LLVMBOLTUtils
-  LLVMTestingSupport
  )


This removal here was interesting. Bolt doesn't support the LLVM dylib build, and it tries to disable linking the LLVM dylib. However, it ends up depending on it transitively through LLVMTestingSupport, resulting in double registration of command line flags due to two copies of Support libraries, static and from the dylib. I think it should be possible to link LLVMTestingSupport as a single static archive and not reference the dylib, but I couldn't figure that out. In the end, I cut the library dependency because it was easier.

This library dependency was introduced in Dec 2024 5100307 and I think it breaks building bolt with cmake and the dylib build enabled, so I think that was actually a regression in functionality, but I need to confirm it.

Anyway, I think this will finally pass premerge checks.

Should split this into a separate PR. Worth noting that #145448 has an alternative approach where a separate statically linked LLVMTestingSupportStatic is introduced. But given that that this is barely used here, just dropping it entirely is fine.

hubert-reinterpretcast · 2025-06-26T15:29:40Z

llvm/CMakeLists.txt

@@ -919,14 +919,22 @@ if(NOT MSVC OR LLVM_BUILD_LLVM_DYLIB_VIS)
  set(CAN_BUILD_LLVM_DYLIB ON)
 endif()

-cmake_dependent_option(LLVM_LINK_LLVM_DYLIB "Link tools against the libllvm dynamic library" OFF
+# Link the tools against the libllvm DSO by default.
+set(LLVM_LINK_LLVM_DYLIB_default ON)


As mentioned on the RFC thread, the deployment story on AIX does not work well with such a configuration (due to lack of relative rpath support).

rnk mentioned this pull request May 1, 2025

MLIR pass test fails when using dylib #138202

Closed

rnk added 2 commits May 2, 2025 16:32

Merge branch 'main' into llvm-dylib-default

53c1338

Merge branch 'main' into llvm-dylib-default

96fddd1

rnk marked this pull request as ready for review June 25, 2025 13:51

rnk requested review from nikic and petrhosek June 25, 2025 13:52

Remove debug prints

1eb025c

rnk requested a review from compnerd June 25, 2025 16:02

compnerd approved these changes Jun 25, 2025

View reviewed changes

Disable linking the dylib from TestingSupport, it only uses Support

788bd48

rnk requested review from aaupov, maksfb, rafaelauler, ayermolo, yota9 and paschalis-mpeis as code owners June 26, 2025 04:01

llvmbot added BOLT llvm:support labels Jun 26, 2025

Add release notes

2f1529f

llvmbot added the clang Clang issues not falling into any other category label Jun 26, 2025

Remove bolt unittest dep on LLVMTestingSupport to fix link

5904206

rnk commented Jun 26, 2025

View reviewed changes

hubert-reinterpretcast reviewed Jun 26, 2025

View reviewed changes

rnk mentioned this pull request Jun 30, 2025

Reapply "[cmake] Refactor clang unittest cmake" #134195

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