bazel: Add GCC compat and CI (#3476)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: phlax <454682+phlax@users.noreply.github.com>

Author: Copilot

.github/workflows/bazel.yml

@@ -87,6 +87,39 @@ jobs:
         - workspace
         - bzlmod
 
+  test-gcc:
+    needs: request
+    if: >-
+      always()
+      && fromJSON(needs.request.outputs.run || 'false')
+    uses: ./.github/workflows/_bazel.yml
+    with:
+      action: build
+      bazel-args: >-
+        --config=ci
+        --config=gcc
+        ${{ matrix.mode == 'bzlmod'
+            && '--enable_bzlmod'
+            || '' }}
+      bazel-mode: ${{ matrix.mode }}-gcc
+      bazel-path: bazel
+      targets: >-
+        //compile/...
+        //coverage/...
+        //dependency/...
+        //format/...
+        //repository/...
+        //sha/...
+        //tarball/...
+        //toolchains/...
+        //:dependency_versions
+    strategy:
+      fail-fast: false
+      matrix:
+        mode:
+        - workspace
+        - bzlmod
+
   status:
     runs-on: ubuntu-24.04
     if: >-
@@ -97,6 +130,7 @@ jobs:
     - request
     - test
     - build
+    - test-gcc
     steps:
     - run: |
         if [[ "${{ contains(needs.*.result, 'failure') }}" == "true" || "${{ contains(needs.*.result, 'cancelled') }}" == "true" ]]; then

bazel/.bazelrc

@@ -20,6 +20,17 @@ common:ci --noshow_loading_progress
 common:ci --test_output=errors
 
 
+#############################################################################
+# gcc - Use system GCC toolchain (similar to BCR testing)
+#############################################################################
+
+# Allow detection of system C++ toolchain for GCC testing
+common:gcc --repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=0
+
+# Enable GCC-compatible flags in compile targets
+common:gcc --//compile:use_gcc_toolchain=true
+
+
 #############################################################################
 # imports
 #############################################################################

bazel/compile/BUILD

@@ -1,3 +1,4 @@
+load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
 load("@rules_foreign_cc//foreign_cc:defs.bzl", "cmake")
 load("@rules_pkg//pkg:mappings.bzl", "pkg_files")
 load("@rules_pkg//pkg:pkg.bzl", "pkg_tar")
@@ -7,20 +8,56 @@ package(default_visibility = ["//visibility:public"])
 
 exports_files(["sanitizer_libs.bzl", "extensions.bzl"])
 
-SANITIZER_ENV = {
+# Build setting to control which toolchain to use
+# This can be set with --//compile:use_gcc_toolchain=true
+bool_flag(
+    name = "use_gcc_toolchain",
+    build_setting_default = False,
+)
+
+config_setting(
+    name = "gcc_build",
+    flag_values = {":use_gcc_toolchain": "true"},
+)
+
+# LLVM/Clang specific environment variables
+LLVM_SANITIZER_ENV = {
     "CXXFLAGS": "-nostdinc++ -nostdlib++",
     "LDFLAGS": "-nostdlib++",
 }
 
-BASE_CMAKE_CACHE = {
-    "CMAKE_BUILD_TYPE": "RelWithDebInfo",
-    "CMAKE_INSTALL_PREFIX": ".",
-    "CMAKE_POSITION_INDEPENDENT_CODE": "ON",
-    "LLVM_ENABLE_RUNTIMES": "libcxxabi;libcxx;libunwind",
+# GCC compatible environment (nostdlib++ is Clang-specific)
+GCC_SANITIZER_ENV = {
+    "CXXFLAGS": "-nostdinc++",
+    "LDFLAGS": "",
+}
+
+SANITIZER_ENV = select({
+    ":gcc_build": GCC_SANITIZER_ENV,
+    "//conditions:default": LLVM_SANITIZER_ENV,
+})
+
+# Linker flags that differ between LLVM (lld) and GCC (default linker)
+LLVM_LINKER_FLAGS = {
     "LLVM_USE_LINKER": "lld",
     "CMAKE_SHARED_LINKER_FLAGS": "-shared -Wl,-S -fuse-ld=lld",
     "CMAKE_MODULE_LINKER_FLAGS": "-shared -Wl,-S -fuse-ld=lld",
     "CMAKE_EXE_LINKER_FLAGS": "-Wl,-S -fuse-ld=lld",
+}
+
+GCC_LINKER_FLAGS = {
+    # Don't specify LLVM_USE_LINKER for GCC, let CMake/compiler choose
+    "CMAKE_SHARED_LINKER_FLAGS": "-shared -Wl,-S",
+    "CMAKE_MODULE_LINKER_FLAGS": "-shared -Wl,-S",
+    "CMAKE_EXE_LINKER_FLAGS": "-Wl,-S",
+}
+
+# Base CMake configuration without linker-specific flags
+BASE_CMAKE_CACHE_COMMON = {
+    "CMAKE_BUILD_TYPE": "RelWithDebInfo",
+    "CMAKE_INSTALL_PREFIX": ".",
+    "CMAKE_POSITION_INDEPENDENT_CODE": "ON",
+    "LLVM_ENABLE_RUNTIMES": "libcxxabi;libcxx;libunwind",
     "BUILD_SHARED_LIBS": "OFF",
     "LIBCXX_ENABLE_SHARED": "OFF",
     "LIBCXX_ENABLE_STATIC": "ON",
@@ -30,6 +67,12 @@ BASE_CMAKE_CACHE = {
     "LIBUNWIND_ENABLE_STATIC": "ON",
 }
 
+# Merge with linker-specific flags based on toolchain
+BASE_CMAKE_CACHE = BASE_CMAKE_CACHE_COMMON | select({
+    ":gcc_build": GCC_LINKER_FLAGS,
+    "//conditions:default": LLVM_LINKER_FLAGS,
+})
+
 cmake(
     name = "libcxx_msan",
     cache_entries = BASE_CMAKE_CACHE | {