Add support for x86_64-unknown-none targets

By default, no sysroot is used.

This is useful for bare-metal targets, firmwares, etc.

We are actively using this patch in https://github.com/project-oak/oak

Author: jblebrun

platforms/BUILD.bazel

@@ -22,6 +22,14 @@ platform(
     ],
 )
 
+platform(
+    name = "none-x86_64",
+    constraint_values = [
+        "@platforms//os:none",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
 platform(
     name = "linux-aarch64",
     constraint_values = [

toolchain/cc_toolchain_config.bzl

@@ -97,6 +97,14 @@ def cc_toolchain_config(
             "clang",
             "glibc_unknown",
         ),
+        "none-x86_64": (
+            "clang-x86_64-none",
+            "k8",
+            "unknown",
+            "clang",
+            "unknown",
+            "unknown",
+        ),
         "wasm32": (
             "clang-wasm32",
             "wasm32",

toolchain/internal/common.bzl

@@ -20,6 +20,7 @@ SUPPORTED_TARGETS = [
     ("darwin", "aarch64"),
     ("none", "wasm32"),
     ("none", "wasm64"),
+    ("none", "x86_64"),
     ("wasip1", "wasm32"),
     ("wasip1", "wasm64"),
 ]
@@ -118,7 +119,7 @@ def os(rctx):
 
     name = rctx.attr.exec_os
     if name:
-        if name in ("linux", "darwin"):
+        if name in ("linux", "darwin", "none"):
             return name
         else:
             fail("Unsupported value for exec_os: %s" % name)

toolchain/internal/configure.bzl

@@ -300,9 +300,12 @@ def _cc_toolchain_str(
         if exec_os == target_os and exec_arch == target_arch:
             # For darwin -> darwin, we can use the macOS SDK path.
             sysroot_path = _default_sysroot_path(rctx, exec_os)
+        elif target_os == "none":
+            # If we are targeting bare metal, we don't need a sysroot.
+            sysroot_path = ""
         else:
             # We are trying to cross-compile without a sysroot, let's bail.
-            # TODO: Are there situations where we can continue?
+            # TODO: Are there other situations where we can continue?
             return ""
 
     extra_files_str = "\":internal-use-files\""
@@ -323,6 +326,7 @@ def _cc_toolchain_str(
         "linux-aarch64": "aarch64-unknown-linux-gnu",
         "linux-armv7": "armv7-unknown-linux-gnueabihf",
         "linux-x86_64": "x86_64-unknown-linux-gnu",
+        "none-x86_64": "x86_64-unknown-none",
         "wasm32": "wasm32-unknown-unknown",
         "wasm64": "wasm64-unknown-unknown",
         "wasip1-wasm32": "wasm32-wasip1",