[bazel] Change how tools are pulled from cc_toolchains

Bazel is moving towards deprecating the `tool_path` style of CC
toolchain which allowed executables to be taken using the `*_executable`
parameters. They want you to use `cc_common.get_tool_for_action`
instead, but unfortunately that only allows accessing the tools used by
particular actions. Some of our rules need to access specific tools
even if they're not used by the presented actions.

Examples:

1. We use `clang` for assembly actions, but OTBN needs
   `riscv32-unknown-elf-as` directly.

2. We need `objdump` which can be registered to the `cc_toolchain`, but
   since there is no action in Bazel that uses it there's no way to
   access it with this function.

To solve this, I've repurposed this Bazel function to pull tools out of
the data for the C toolchain.

Signed-off-by: James Wainwright <james.wainwright@lowrisc.org>

Author: jwnrt

rules/opentitan/static_library.bzl

@@ -9,6 +9,7 @@
 
 load("@rules_cc//cc:find_cc_toolchain.bzl", "find_cc_toolchain")
 load("@lowrisc_opentitan//rules:rv.bzl", "rv_rule")
+load("@lowrisc_opentitan//rules/opentitan:toolchain.bzl", "get_tool")
 
 def _ot_static_library_impl(ctx):
     output_lib = ctx.actions.declare_file("lib{}.a".format(ctx.attr.name))
@@ -42,7 +43,7 @@ def _ot_static_library_impl(ctx):
 
     lib_paths = [lib.path for lib in libs]
 
-    ar_path = cc_toolchain.ar_executable
+    ar_path = get_tool(cc_toolchain, "riscv32-unknown-elf-ar").path
 
     ctx.actions.run_shell(
         command = "\"{0}\" rcT {1} {2} && echo -e 'create {1}\naddlib {1}\nsave\nend' | \"{0}\" -M".format(

rules/opentitan/toolchain.bzl

@@ -15,6 +15,16 @@ LocalToolInfo = provider(fields = [
     "update_manifest_json",
 ])
 
+def get_tool(cc_toolchain, name):
+    """Find the path to a tool within the C toolchain."""
+
+    # Note: we should really be using `cc.get_tool_for_action`, but that only exposes
+    # tools with corresponding C compilation actions. This does not include tools like
+    # `objdump` despite them being in the C toolchain. It also doesn't allow for
+    # accessing specific tools such as `riscv32-unknown-elf-as` when the toolchain is
+    # configured to use a wrapper like `clang` for assembly actions instead.
+    return [f for f in cc_toolchain.all_files.to_list() if f.basename == name][0]
+
 def _localtools_toolchain(ctx):
     tools = LocalToolInfo(
         opentitantool = ctx.attr.opentitantool[0].files_to_run,

rules/opentitan/transform.bzl

@@ -6,6 +6,7 @@ load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
 load("@rules_cc//cc:find_cc_toolchain.bzl", "find_cc_toolchain")
 load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@lowrisc_opentitan//rules/opentitan:util.bzl", "get_override")
+load("@lowrisc_opentitan//rules/opentitan:toolchain.bzl", "get_tool")
 
 def obj_transform(ctx, **kwargs):
     """Transform an object file via objcopy.
@@ -40,7 +41,7 @@ def obj_transform(ctx, **kwargs):
             src.path,
             output.path,
         ],
-        executable = cc_toolchain.objcopy_executable,
+        executable = get_tool(cc_toolchain, "riscv32-unknown-elf-objcopy"),
     )
     return output
 
@@ -69,7 +70,7 @@ def obj_disassemble(ctx, **kwargs):
         outputs = [output],
         inputs = [src] + cc_toolchain.all_files.to_list(),
         arguments = [
-            cc_toolchain.objdump_executable,
+            get_tool(cc_toolchain, "riscv32-unknown-elf-objdump").path,
             src.path,
             output.path,
         ],

rules/otbn.bzl

@@ -3,25 +3,16 @@
 # SPDX-License-Identifier: Apache-2.0
 
 load("//rules:rv.bzl", "rv_rule")
+load("//rules/opentitan:toolchain.bzl", "get_tool")
 load("@rules_cc//cc:find_cc_toolchain.bzl", "find_cc_toolchain")
 
