fix: isolate probe counters from JIT tier transitions

Author: KSemenenko

.github/workflows/ci.yml

@@ -37,6 +37,7 @@ jobs:
       uses: actions/upload-artifact@v4
       with:
         name: full-test-results
+        overwrite: true
         path: ./**/TestResults/*.trx
         retention-days: 5
 

.github/workflows/release.yml

@@ -46,6 +46,7 @@ jobs:
       uses: actions/upload-artifact@v4
       with:
         name: full-test-results
+        overwrite: true
         path: ./**/TestResults/*.trx
         retention-days: 5
 

ManagedCode.MimeTypes.StartupProbe/Program.cs

@@ -2,22 +2,37 @@
 using ManagedCode.MimeTypes;
 
 var input = args.Length == 0 ? "https://cdn.example.test/assets/photo.png?v=1" : args[0];
+var allocationControl = args.Length > 1 && args[1] == "--allocation-control";
 var mime = MimeHelper.GetMimeType("asset.png");
 var extensions = MimeHelper.GetExtensions(mime);
 var knownMimeTypes = MimeHelper.GetKnownMimeTypes();
-var before = GC.GetAllocatedBytesForCurrentThread();
-for (var index = 0; index < 100_000; index++)
-{
-    mime = MimeHelper.GetMimeType(input);
-}
-var allocated = GC.GetAllocatedBytesForCurrentThread() - before;
+var result = MeasureLookups(input, allocationControl);
 
-Report(mime, extensions.Count, knownMimeTypes.Count, allocated);
+Report(result.Mime, extensions.Count, knownMimeTypes.Count, result.Allocated);
 
-// Keep interpolated output in a separate JIT body. Otherwise OSR can initialize
-// formatting infrastructure while the lookup allocation counter is running.
+// Keep output infrastructure outside the lookup allocation measurement.
 [MethodImpl(MethodImplOptions.NoInlining)]
 static void Report(string mime, int extensionCount, int knownCount, long allocated)
 {
     Console.WriteLine($"{mime}|{extensionCount}|{knownCount}|{allocated}");
 }
+
+// Only the harness bypasses tiering: the library keeps the requested JIT mode.
+// This prevents tier transitions of the measuring loop from polluting its counter.
+[MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+static (string Mime, long Allocated) MeasureLookups(string input, bool allocationControl)
+{
+    var before = GC.GetAllocatedBytesForCurrentThread();
+    if (allocationControl)
+    {
+        GC.KeepAlive(new byte[1024]);
+    }
+
+    var mime = string.Empty;
+    for (var index = 0; index < 100_000; index++)
+    {
+        mime = MimeHelper.GetMimeType(input);
+    }
+
+    return (mime, GC.GetAllocatedBytesForCurrentThread() - before);
+}

ManagedCode.MimeTypes.Tests/MimeHelperPerformanceTests.cs

@@ -147,7 +147,18 @@ public void ColdProcessLookups_ShouldPreserveAllocationBudgetAcrossJitModes(
         result.Allocated.ShouldBeLessThanOrEqualTo(256);
     }
 
-    private (TimeSpan Duration, long Allocated, string Mime) RunStartupProbe(string? input, string runtimeMode)
+    [Fact]
+    [Trait("Category", "Performance")]
+    public void AllocationProbe_ShouldDetectRealManagedAllocations()
+    {
+        var result = RunStartupProbe("assets/photo.png", "default", allocationControl: true);
+
+        result.Mime.ShouldBe("image/png");
+        result.Allocated.ShouldBeGreaterThanOrEqualTo(1024);
+    }
+
+    private (TimeSpan Duration, long Allocated, string Mime) RunStartupProbe(
+        string? input, string runtimeMode, bool allocationControl = false)
     {
         var startInfo = new ProcessStartInfo("dotnet")
         {
@@ -161,6 +172,11 @@ public void ColdProcessLookups_ShouldPreserveAllocationBudgetAcrossJitModes(
             startInfo.ArgumentList.Add(input);
         }
 
+        if (allocationControl)
+        {
+            startInfo.ArgumentList.Add("--allocation-control");
+        }
+
         if (runtimeMode == "tiered")
         {
             startInfo.Environment["DOTNET_TieredCompilation"] = "1";