Determinism: SEQ=PAR byte-identical FSharp.Compiler.Service.dll (fixes #19928)

.gitignore

@@ -161,3 +161,4 @@ tests/projects/CompilerCompat/local-nuget-packages/
 tests/projects/CompilerCompat/lib-output-*/
 tests/projects/CompilerCompat/**/bin/
 tests/projects/CompilerCompat/**/obj/
+.scratch/

azure-pipelines-PR.yml

@@ -100,7 +100,7 @@ stages:
         helixRepo: dotnet/fsharp
         jobs:
           # Determinism, we want to run it only in PR builds
-        - job: Determinism_Debug
+        - job: Determinism_Release
           condition: eq(variables['Build.Reason'], 'PullRequest')
           variables:
           - name: _SignType
@@ -109,13 +109,6 @@ stages:
             name: $(DncEngPublicBuildPool)
             demands: ImageOverride -equals $(_WindowsMachineQueueName)
           timeoutInMinutes: 90
-          strategy:
-            maxParallel: 2
-            matrix:
-              regular:
-                _experimental_flag: ''
-              experimental_features:
-                _experimental_flag: ''
           steps:
           - checkout: self
             clean: true
@@ -129,15 +122,15 @@ stages:
               workingDirectory: $(Build.SourcesDirectory)
               installationPath: $(Build.SourcesDirectory)/.dotnet
           - script: .\eng\common\dotnet.cmd
-          - script: .\eng\test-determinism.cmd -configuration Debug
-            env:
-              FSHARP_EXPERIMENTAL_FEATURES: $(_experimental_flag)
-            displayName: Determinism tests with Debug configuration
+          - script: .\eng\test-determinism.cmd -configuration Release
+            displayName: Determinism tests (race detector — same flags both builds)
+          - script: .\eng\test-determinism.cmd -configuration Release -mode seq-vs-par
+            displayName: Determinism tests (1-shot diff — sequential vs parallel)
           - task: PublishPipelineArtifact@1
             displayName: Publish Determinism Logs
             inputs:
-              targetPath: '$(Build.SourcesDirectory)/artifacts/log/Debug'
-              artifactName: 'Determinism_Debug Attempt $(System.JobAttempt) Logs'
+              targetPath: '$(Build.SourcesDirectory)/artifacts/log/Release'
+              artifactName: 'Determinism_Release Attempt $(System.JobAttempt) Logs'
             continueOnError: true
             condition: not(succeeded())
 

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -1,9 +1,12 @@
 ### Fixed
 
+* Stabilize codegen order under `--parallelcompilation+` so `--deterministic` Release builds produce byte-identical IL across rebuilds: optimizer Val iteration, IlxGen type/method/field/event emit order, anonymous-record extra-binding drain, and `FileIndex` assignment now follow source position rather than thread-scheduling order. Compiler-generated names are also bucketed by the file being optimized/emitted (per-file naming scope) rather than by inlined source location, so parallel optimization and code generation no longer race on a shared name counter. ([Issue #19732](https://github.com/dotnet/fsharp/issues/19732), [PR #19810](https://github.com/dotnet/fsharp/pull/19810))
 * Suppress hover/symbol resolution for wildcard `_` patterns inside `member _.…` bodies that incorrectly showed `val _: T` tooltip. ([PR #19760](https://github.com/dotnet/fsharp/pull/19760))
 * Deduplicate format specifier locations in computation expressions so editor tooling no longer reports duplicate entries for the same `%` specifier. ([Issue #16419](https://github.com/dotnet/fsharp/issues/16419), [PR #19791](https://github.com/dotnet/fsharp/pull/19791))
+* Stabilize codegen order under `--parallelcompilation+` so `--deterministic` Release builds produce byte-identical IL across rebuilds: optimizer Val iteration, IlxGen type/method/field/event emit order, anonymous-record extra-binding drain, and `FileIndex` assignment now follow source position rather than thread-scheduling order. ([Issue #19732](https://github.com/dotnet/fsharp/issues/19732), [PR #19810](https://github.com/dotnet/fsharp/pull/19810))
 * Reject non-function bindings for single-case and partial active pattern names with FS1209, matching the existing multi-case behavior. ([PR #19763](https://github.com/dotnet/fsharp/pull/19763))
 * Fix FS0421 "The address of the variable cannot be used at this point" incorrectly raised for the discard pattern `let _ = &expr` when `let x = &expr` compiles. ([Issue #18841](https://github.com/dotnet/fsharp/issues/18841), [PR #19811](https://github.com/dotnet/fsharp/pull/19811))
+* Make `--deterministic` Release builds byte-reproducible under `--parallelcompilation+`. Compiler-generated names produced by the Detuple/TLR optimizer passes and by closure code-generation are now bucketed by the emitting file rather than by the (possibly inlined) source range, so parallel codegen of distinct consumer files no longer races on a shared name-counter. ([Issue #19732](https://github.com/dotnet/fsharp/issues/19732), [PR #19810](https://github.com/dotnet/fsharp/pull/19810))
 * Honor `--nowarn` and `--warnaserror` for warnings emitted during command-line option parsing ([Issue #19576](https://github.com/dotnet/fsharp/issues/19576), [PR #19776](https://github.com/dotnet/fsharp/pull/19776))
 * Fix `[<return: X>]` prefix attributes being silently dropped on class members, and fix false-positive `AllowMultiple=false` errors when `[<X>]` and `[<return: X>]` are applied to the same binding. ([Issue #17904](https://github.com/dotnet/fsharp/issues/17904), [Issue #19020](https://github.com/dotnet/fsharp/issues/19020), [PR #19738](https://github.com/dotnet/fsharp/pull/19738))
 * Preserve type abbreviations (`string`, user-defined aliases) in the refined type of bindings introduced after a `| null` pattern in a `match` expression. ([Issue #19646](https://github.com/dotnet/fsharp/issues/19646), [PR #19745](https://github.com/dotnet/fsharp/pull/19745))

