[llvm] build Blake3 source with LLVM_EXPORTS defined

[andrurogerz]

Purpose

This patch ensures that the BLAKE3 implementation in the LLVM Support library exports its public interface with __declspec(dllexport) when building LLVM as a Windows DLL.

Background

The effort to support building LLVM as a Windows DLL is tracked in #109483. Additional context is provided in this discourse.

Overview

Replicate this logic from llvm_add_library() for the LLVMSupportBlake3 target. Without this change, the llvm_blake_ functions will only be annotated with __declspec(dllimport) when building LLVM as a Windows DLL which leads to inconsistent DLL linkage warnings from MSVC and clang-cl.

[llvmbot]

@llvm/pr-subscribers-llvm-support

Author: Andrew Rogers (andrurogerz)

Changes

Purpose

This patch ensures that the BLAKE3 implementation in the LLVM Support library exports its public interface with __declspec(dllexport) when building LLVM as a Windows DLL.

Background

The effort to support building LLVM as a Windows DLL is tracked in #109483. Additional context is provided in this discourse.

Overview

Replicate this logic from llvm_add_library() for the LLVMSupportBlake3 target. Without this change, the llvm_blake_ functions will only be annotated with __declspec(dllimport) when building LLVM as a Windows DLL which leads to inconsistent DLL linkage warnings from MSVC and clang-cl.

---

Full diff: https://github.com/llvm/llvm-project/pull/144753.diff

1 Files Affected:

• (modified) llvm/lib/Support/BLAKE3/CMakeLists.txt (+6)

diff --git a/llvm/lib/Support/BLAKE3/CMakeLists.txt b/llvm/lib/Support/BLAKE3/CMakeLists.txt
index 99cb78881ec47..eae2b0280e5d3 100644
--- a/llvm/lib/Support/BLAKE3/CMakeLists.txt
+++ b/llvm/lib/Support/BLAKE3/CMakeLists.txt
@@ -85,3 +85,9 @@ endif()
 add_library(LLVMSupportBlake3 OBJECT EXCLUDE_FROM_ALL ${LLVM_BLAKE3_FILES})
 set_target_properties(LLVMSupportBlake3 PROPERTIES FOLDER "LLVM/Libraries")
 llvm_update_compile_flags(LLVMSupportBlake3)
+if(LLVM_BUILD_LLVM_DYLIB OR BUILD_SHARED_LIBS)
+  # Since LLVMSupportBlake3 is not defined using llvm_add_library(), we must
+  # define LLVM_EXPORTS here so its public interface is annotated with
+  # __declspec(dllexport) when building as a DLL on Windows.
+  target_compile_definitions(LLVMSupportBlake3 PRIVATE LLVM_EXPORTS)
+endif()

[andrurogerz]

@compnerd, @vgvassilev this one should be straight-forward, but I thought it was worth making into its own PR for discussion if needed. Thanks!

[compnerd]

It is unfortunate that we cannot use standard CMake tooling to control the behaviour here. I think that given the state of the build, this is reasonable.