[libc-commits] [libc] [libc] Tweak the runtimes cross-build for GPU (PR #178548)

[Joseph Huber via libc-commits]

https://github.com/jhuber6 updated https://github.com/llvm/llvm-project/pull/178548

>From b2c3c13b5748190a99c02fcae3e9f7c8b35f156d Mon Sep 17 00:00:00 2001
From: Joseph Huber <huberjn at outlook.com>
Date: Wed, 28 Jan 2026 17:54:24 -0600
Subject: [PATCH] [libc] Tweak the runtimes cross-build for GPU

Summary:
We should likely use `-DLLVM_DEFAULT_TARGET_TRIPLE` as the general
source of truth, make the handling work with that since we use it for
the output directories. Fix the creation of startup files in this mode
and make sure it can detect the GPU properly.
---
 .../cmake/modules/LLVMLibCArchitectures.cmake |  2 +
 .../modules/prepare_libc_gpu_build.cmake      |  4 +-
 libc/docs/gpu/building.rst                    | 40 +++----------------
 libc/startup/gpu/CMakeLists.txt               |  1 +
 4 files changed, 11 insertions(+), 36 deletions(-)

diff --git a/libc/cmake/modules/LLVMLibCArchitectures.cmake b/libc/cmake/modules/LLVMLibCArchitectures.cmake
index 939fc1226a4e9..b27a69baf3746 100644
--- a/libc/cmake/modules/LLVMLibCArchitectures.cmake
+++ b/libc/cmake/modules/LLVMLibCArchitectures.cmake
@@ -108,6 +108,8 @@ if(LLVM_RUNTIMES_TARGET)
   set(explicit_target_triple ${LLVM_RUNTIMES_TARGET})
 elseif(LIBC_TARGET_TRIPLE)
   set(explicit_target_triple ${LIBC_TARGET_TRIPLE})
+elseif(LLVM_DEFAULT_TARGET_TRIPLE AND NOT CMAKE_COMPILER_IS_GNUCXX)
+  set(explicit_target_triple ${LLVM_DEFAULT_TARGET_TRIPLE})
 endif()
 
 # The libc's target architecture and OS are set to match the compiler's default
diff --git a/libc/cmake/modules/prepare_libc_gpu_build.cmake b/libc/cmake/modules/prepare_libc_gpu_build.cmake
index 4d12a5917a56f..55cd0242eedeb 100644
--- a/libc/cmake/modules/prepare_libc_gpu_build.cmake
+++ b/libc/cmake/modules/prepare_libc_gpu_build.cmake
@@ -17,8 +17,8 @@ if(NOT LLVM_LIBC_FULL_BUILD)
 endif()
 
 # Set the required flags globally so standard CMake utilities can compile.
-if(LIBC_TARGET_TRIPLE)
-  set(CMAKE_REQUIRED_FLAGS "--target=${LIBC_TARGET_TRIPLE}")
+if(NOT CMAKE_CXX_COMPILER_TARGET)
+  set(CMAKE_REQUIRED_FLAGS "${LIBC_COMPILE_OPTIONS_DEFAULT}")
 endif()
 
 # Optionally set up a job pool to limit the number of GPU tests run in parallel.
diff --git a/libc/docs/gpu/building.rst b/libc/docs/gpu/building.rst
index 9f9528b30d9bf..a7791331423dc 100644
--- a/libc/docs/gpu/building.rst
+++ b/libc/docs/gpu/building.rst
@@ -100,12 +100,12 @@ targeting a GPU architecture.
   $> TARGET_C_COMPILER=</path/to/clang>
   $> TARGET_CXX_COMPILER=</path/to/clang++>
   $> cmake ../runtimes \ # Point to the runtimes build
-     -G Ninja                                  \
-     -DLLVM_ENABLE_RUNTIMES=libc               \
-     -DCMAKE_C_COMPILER=$TARGET_C_COMPILER     \
-     -DCMAKE_CXX_COMPILER=$TARGET_CXX_COMPILER \
-     -DLLVM_LIBC_FULL_BUILD=ON                 \
-     -DLLVM_RUNTIMES_TARGET=$TARGET_TRIPLE     \
+     -G Ninja                                    \
+     -DLLVM_ENABLE_RUNTIMES=libc                 \
+     -DCMAKE_C_COMPILER=$TARGET_C_COMPILER       \
+     -DCMAKE_CXX_COMPILER=$TARGET_CXX_COMPILER   \
+     -DLLVM_LIBC_FULL_BUILD=ON                   \
+     -DLLVM_DEFAULT_TARGET_TRIPLE=$TARGET_TRIPLE \
      -DCMAKE_BUILD_TYPE=Release
   $> ninja install
 
@@ -113,34 +113,6 @@ The above steps will result in a build targeting one of the supported GPU
 architectures. Building for multiple targets requires separate CMake
 invocations.
 
-Standalone cross build
-----------------------
-
-The GPU build can also be targeted directly as long as the compiler used is a
-supported ``clang`` compiler. This method is generally not recommended as it can
-only target a single GPU architecture.
-
-.. code-block:: sh
-
-  $> cd llvm-project  # The llvm-project checkout
-  $> mkdir build # A different build directory for the build tools
-  $> cd build
-  $> CLANG_C_COMPILER=</path/to/clang> # Must be a trunk build
-  $> CLANG_CXX_COMPILER=</path/to/clang++> # Must be a trunk build
-  $> TARGET_TRIPLE=<amdgcn-amd-amdhsa or nvptx64-nvidia-cuda>
-  $> cmake ../llvm \ # Point to the llvm directory
-     -G Ninja                                 \
-     -DLLVM_ENABLE_PROJECTS=libc              \
-     -DCMAKE_C_COMPILER=$CLANG_C_COMPILER     \
-     -DCMAKE_CXX_COMPILER=$CLANG_CXX_COMPILER \
-     -DLLVM_LIBC_FULL_BUILD=ON                \
-     -DLIBC_TARGET_TRIPLE=$TARGET_TRIPLE      \
-     -DCMAKE_BUILD_TYPE=Release
-  $> ninja install
-
-This will build and install the GPU C library along with all the other LLVM
-libraries.
-
 Build overview
 ==============
 
diff --git a/libc/startup/gpu/CMakeLists.txt b/libc/startup/gpu/CMakeLists.txt
index fa326ef46a9d1..63e2a6c5dee1a 100644
--- a/libc/startup/gpu/CMakeLists.txt
+++ b/libc/startup/gpu/CMakeLists.txt
@@ -34,6 +34,7 @@ function(add_startup_object name)
       RUNTIME_OUTPUT_DIRECTORY ${LIBC_LIBRARY_DIR}
       RUNTIME_OUTPUT_NAME ${name}.o)
     target_link_options(${fq_target_name}.exe PRIVATE
+                        ${LIBC_COMPILE_OPTIONS_DEFAULT}
                         "-r" "-nostdlib" "-flto" "-Wl,--lto-emit-llvm")
   endif()
 endfunction()