[llvm] [CMake] Set LLVM_USE_LINKER for external projects when using lld (PR #170401)

[Zachary Fogg via llvm-commits]

https://github.com/zfogg created https://github.com/llvm/llvm-project/pull/170401

## Summary

   When lld is in TOOLCHAIN_TOOLS, LLVMExternalProjectUtils sets CMAKE_LINKER to ld.lld for external projects. However, clang
    does not use CMAKE_LINKER on Linux - it requires  to be passed via linker flags.

   This causes build failures when , because the runtimes build ends up using the system linker (e.g.,
   ) which cannot process LLVM bitcode in static archives:

   

   Fix this by setting , which allows HandleLLVMOptions.cmake to properly append  to the
   linker flags.

   ## Test plan

   - [x] Build LLVM with  (previously failed, now succeeds)
   - [ ] Verify runtimes link with lld instead of system linker

>From b4d3854a5658cb6b198714c0c36057ed0dd64269 Mon Sep 17 00:00:00 2001
From: Zachary Fogg <me at zfo.gg>
Date: Tue, 2 Dec 2025 21:13:35 -0500
Subject: [PATCH] [CMake] Set LLVM_USE_LINKER for external projects when using
 lld

   When lld is in TOOLCHAIN_TOOLS, LLVMExternalProjectUtils sets
   CMAKE_LINKER to ld.lld for external projects. However, clang does not
   use CMAKE_LINKER on Linux - it requires -fuse-ld=lld to be passed
   via linker flags.

   This causes build failures when LLVM_ENABLE_LTO is enabled, because
   the runtimes build ends up using the system linker (e.g., /usr/bin/ld)
   which cannot process LLVM bitcode in static archives:

     /usr/bin/ld: libclangTidy.a: error adding symbols: file format not recognized

   Fix this by setting LLVM_USE_LINKER=lld, which allows
   HandleLLVMOptions.cmake to properly append -fuse-ld=lld to the
   linker flags.
---
 llvm/cmake/modules/LLVMExternalProjectUtils.cmake | 1 +
 1 file changed, 1 insertion(+)

diff --git a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
index 975798a8c2636..bcd60f6336232 100644
--- a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
+++ b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
@@ -242,6 +242,7 @@ function(llvm_ExternalProject_Add name source_dir)
         list(APPEND compiler_args -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link${CMAKE_EXECUTABLE_SUFFIX})
       elseif(NOT _cmake_system_name STREQUAL Darwin)
         list(APPEND compiler_args -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/ld.lld${CMAKE_EXECUTABLE_SUFFIX})
+        list(APPEND compiler_args -DLLVM_USE_LINKER=lld)
       endif()
     endif()
     if(llvm-ar IN_LIST TOOLCHAIN_TOOLS)