twister: Set a default gcov-tool for llvm

When using llvm we need to set the gcov-tool to "llvm-cov gcov" but
the lcov tool is incapable of passing arguments to the gcov-tool. i.e.
the following cannot work:

$ lcov --gcov-tool "llvm-cov gcov" ...

Instead, create a symlink to llvm-cov prefixed as `gcov` which by the
documentation of llvm-cov will alias to `llvm-cov gcov` subcommand.

Signed-off-by: Yuval Peress <peress@google.com>

doc/develop/test/coverage.rst

@@ -153,3 +153,18 @@ which produces a report in the same location as non-unit testing.
 
 .. _gcov:
    https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
+
+Using different toolchains
+==========================
+
+Twister looks at the environment variable ``ZEPHYR_TOOLCHAIN_VARIANT``
+to check which gcov tool to use by default. The following are used as the
+default for the Twister ``--gcov-tool`` argument default:
+
++-----------+-----------------------+
+| Toolchain | ``--gcov-tool`` value |
++-----------+-----------------------+
+| host      | ``gcov``              |
+| llvm      | ``llvm-cov gcov``     |
+| zephyr    | ``gcov``              |
++-----------+-----------------------+

scripts/pylib/twister/twisterlib/coverage.py

@@ -5,6 +5,8 @@
 
 import os
 import logging
+import pathlib
+import shutil
 import subprocess
 import glob
 import re
@@ -125,7 +127,7 @@ class Lcov(CoverageTool):
     def _generate(self, outdir, coveragelog):
         coveragefile = os.path.join(outdir, "coverage.info")
         ztestfile = os.path.join(outdir, "ztest.info")
-        cmd = ["lcov", "--gcov-tool", self.gcov_tool,
+        cmd = ["lcov", "--gcov-tool", str(self.gcov_tool),
                          "--capture", "--directory", outdir,
                          "--rc", "lcov_branch_coverage=1",
                          "--output-file", coveragefile]
@@ -195,8 +197,10 @@ class Gcovr(CoverageTool):
 
         # We want to remove tests/* and tests/ztest/test/* but save tests/ztest
         cmd = ["gcovr", "-r", self.base_dir, "--gcov-executable",
-               self.gcov_tool, "-e", "tests/*"] + excludes + ["--json", "-o",
-               coveragefile, outdir]
+               str(self.gcov_tool), "-e", "tests/*"] + excludes + ["--json",
+                                                                   "-o",
+                                                                   coveragefile,
+                                                                   outdir]
         cmd_str = " ".join(cmd)
         logger.debug(f"Running {cmd_str}...")
         subprocess.call(cmd, stdout=coveragelog)
@@ -234,18 +238,25 @@ class Gcovr(CoverageTool):
 
 def run_coverage(testplan, options):
     if not options.gcov_tool:
-        use_system_gcov = False
-
-        for plat in options.coverage_platform:
-            ts_plat = testplan.get_platform(plat)
-            if ts_plat and (ts_plat.type in {"native", "unit"}):
-                use_system_gcov = True
-
-        if use_system_gcov or "ZEPHYR_SDK_INSTALL_DIR" not in os.environ:
-            options.gcov_tool = "gcov"
+        zephyr_sdk_gcov_tool = os.path.join(
+            os.environ.get("ZEPHYR_SDK_INSTALL_DIR", default=""),
+            "x86_64-zephyr-elf/bin/x86_64-zephyr-elf-gcov")
+        if os.environ.get("ZEPHYR_TOOLCHAIN_VARIANT") == "llvm":
+            llvm_path = os.environ.get("LLVM_TOOLCHAIN_PATH")
+            if llvm_path is not None:
+                llvm_path = os.path.join(llvm_path, "bin")
+            llvm_cov = shutil.which("llvm-cov", path=llvm_path)
+            llvm_cov_ext = pathlib.Path(llvm_cov).suffix
+            gcov_lnk = os.path.join(options.outdir, f"gcov{llvm_cov_ext}")
+            try:
+                os.symlink(llvm_cov, gcov_lnk)
+            except OSError:
+                shutil.copy(llvm_cov, gcov_lnk)
+            options.gcov_tool = gcov_lnk
+        elif os.path.exists(zephyr_sdk_gcov_tool):
+            options.gcov_tool = zephyr_sdk_gcov_tool
         else:
-            options.gcov_tool = os.path.join(os.environ["ZEPHYR_SDK_INSTALL_DIR"],
-                                                "x86_64-zephyr-elf/bin/x86_64-zephyr-elf-gcov")
+            options.gcov_tool = "gcov"
 
     logger.info("Generating coverage files...")
     coverage_tool = CoverageTool.factory(options.coverage_tool)

scripts/pylib/twister/twisterlib/environment.py

@@ -340,7 +340,7 @@ structure in the main Zephyr tree: boards/<arch>/<board_name>/""")
                         help="Do not filter based on toolchain, use the set "
                              " toolchain unconditionally")
 
-    parser.add_argument("--gcov-tool", default=None,
+    parser.add_argument("--gcov-tool", type=Path, default=None,
                         help="Path to the gcov tool to use for code coverage "
                              "reports")