Make the c feature for compiler-builtins opt-in instead of inferred

The build script for `compiler_builtins` doesn't support cross-compilation. I tried fixing it, but the cc crate itself doesn't appear to support cross-compiling to windows either unless you use the -gnu toolchain: ``` error occurred: Failed to find tool. Is `lib.exe` installed? ``` Rather than trying to fix it or special-case the platforms without bugs, make it opt-in instead of automatic.

config.toml.example

@@ -291,6 +291,10 @@ changelog-seen = 2
 # on this runtime, such as `-C profile-generate` or `-C instrument-coverage`).
 #profiler = false
 
+# Use the optimized LLVM C intrinsics for `compiler_builtins`, rather than Rust intrinsics.
+# Requires the LLVM submodule to be managed by bootstrap (i.e. not external).
+#optimized-compiler-builtins = false
+
 # Indicates whether the native libraries linked into Cargo will be statically
 # linked or not.
 #cargo-native-static = false

src/bootstrap/compile.rs

@@ -299,9 +299,7 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
 
  // Determine if we're going to compile in optimized C intrinsics to
  // the `compiler-builtins` crate. These intrinsics live in LLVM's
- // `compiler-rt` repository, but our `src/llvm-project` submodule isn't
- // always checked out, so we need to conditionally look for this. (e.g. if
- // an external LLVM is used we skip the LLVM submodule checkout).
+ // `compiler-rt` repository.
  //
  // Note that this shouldn't affect the correctness of `compiler-builtins`,
  // but only its speed. Some intrinsics in C haven't been translated to Rust
@@ -312,8 +310,15 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
  // If `compiler-rt` is available ensure that the `c` feature of the
  // `compiler-builtins` crate is enabled and it's configured to learn where
  // `compiler-rt` is located.
- let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
- let compiler_builtins_c_feature = if compiler_builtins_root.exists() {
+ let compiler_builtins_c_feature = if builder.config.optimized_compiler_builtins {
+ if !builder.is_rust_llvm(target) {
+ panic!(
+ "need a managed LLVM submodule for optimized intrinsics support; unset `llvm-config` or `optimized-compiler-builtins`"
+ );
+ }
+
+ builder.update_submodule(&Path::new("src").join("llvm-project"));
+ let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
  // Note that `libprofiler_builtins/build.rs` also computes this so if
  // you're changing something here please also change that.
  cargo.env("RUST_COMPILER_RT_ROOT", &compiler_builtins_root);

src/bootstrap/config.rs

@@ -73,6 +73,8 @@ pub struct Config {
  pub color: Color,
  pub patch_binaries_for_nix: bool,
  pub stage0_metadata: Stage0Metadata,
+ /// Whether to use the `c` feature of the `compiler_builtins` crate.
+ pub optimized_compiler_builtins: bool,
 
  pub on_fail: Option<String>,
  pub stage: u32,
@@ -597,6 +599,7 @@ define_config! {
  bench_stage: Option<u32> = "bench-stage",
  patch_binaries_for_nix: Option<bool> = "patch-binaries-for-nix",
  metrics: Option<bool> = "metrics",
+ optimized_compiler_builtins: Option<bool> = "optimized-compiler-builtins",
  }
 }
 
@@ -916,6 +919,7 @@ impl Config {
  set(&mut config.print_step_timings, build.print_step_timings);
  set(&mut config.print_step_rusage, build.print_step_rusage);
  set(&mut config.patch_binaries_for_nix, build.patch_binaries_for_nix);
+ set(&mut config.optimized_compiler_builtins, build.optimized_compiler_builtins);
 
  config.verbose = cmp::max(config.verbose, flags.verbose);
 

