Fix memory leak in ImportILTypeDef and profile large F# project builds

docs/perf-analysis/ANALYSIS.md

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+# Performance Analysis - Issue #19132
+
+## Problem Statement
+
+Building a synthetic F# project with 5,000+ modules takes excessive time and memory.
+
+Related Issue: https://github.com/dotnet/fsharp/issues/19132
+
+## Test Environment
+
+- **F# Compiler**: Built from main branch
+- **Compiler Path**: `/home/runner/work/fsharp/fsharp/artifacts/bin/fsc/Release/net10.0/fsc.dll`
+- **FSharp.Core**: `/home/runner/work/fsharp/fsharp/artifacts/bin/FSharp.Core/Release/netstandard2.0/FSharp.Core.dll`
+- **.NET SDK**: 10.0.100-rc.2
+- **Platform**: Linux (Ubuntu)
+
+## Test Project Structure
+
+Each module (`FooN.fs`) contains:
+```fsharp
+namespace TestProject
+
+[<RequireQualifiedAccess>]
+type FooN = Foo of int | Bar
+
+[<RequireQualifiedAccess>]
+module FooN =
+    let foo: FooN = FooN.Bar
+```
+
+## 5000 Module Build - Detailed Analysis
+
+### Build Result
+- **Total Time**: 14 minutes 11 seconds (851.19s)
+- **Configuration**: Release, ParallelCompilation=true
+- **Result**: Build succeeded
+
+### Memory Growth Over Time (Measured Every Minute)
+
+| Elapsed Time | CPU % | Memory % | RSS (MB) |
+|--------------|-------|----------|----------|
+| 1 min | 380% | 6.0% | 969 |
+| 2 min | 387% | 6.5% | 1,050 |
+| 3 min | 336% | 14.2% | 2,287 |
+| 4 min | 286% | 23.7% | 3,805 |
+| 5 min | 255% | 32.1% | 5,144 |
+| 6 min | 234% | 39.5% | 6,331 |
+| 7 min | 218% | 46.9% | 7,513 |
+| 8 min | 207% | 53.5% | 8,561 |
+| 9 min | 197% | 60.4% | 9,664 |
+| 10 min | 189% | 67.1% | 10,746 |
+| 11 min | 183% | 79.6% | 12,748 |
+| 12 min | 175% | 90.4% | 14,473 |
+| 13 min | 165% | 90.6% | 14,498 |
+| 14 min | - | - | Build completed |
+
+### Key Observations From Measured Data
+
+1. **Memory growth is linear**: ~1.1 GB per minute for first 12 minutes
+2. **Peak memory**: 14.5 GB (90.6% of system RAM)
+3. **CPU utilization decreases over time**: 380% → 165%
+4. **Memory plateaus at ~90%**: Possible GC pressure at 12-13 min mark
+
+### Build Log Evidence
+```
+Determining projects to restore...
+  Restored /tmp/perf-testing/fsharp-5000/src/TestProject.fsproj (in 78 ms).
+  TestProject -> /tmp/perf-testing/fsharp-5000/src/bin/Release/net8.0/TestProject.dll
+
+Build succeeded.
+    0 Warning(s)
+    0 Error(s)
+
+Time Elapsed 00:14:11.19
+```
+
+### Compiler Process Evidence
+```
+runner 35321 ... /usr/share/dotnet/dotnet /home/runner/work/fsharp/fsharp/artifacts/bin/fsc/Release/net10.0/fsc.dll @/tmp/MSBuildTempQZbd6p/tmp24fcc0624ca6474f8fc7ddd8ab0874ef.rsp
+```
+
+## Trace Collection
+
+A 2-minute trace was collected during the build using:
+```bash
+dotnet-trace collect --process-id <FSC_PID> --format speedscope --output fsc-trace --duration 00:02:00
+```
+
+Trace file: `fsc-trace` (44,537 bytes)
+Converted to speedscope format: `fsc-trace.speedscope.speedscope.json` (92,797 bytes)
+
+Note: Trace shows high proportion of unmanaged code time, indicating native code execution or JIT compilation overhead.
+
+## Experiment Reproduction Steps
+
+### Using Compiler Timing Output
+
+The F# compiler supports detailed timing output via project properties:
+
+1. **Add to project file** (or pass via command line):
+```xml
+<PropertyGroup>
+  <OtherFlags>--times --times:compilationTiming.csv</OtherFlags>
+</PropertyGroup>
+```
+
+Or via command line:
+```bash
+dotnet build -c Release -p:OtherFlags="--times --times:compilationTiming.csv"
+```
+
+2. **Collect traces and dumps**:
+```bash
+# Start build with tracing
+dotnet-trace collect --output build-trace.nettrace -- dotnet build -c Release -p:OtherFlags="--times --times:compilationTiming.csv"
+
+# Or attach to running fsc.dll process
+FSC_PID=$(pgrep -f fsc.dll)
+dotnet-trace collect -p $FSC_PID --output fsc-trace.nettrace --duration 00:05:00 &
+dotnet-gcdump collect -p $FSC_PID --output heap.gcdump
+
+# For memory dump at specific time (e.g., 15 min mark)
+sleep 900 && dotnet-dump collect -p $FSC_PID --output memory.dmp
+```
+
+3. **Analyze results**:
+```bash
+# Convert trace to speedscope format
+dotnet-trace convert fsc-trace.nettrace --format speedscope
+
+# Report on GC dump
+dotnet-gcdump report heap.gcdump
+
+# Analyze timing CSV
+python3 -c "
+import csv
+with open('compilationTiming.csv') as f:
+    reader = csv.DictReader(f)
+    for row in sorted(reader, key=lambda x: float(x.get('Duration', 0)), reverse=True)[:20]:
+        print(row)
+"
+```
+
+### Validation After Memory Leak Fix
+
+After CI builds the fixed compiler, repeat the 5000-module experiment with:
+
+1. Same build configuration (Release, ParallelCompilation=true)
+2. Same measurement methodology (memory/CPU every minute)
+3. Enable `--times` output for detailed phase breakdown
+4. Collect GC dumps at 1, 5, and 10 minute marks
+5. Compare `ImportILTypeDef@*` closure counts with previous run