-def _get_assembler(cc_toolchain):
-    """Find the path to riscv-unknown-elf-as."""
-
-    # Note: the toolchain config doesn"t appear to have a good way to get
-    # access to the assembler.  We should be able to access it via the
-    # the compiler, but I had trouble with //hw/ip/otbn/util/otbn_as.py invoking
-    # the compiler as assembler.
-    return [f for f in cc_toolchain.all_files.to_list() if f.basename.endswith("as")][0]
-
 def _otbn_assemble_sources(ctx, additional_srcs = []):
     """Helper function that, for each source file in the provided context, adds
     an action to the context that invokes the otbn assember (otbn_as.py),
     producing a corresponding object file. Returns a list of all object files
     that will be generated by these actions.
     """
     cc_toolchain = find_cc_toolchain(ctx)
-    assembler = _get_assembler(cc_toolchain)
 
     objs = []
     for src in ctx.files.srcs + additional_srcs:
@@ -33,7 +24,7 @@ def _otbn_assemble_sources(ctx, additional_srcs = []):
                       cc_toolchain.all_files.to_list() +
                       [ctx.executable._otbn_as]),
             env = {
-                "RV32_TOOL_AS": assembler.path,
+                "RV32_TOOL_AS": get_tool(cc_toolchain, "riscv32-unknown-elf-as").path,
             },
             arguments = ["-o", obj.path, src.path] + ctx.attr.args,
             executable = ctx.executable._otbn_as,
@@ -78,7 +69,6 @@ def _otbn_binary(ctx, additional_srcs = []):
     that other rules can depend on in their `deps`.
     """
     cc_toolchain = find_cc_toolchain(ctx)
-    assembler = _get_assembler(cc_toolchain)
 
     # Run the otbn assembler on source files to produce object (.o) files.
     objs = _otbn_assemble_sources(ctx, additional_srcs)
@@ -100,10 +90,10 @@ def _otbn_binary(ctx, additional_srcs = []):
                   ctx.files._otbn_data +
                   [ctx.executable._wrapper]),
         env = {
-            "RV32_TOOL_AS": assembler.path,
-            "RV32_TOOL_AR": cc_toolchain.ar_executable,
-            "RV32_TOOL_LD": cc_toolchain.ld_executable,
-            "RV32_TOOL_OBJCOPY": cc_toolchain.objcopy_executable,
+            "RV32_TOOL_AS": get_tool(cc_toolchain, "riscv32-unknown-elf-as").path,
+            "RV32_TOOL_AR": get_tool(cc_toolchain, "riscv32-unknown-elf-ar").path,
+            "RV32_TOOL_LD": get_tool(cc_toolchain, "riscv32-unknown-elf-ld").path,
+            "RV32_TOOL_OBJCOPY": get_tool(cc_toolchain, "riscv32-unknown-elf-objcopy").path,
         },
         arguments = [
             "--app-name={}".format(ctx.attr.name),

rules/tock.bzl

@@ -6,6 +6,7 @@
 """
 
 load("@rules_cc//cc:find_cc_toolchain.bzl", "find_cc_toolchain")
+load("//rules/opentitan:toolchain.bzl", "get_tool")
 load(
     "//rules:rv.bzl",
     "rv_rule",
@@ -110,7 +111,7 @@ def _tock_image_impl(ctx):
             ctx.file.kernel.path,
             kernel_binary.path,
         ],
-        executable = cc_toolchain.objcopy_executable,
+        executable = get_tool(cc_toolchain, "riscv32-unknown-elf-objcopy").path,
     )
 
     ctx.actions.run(

sw/device/silicon_creator/imm_rom_ext/utils.bzl

@@ -4,6 +4,7 @@
 
 load("@rules_cc//cc:find_cc_toolchain.bzl", "find_cc_toolchain")
 load("@lowrisc_opentitan//rules:rv.bzl", "rv_rule")
+load("@lowrisc_opentitan//rules/opentitan:toolchain.bzl", "get_tool")
 
 def _bin_to_imm_rom_ext_object_impl(ctx):
     cc_toolchain = find_cc_toolchain(ctx)
@@ -30,7 +31,7 @@ def _bin_to_imm_rom_ext_object_impl(ctx):
                 src.path,
                 object.path,
             ],
-            executable = cc_toolchain.objcopy_executable,
+            executable = get_tool(cc_toolchain, "riscv32-unknown-elf-objcopy").path,
         )
         outputs.append(object)
     return [DefaultInfo(files = depset(outputs), runfiles = ctx.runfiles(files = outputs))]