eng/test-determinism.ps1

@@ -2,6 +2,8 @@
 param([string]$configuration = "Debug",
   [string]$msbuildEngine = "vs",
   [string]$altRootDrive = "q:",
+  [ValidateSet("same", "seq-vs-par")]
+  [string]$mode = "same",
   [switch]$help,
   [switch]$norestore,
   [switch]$rebuild)
@@ -15,6 +17,8 @@ function Print-Usage() {
   Write-Host "  -msbuildEngine <value>    Msbuild engine to use to run build ('dotnet', 'vs', or unspecified)."
   Write-Host "  -bootstrapDir             Directory containing the bootstrap compiler"
   Write-Host "  -altRootDrive             The drive we build on (via subst) for verifying pathmap implementation"
+  Write-Host "  -mode <value>             'same' (default): build twice with identical flags (race-detector)"
+  Write-Host "                            'seq-vs-par': first build sequential, second build parallel (deterministic 1-shot diff)"
 }
 
 if ($help) {
@@ -25,7 +29,7 @@ if ($help) {
 # List of binary names that should be skipped because they have a known issue that
 # makes them non-deterministic.
 $script:skipList = @()
-function Run-Build([string]$rootDir, [string]$increment) {
+function Run-Build([string]$rootDir, [string]$increment, [string]$additionalFscFlags = "") {
 
   $logFileName = $increment
 
@@ -62,6 +66,9 @@ function Run-Build([string]$rootDir, [string]$increment) {
   Stop-Processes
 
   Write-Host "Building $solution using $bootstrapDir into '$increment' $incrementDir"
+  if ($additionalFscFlags -ne "") {
+    Write-Host "  AdditionalFscCmdFlags = '$additionalFscFlags'"
+  }
   MSBuild $toolsetBuildProj `
     /p:Configuration=$configuration `
     /p:Projects=$solution `
@@ -86,6 +93,7 @@ function Run-Build([string]$rootDir, [string]$increment) {
     /p:RunAnalyzers=false `
     /p:RunAnalyzersDuringBuild=false `
     /p:BUILDING_USING_DOTNET=false `
+    /p:AdditionalFscCmdFlags="$additionalFscFlags" `
     /bl:$logFilePath
 
   Write-Host "Copy-Item -Path $binDir -Destination $incrementDir -ErrorAction SilentlyContinue -Recurse"
@@ -202,9 +210,9 @@ function Test-MapContents($dataMap) {
   }
 }
 
-function Test-Build([string]$rootDir, $dataMap, [string]$increment) {
+function Test-Build([string]$rootDir, $dataMap, [string]$increment, [string]$additionalFscFlags = "") {
   $logFileName = $increment
-  Run-Build $rootDir -increment $increment
+  Run-Build $rootDir -increment $increment -additionalFscFlags $additionalFscFlags
 
   $errorList = @()
   $allGood = $true
@@ -273,14 +281,31 @@ function Test-Build([string]$rootDir, $dataMap, [string]$increment) {
 }
 
 function Run-Test() {
+  $seqFlags = ""
+  $parFlags = ""
+  if ($mode -eq "seq-vs-par") {
+    # First build: sequential (force single-threaded, disable any parallel test paths)
+    $seqFlags = "--parallelcompilation- --test:ParallelOff --nowarn:75"
+    # Second build: parallel (default in modern fsc, made explicit for clarity)
+    $parFlags = "--parallelcompilation+"
+    Write-Host "Determinism mode: seq-vs-par"
+    Write-Host "  Initial (seq): $seqFlags"
+    Write-Host "  Test1   (par): $parFlags"
+  }
+  else {
+    Write-Host "Determinism mode: same (race-detector; both builds identical)"
+  }
+
   # Run the initial build so that we can populate the maps
-  Run-Build $RepoRoot -increment "Initial" -useBootstrap
+  Run-Build $RepoRoot -increment "Initial" -additionalFscFlags $seqFlags
 
   $dataMap = Record-Binaries $RepoRoot "Initial"
   Test-MapContents $dataMap
 
-  # Run a test against the source in the same directory location
-  Test-Build -rootDir $RepoRoot -dataMap $dataMap -increment "Test1"
+  # Run a test against the source in the same directory location.
+  # In 'same' mode: same flags as Initial (probabilistic race detector).
+  # In 'seq-vs-par' mode: parallel flags, contrasting against the sequential Initial.
+  Test-Build -rootDir $RepoRoot -dataMap $dataMap -increment "Test1" -additionalFscFlags $parFlags
 
   # Run another build in a different source location and verify that path mapping
   # allows the build to be identical.  To do this we'll copy the entire source