Support WebAssembly target platforms wasm{32,64}-unknown-unknown

platforms/BUILD.bazel

@@ -45,3 +45,19 @@ platform(
         "@platforms//cpu:aarch64",
     ],
 )
+
+platform(
+    name = "wasm32",
+    constraint_values = [
+        "@platforms//os:none",
+        "@platforms//cpu:wasm32",
+    ],
+)
+
+platform(
+    name = "wasm64",
+    constraint_values = [
+        "@platforms//os:none",
+        "@platforms//cpu:wasm64",
+    ],
+)

toolchain/BUILD.llvm_repo

@@ -41,6 +41,7 @@ filegroup(
     srcs = [
         "bin/ld.lld",
         "bin/ld64.lld",
+        "bin/wasm-ld",
     ],
 )
 

toolchain/cc_toolchain_config.bzl

@@ -89,6 +89,22 @@ def cc_toolchain_config(
             "clang",
             "glibc_unknown",
         ),
+        "wasm32": (
+            "clang-wasm32",
+            "wasm32",
+            "unknown",
+            "clang",
+            "unknown",
+            "unknown",
+        ),
+        "wasm64": (
+            "clang-wasm64",
+            "wasm64",
+            "unknown",
+            "clang",
+            "unknown",
+            "unknown",
+        ),
     }[target_os_arch_key]
 
     # Unfiltered compiler flags; these are placed at the end of the command
@@ -171,6 +187,10 @@ def cc_toolchain_config(
         archive_flags.extend([
             "-static",
         ])
+    elif target_arch in ["wasm32", "wasm64"]:
+        # lld is invoked as wasm-ld for WebAssembly targets.
+        use_lld = True
+        use_libtool = False
     else:
         # Note that for xcompiling from darwin to linux, the native ld64 is
         # not an option because it is not a cross-linker, so lld is the

toolchain/internal/common.bzl

@@ -12,7 +12,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-SUPPORTED_TARGETS = [("linux", "x86_64"), ("linux", "aarch64"), ("darwin", "x86_64"), ("darwin", "aarch64")]
+SUPPORTED_TARGETS = [
+    ("linux", "x86_64"),
+    ("linux", "aarch64"),
+    ("darwin", "x86_64"),
+    ("darwin", "aarch64"),
+    ("none", "wasm32"),
+    ("none", "wasm64"),
+]
 
 # Map of tool name to its symlinked name in the tools directory.
 # See tool_paths in toolchain/cc_toolchain_config.bzl.
@@ -120,7 +127,7 @@ def os(rctx):
 
 def os_bzl(os):
     # Return the OS string as used in bazel platform constraints.
-    return {"darwin": "osx", "linux": "linux"}[os]
+    return {"darwin": "osx", "linux": "linux", "none": "none"}[os]
 
 def arch(rctx):
     arch = rctx.attr.exec_arch
@@ -139,7 +146,12 @@ def arch(rctx):
         return "x86_64"
     return arch
 
+def is_standalone_arch(os, arch):
+    return os == "none" and arch in ["wasm32", "wasm64"]
+
 def os_arch_pair(os, arch):
+    if is_standalone_arch(os, arch):
+        return arch
     return "{}-{}".format(os, arch)
 
 _supported_os_arch = [os_arch_pair(os, arch) for (os, arch) in SUPPORTED_TARGETS]

toolchain/internal/configure.bzl

@@ -25,6 +25,7 @@ load(
     _check_os_arch_keys = "check_os_arch_keys",
     _exec_os_arch_dict_value = "exec_os_arch_dict_value",
     _is_absolute_path = "is_absolute_path",
+    _is_standalone_arch = "is_standalone_arch",
     _list_to_string = "list_to_string",
     _os = "os",
     _os_arch_pair = "os_arch_pair",
@@ -243,7 +244,10 @@ def _cc_toolchains_str(
     cc_toolchains_str = ""
     toolchain_names = []
     for (target_os, target_arch) in _supported_targets:
-        suffix = "{}-{}".format(target_arch, target_os)
+        if _is_standalone_arch(target_os, target_arch):
+            suffix = target_arch
+        else:
+            suffix = "{}-{}".format(target_arch, target_os)
         cc_toolchain_str = _cc_toolchain_str(
             rctx,
             suffix,
@@ -315,6 +319,8 @@ def _cc_toolchain_str(
         "darwin-aarch64": "aarch64-apple-macosx",
         "linux-aarch64": "aarch64-unknown-linux-gnu",
         "linux-x86_64": "x86_64-unknown-linux-gnu",
+        "wasm32": "wasm32-unknown-unknown",
+        "wasm64": "wasm64-unknown-unknown",
     }[target_pair]
     cxx_builtin_include_directories = [
         toolchain_path_prefix + "include/c++/v1",
@@ -341,6 +347,11 @@ def _cc_toolchain_str(
             _join(sysroot_prefix, "/usr/include"),
             _join(sysroot_prefix, "/System/Library/Frameworks"),
         ])
+    elif target_os == "none":
+        if sysroot_prefix:
+            cxx_builtin_include_directories.extend([
+                _join(sysroot_prefix, "/include"),
+            ])
     else:
         fail("Unreachable")