Generate .ifso libraries with llvm-ifs

.bazelrc

@@ -42,6 +42,8 @@ common --check_direct_dependencies=error
 # Add mirrors for certain download URLs
 common --experimental_downloader_config=bazel_downloader.cfg
 
+common --noincompatible_sandbox_hermetic_tmp
+
 # Enable modern C++ features
 build:linux --cxxopt=-std=c++17
 build:linux --host_cxxopt=-std=c++17

site/en/docs/user-manual.md

@@ -576,6 +576,15 @@ Modes:
   [mostly static](/reference/be/c-cpp#cc_binary.linkstatic) mode.
   If `-static` is set in linkopts, targets will change to fully static.
 
+#### `--interface_shared_objects` {:#interface-shared-objects}
+
+If enabled, Bazel will link against interface libraries instead of dynamic libraries (if
+[dynamic linking](#dynamic-mode) is enabled). This avoids relinking when changes only affect the
+implementation of the library, but not its interface. Interface libraries are always available on
+Windows. The default Linux C++ toolchain automatically generates interface libraries if the
+[`llvm-ifs`](https://llvm.org/docs/CommandGuide/llvm-ifs.html) tool is provided via
+the `BAZEL_LLVM_IFS` environment variable or can be found in `PATH`.
+
 #### `--fission (yes|no|[dbg][,opt][,fastbuild])` {:#fission}
 
 Enables [Fission](https://gcc.gnu.org/wiki/DebugFission){: .external},

src/test/shell/bazel/cc_integration_test.sh

@@ -1912,4 +1912,127 @@ EOF
   fi
 }
 
+function test_ifso_generation() {
+  type -P llvm-ifs-14 || return 0
+
+  mkdir pkg
+  cat > pkg/BUILD <<'EOF'
+cc_library(
+  name = "lib",
+  srcs = ["lib.c"],
+  hdrs = ["lib.h"],
+)
+cc_test(
+  name = "test",
+  srcs = ["test.c"],
+  deps = [":lib"],
+)
+EOF
+  cat > pkg/lib.c <<'EOF'
+#include "lib.h"
+int func() { return 42; }
+EOF
+  cat > pkg/lib.h <<'EOF'
+int func();
+EOF
+  cat > pkg/test.c <<'EOF'
+#include "lib.h"
+#include "stdio.h"
+int main() {
+  printf("func(): %d\n", func());
+}
+EOF
+
+  bazel test //pkg:test -s --test_output=all \
+    --repo_env=BAZEL_LLVM_IFS=llvm-ifs-14 \
+    &> "$TEST_log" || fail "Build failed"
+  expect_log "action 'Compiling pkg/lib.c'"
+  expect_log "action 'Linking pkg/liblib.so'"
+  expect_log "action 'Linking pkg/test'"
+  expect_log "func(): 42"
+  [[ -f ${PRODUCT_NAME}-bin/pkg/liblib.ifso ]] || fail "liblib.ifso not found"
+
+  # Only modify the implementation of func(). This should not trigger a relink
+  # as the .ifso file isn't modified.
+  cat > pkg/lib.c <<'EOF'
+#include "lib.h"
+int func() { return 43; }
+EOF
+  bazel test //pkg:test -s --test_output=all \
+    --repo_env=BAZEL_LLVM_IFS=llvm-ifs-14 \
+    &> "$TEST_log" || fail "Build failed"
+  expect_log "action 'Compiling pkg/lib.c'"
+  expect_log "action 'Linking pkg/liblib.so'"
+  expect_not_log "action 'Linking pkg/test'"
+  expect_log "func(): 43"
+}
+
+function test_ifso_symbol_versioning() {
+  type -P llvm-ifs-14 || return 0
+
+  mkdir pkg
+  cat > pkg/BUILD <<'EOF'
+cc_library(
+  name = "lib",
+  srcs = ["lib.c"],
+  hdrs = ["lib.h"],
+  additional_linker_inputs = ["version_script.lds"],
+  linkopts = ["-Wl,--version-script,$(execpath version_script.lds)"],
+)
+cc_test(
+  name = "test",
+  srcs = ["test.c"],
+  deps = [":lib"],
+)
+EOF
+  cat > pkg/version_script.lds <<'EOF'
+v1 {};
+v2 {};
+EOF
+  cat > pkg/lib.c <<'EOF'
+#include "lib.h"
+__asm__(".symver old_func, func@@v1, remove");
+int old_func() { return 42; }
+__asm__(".symver new_func, func@v2, remove");
+int new_func() { return 43; }
+EOF
+  cat > pkg/lib.h <<'EOF'
+int func();
+EOF
+  cat > pkg/test.c <<'EOF'
+#include "lib.h"
+#include "stdio.h"
+int main() {
+  printf("func(): %d\n", func());
+}
+EOF
+
+  bazel test //pkg:test -s --test_output=all \
+    --repo_env=BAZEL_LLVM_IFS=llvm-ifs-14 \
+    &> "$TEST_log" || fail "Build failed"
+  expect_log "action 'Compiling pkg/lib.c'"
+  expect_log "action 'Linking pkg/liblib.so'"
+  expect_log "action 'Linking pkg/test'"
+  expect_log "func(): 42"
+  [[ -f ${PRODUCT_NAME}-bin/pkg/liblib.ifso ]] || fail "liblib.ifso not found"
+
+  # Only modify the default symbol version of func(). This results in a
+  # different implementation being at runtime after a relink. Without the
+  # relink, the runtime behavior will be incorrect.
+  cat > pkg/lib.c <<'EOF'
+#include "lib.h"
+__asm__(".symver old_func, func@v1, remove");
+int old_func() { return 42; }
+__asm__(".symver new_func, func@@v2, remove");
+int new_func() { return 43; }
+EOF
+  bazel test //pkg:test -s --test_output=all \
+    --repo_env=BAZEL_LLVM_IFS=llvm-ifs-14 \
+    &> "$TEST_log" || fail "Build failed"
+  expect_log "action 'Compiling pkg/lib.c'"
+  expect_log "action 'Linking pkg/liblib.so'"
+  expect_log "action 'Linking pkg/test'"
+  expect_log "func(): 43"
+}
+
 run_suite "cc_integration_test"

