From 87daace271a6b571add2744f6e834b74838f6088 Mon Sep 17 00:00:00 2001
From: Parker Bibus <parker.bibus@microsoft.com>
Date: Tue, 29 Sep 2026 12:47:36 -0700
Subject: [PATCH] Set CoreCLR runtime flavor for WASM Webcil conversion

Apply RuntimeFlavor after BenchmarkDotNet's UseMonoRuntime override so Webcil conversion selects the CoreCLR format. Cover R2R and non-R2R property evaluation while preserving Mono defaults.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
---
 docs/benchmarking-workflow-dotnet-runtime.md  |  5 ++
 scripts/tests/test_wasm_coreclr_r2r.py        | 60 +++++++++++++++++++
 .../micro/MicroBenchmarks.Wasm.targets        |  5 ++
 3 files changed, 70 insertions(+)

diff --git a/docs/benchmarking-workflow-dotnet-runtime.md b/docs/benchmarking-workflow-dotnet-runtime.md
index 8980660135b..e10416a0078 100644
--- a/docs/benchmarking-workflow-dotnet-runtime.md
+++ b/docs/benchmarking-workflow-dotnet-runtime.md
@@ -215,6 +215,11 @@ Essentially, add `--aotcompilermode wasm` to the `--bdn-arguments=".."`:
 
 #### Run CoreCLR WASM ReadyToRun with a nightly or VMR SDK
 
+For CoreCLR WASM, `MicroBenchmarks.Wasm.targets` sets `RuntimeFlavor=CoreCLR`
+after BenchmarkDotNet applies `UseMonoRuntime=false`. This lets the WebAssembly
+SDK select the correct Webcil format, with or without ReadyToRun, while leaving
+Mono configuration unchanged.
+
 Use an exact SDK version together with a NuGet source containing the matching
 browser runtime/ref, WebAssembly SDK, host-specific Crossgen2, and ILLink
 packages, plus the complete `wasm-tools` pack closure. The source can be a
diff --git a/scripts/tests/test_wasm_coreclr_r2r.py b/scripts/tests/test_wasm_coreclr_r2r.py
index e59f34a4327..2ae08bcbf51 100644
--- a/scripts/tests/test_wasm_coreclr_r2r.py
+++ b/scripts/tests/test_wasm_coreclr_r2r.py
@@ -1,4 +1,7 @@
+import json
 import os
+import shutil
+import subprocess
 import sys
 import xml.etree.ElementTree as ET
 from argparse import Namespace
@@ -136,6 +139,63 @@ def test_ready_to_run_validates_resolved_runtime_pack_items():
     assert all("_PerformanceWasmRuntimePackName" not in condition for condition in conditions)
 
 
+@pytest.mark.parametrize("ready_to_run", ["false", "true"])
+@pytest.mark.parametrize(
+    ("use_mono_runtime", "initial_flavor", "expected_flavor"),
+    [
+        ("false", "", "CoreCLR"),
+        ("false", "Mono", "CoreCLR"),
+        ("true", "", ""),
+        ("true", "Mono", "Mono"),
+        ("", "", ""),
+        ("", "Mono", "Mono"),
+    ],
+)
+def test_wasm_runtime_flavor_after_generated_project_imports(
+        tmp_path, ready_to_run, use_mono_runtime, initial_flavor, expected_flavor):
+    dotnet_cli = shutil.which("dotnet")
+    if dotnet_cli is None:
+        pytest.skip("A .NET SDK is required for MSBuild property evaluation.")
+
+    micro_dir = scripts_dir.parent / "src" / "benchmarks" / "micro"
+    project = ET.Element("Project")
+    properties = ET.SubElement(project, "PropertyGroup")
+    for name, value in {
+        "UseMonoRuntime": "true",
+        "RuntimeFlavor": initial_flavor,
+        "PERFLAB_WASM_READY_TO_RUN": ready_to_run,
+        "PublishReadyToRun": "false",
+    }.items():
+        ET.SubElement(properties, name).text = value
+
+    # BenchmarkDotNet overrides UseMonoRuntime after .props but before .targets.
+    ET.SubElement(project, "Import", Project=str(micro_dir / "MicroBenchmarks.Wasm.props"))
+    properties = ET.SubElement(project, "PropertyGroup")
+    ET.SubElement(properties, "UseMonoRuntime").text = use_mono_runtime
+    ET.SubElement(project, "Import", Project=str(micro_dir / "MicroBenchmarks.Wasm.targets"))
+    project_path = tmp_path / "GeneratedWasm.proj"
+    ET.ElementTree(project).write(project_path, encoding="utf-8")
+
+    result = subprocess.run(
+        [
+            dotnet_cli, "msbuild", str(project_path), "-nologo",
+            "-getProperty:UseMonoRuntime,RuntimeFlavor,PublishReadyToRun",
+        ],
+        cwd=tmp_path,
+        capture_output=True,
+        text=True,
+        timeout=60,
+    )
+
+    assert result.returncode == 0, result.stdout + result.stderr
+    evaluated = json.loads(result.stdout)["Properties"]
+    assert evaluated["UseMonoRuntime"] == use_mono_runtime
+    assert evaluated["RuntimeFlavor"] == expected_flavor
+    assert evaluated["PublishReadyToRun"] == (
+        "true" if use_mono_runtime == "false" and ready_to_run == "true" else "false"
+    )
+
+
 def test_coreclr_payload_detects_local_toolchain_package_version(tmp_path):
     artifact = tmp_path / "artifact" / "staging"
     ref_pack = artifact / "dotnet-none" / "packs" / "Microsoft.NETCore.App.Ref" / "11.0.0-rc.1.26431.109"
diff --git a/src/benchmarks/micro/MicroBenchmarks.Wasm.targets b/src/benchmarks/micro/MicroBenchmarks.Wasm.targets
index b6e4216cf79..e1a4340fe4c 100644
--- a/src/benchmarks/micro/MicroBenchmarks.Wasm.targets
+++ b/src/benchmarks/micro/MicroBenchmarks.Wasm.targets
@@ -10,6 +10,11 @@
     <_WasmStrictVersionMatch>false</_WasmStrictVersionMatch>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(UseMonoRuntime)' == 'false'">
+    <!-- BenchmarkDotNet sets UseMonoRuntime after importing .Wasm.props. -->
+    <RuntimeFlavor>CoreCLR</RuntimeFlavor>
+  </PropertyGroup>
+
   <PropertyGroup Condition="'$(PERFLAB_WASM_READY_TO_RUN)' == 'true' and '$(UseMonoRuntime)' == 'false'">
     <PublishReadyToRun>true</PublishReadyToRun>
     <PublishReadyToRunComposite>false</PublishReadyToRunComposite>