src/bootstrap/dist.rs

@@ -1838,23 +1838,21 @@ fn add_env(builder: &Builder<'_>, cmd: &mut Command, target: TargetSelection) {
 ///
 /// Returns whether the files were actually copied.
 fn maybe_install_llvm(builder: &Builder<'_>, target: TargetSelection, dst_libdir: &Path) -> bool {
- if let Some(config) = builder.config.target_config.get(&target) {
- if config.llvm_config.is_some() && !builder.config.llvm_from_ci {
- // If the LLVM was externally provided, then we don't currently copy
- // artifacts into the sysroot. This is not necessarily the right
- // choice (in particular, it will require the LLVM dylib to be in
- // the linker's load path at runtime), but the common use case for
- // external LLVMs is distribution provided LLVMs, and in that case
- // they're usually in the standard search path (e.g., /usr/lib) and
- // copying them here is going to cause problems as we may end up
- // with the wrong files and isn't what distributions want.
- //
- // This behavior may be revisited in the future though.
- //
- // If the LLVM is coming from ourselves (just from CI) though, we
- // still want to install it, as it otherwise won't be available.
- return false;
- }
+ if builder.is_rust_llvm(target) {
+ // If the LLVM was externally provided, then we don't currently copy
+ // artifacts into the sysroot. This is not necessarily the right
+ // choice (in particular, it will require the LLVM dylib to be in
+ // the linker's load path at runtime), but the common use case for
+ // external LLVMs is distribution provided LLVMs, and in that case
+ // they're usually in the standard search path (e.g., /usr/lib) and
+ // copying them here is going to cause problems as we may end up
+ // with the wrong files and isn't what distributions want.
+ //
+ // This behavior may be revisited in the future though.
+ //
+ // If the LLVM is coming from ourselves (just from CI) though, we
+ // still want to install it, as it otherwise won't be available.
+ return false;
  }
 
  // On macOS, rustc (and LLVM tools) link to an unversioned libLLVM.dylib

src/ci/docker/host-x86_64/disabled/dist-x86_64-haiku/Dockerfile

@@ -47,4 +47,6 @@ ENV RUST_CONFIGURE_ARGS --disable-jemalloc \
  --set=$TARGET.cc=x86_64-unknown-haiku-gcc \
  --set=$TARGET.cxx=x86_64-unknown-haiku-g++ \
  --set=$TARGET.llvm-config=/bin/llvm-config-haiku
+ENV EXTERNAL_LLVM 1
+
 ENV SCRIPT python3 ../x.py dist --host=$HOST --target=$HOST

src/ci/docker/host-x86_64/dist-various-2/Dockerfile

@@ -129,4 +129,6 @@ ENV RUST_CONFIGURE_ARGS --enable-extended --enable-lld --disable-docs \
  --set target.wasm32-wasi.wasi-root=/wasm32-wasi \
  --musl-root-armv7=/musl-armv7
 
+ENV EXTERNAL_LLVM 1
+
 ENV SCRIPT python3 ../x.py dist --host='' --target $TARGETS

src/ci/docker/host-x86_64/x86_64-gnu-llvm-13-stage1/Dockerfile

@@ -29,6 +29,7 @@ RUN sh /scripts/sccache.sh
 # We are disabling CI LLVM since this builder is intentionally using a host
 # LLVM, rather than the typical src/llvm-project LLVM.
 ENV NO_DOWNLOAD_CI_LLVM 1
+ENV EXTERNAL_LLVM 1
 
 # Using llvm-link-shared due to libffi issues -- see #34486
 ENV RUST_CONFIGURE_ARGS \

src/ci/docker/host-x86_64/x86_64-gnu-llvm-13/Dockerfile

@@ -40,6 +40,7 @@ RUN sh /scripts/sccache.sh
 # We are disabling CI LLVM since this builder is intentionally using a host
 # LLVM, rather than the typical src/llvm-project LLVM.
 ENV NO_DOWNLOAD_CI_LLVM 1
+ENV EXTERNAL_LLVM 1
 
 # Using llvm-link-shared due to libffi issues -- see #34486
 ENV RUST_CONFIGURE_ARGS \

src/ci/run.sh

@@ -69,6 +69,11 @@ RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.codegen-units-std=1"
 # space required for CI artifacts.
 RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --dist-compression-formats=xz"
 
+# Enable the `c` feature for compiler_builtins, but only when the `compiler-rt` source is available.
+if [ "$EXTERNAL_LLVM" = "" ]; then
+ RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set build.optimized-compiler-builtins"
+fi
+
 if [ "$DIST_SRC" = "" ]; then
  RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-dist-src"
 fi