tools/cpp/build_interface_so

@@ -1,8 +1,19 @@
-#!/bin/bash
+#!/bin/sh
 
-if [[ $# != 2 ]]; then
+set -eu
+
+if [ $# != 2 ]; then
    echo "Usage: $0 <so> <interface so>" 1>&2
    exit 1
 fi
 
-exec cp $1 $2
+has_versioned_symbol() {
+  "$NM" --dynamic --format=just-symbols --no-sort "$1" \
+    | grep --quiet --max-count=1 --fixed-strings @
+}
+
+if has_versioned_symbol "$1"; then
+  exec cp "$1" "$2"
+else
+  "$LLVM_IFS" --input-format=ELF "$1" --output-elf "$2"
+fi

tools/cpp/unix_cc_configure.bzl

@@ -83,6 +83,7 @@ def _get_tool_paths(repository_ctx, overriden_tools):
             "ld",
             "llvm-cov",
             "llvm-profdata",
+            "llvm-ifs",
             "cpp",
             "gcc",
             "dwp",
@@ -373,6 +374,14 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
         warn = True,
         silent = True,
     )
+    overriden_tools["llvm-ifs"] = _find_generic(
+        repository_ctx,
+        "llvm-ifs",
+        "BAZEL_LLVM_IFS",
+        overriden_tools,
+        warn = True,
+        silent = True,
+    )
     overriden_tools["ar"] = _find_generic(
         repository_ctx,
         "ar",
@@ -545,7 +554,10 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
             "%{cc_toolchain_identifier}": cc_toolchain_identifier,
             "%{name}": cpu_value,
             "%{modulemap}": ("\":module.modulemap\"" if generate_modulemap else "None"),
-            "%{cc_compiler_deps}": get_starlark_list([":builtin_include_directory_paths"] + (
+            "%{cc_compiler_deps}": get_starlark_list([
+                ":builtin_include_directory_paths",
+                "@bazel_tools//tools/cpp:link_dynamic_library",
+            ] + (
                 [":cc_wrapper"] if darwin else []
             )),
             "%{compiler}": escape_string(get_env_var(
@@ -591,7 +603,7 @@ def configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools):
                 False,
             )),
             "%{tool_paths}": ",\n        ".join(
-                ['"%s": "%s"' % (k, v) for k, v in tool_paths.items()],
+                ['"%s": "%s"' % (k, v) for k, v in tool_paths.items() if v != None],
             ),
             "%{cxx_builtin_include_directories}": get_starlark_list(builtin_include_directories),
             "%{compile_flags}": get_starlark_list(