docs/perf-analysis/GC_ANALYSIS.md

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+# GC Analysis - Issue #19132
+
+## Overview
+
+GC heap analysis during 5000-module F# project build using `dotnet-gcdump`.
+
+Related Issue: https://github.com/dotnet/fsharp/issues/19132
+
+## Collection Methodology
+
+```bash
+dotnet-gcdump collect -p <FSC_PID> -o <output>.gcdump
+dotnet-gcdump report <gcdump_file>
+```
+
+## GC Dump Timeline
+
+| Dump | Elapsed Time | RSS Memory | GC Heap Size | GC Heap Objects |
+|------|--------------|------------|--------------|-----------------|
+| 1 | ~1 min | 1,058 MB | 361 MB | 10,003,470 |
+| 2 | ~4 min | 3,828 MB | 291 MB | 10,000,000 |
+| 3 | ~10 min | 10,406 MB | 302 MB | 10,000,000 |
+
+## Key Observation
+
+**RSS grows from 1 GB to 10 GB but GC Heap remains ~300 MB**
+
+This indicates memory is being consumed outside the managed GC heap, likely in:
+- Native allocations
+- Memory-mapped files
+- Large Object Heap (LOH) that was collected between dumps
+- Unmanaged code buffers
+
+## GC Dump 1 - Top Types (1 min mark, 1 GB RSS)
+
+```
+361,256,014  GC Heap bytes
+ 10,003,470  GC Heap objects
+
+   Object Bytes     Count  Type
+      2,376,216         1  System.Byte[] (Bytes > 1M)
+        131,096        11  System.Byte[] (Bytes > 100K)
+         80,848         1  VolatileNode<System.String,FSharp.Compiler.Parser+token>[]
+         64,808        16  Entry<FSharp.Compiler.ParseAndCheckInputs+NodeToTypeCheck>[]
+         49,200     1,848  System.Int32[]
+         40,048         1  System.Tuple<FSharp.Compiler.Syntax.ParsedInput,...>[]
+         40,048         1  FSharp.Compiler.Syntax.ParsedInput[]
+         40,048         1  FSharp.Compiler.GraphChecking.FileInProject[]
+```
+
+## GC Dump 2 - Top Types (4 min mark, 3.8 GB RSS)
+
+```
+290,770,269  GC Heap bytes
+ 10,000,000  GC Heap objects
+
+   Object Bytes     Count  Type
+         12,140       824  System.Int32[]
+         11,288     2,237  NodeToTypeCheck[]
+            136       713  TcEnv
+            136       702  NameResolutionEnv
+            136       677  ILTypeDef
+            128     5,671  ValOptionalData
+            128     2,431  Entity
+            120       729  FSharp.Compiler.Syntax.SynBinding
+            112     2,446  ModuleOrNamespaceType
+             96     1,183  EntityOptionalData
+             72    28,691  Val
+```
+
+## GC Dump 3 - Top Types (10 min mark, 10.4 GB RSS)
+
+```
+301,948,795  GC Heap bytes
+ 10,000,000  GC Heap objects
+
+   Object Bytes     Count  Type
+         45,080         1  System.String[]
+         35,200         2  System.UInt16[]
+         15,656     2,258  NodeToTypeCheck[]
+         10,016     1,416  System.Int32[]
+            136     1,225  ILTypeDef
+            136       686  TcEnv
+            136       674  NameResolutionEnv
+            128     5,480  ValOptionalData
+            128     2,941  Entity
+            120     1,144  FSharp.Compiler.Syntax.SynBinding
+            112     2,385  ModuleOrNamespaceType
+             88     5,890  Match
+             88     4,964  Lambda
+             88     2,659  TyconAugmentation
+             72    27,888  Val
+```
+
+## Type Growth Analysis
+
+Comparing object counts between Dump 2 (4 min) and Dump 3 (10 min):
+
+| Type | Dump 2 Count | Dump 3 Count | Change |
+|------|--------------|--------------|--------|
+| Val | 28,691 | 27,888 | -803 |
+| Entity | 2,431 | 2,941 | +510 |
+| ILTypeDef | 677 | 1,225 | +548 |
+| TcEnv | 713 | 686 | -27 |
+| ModuleOrNamespaceType | 2,446 | 2,385 | -61 |
+| Match | 6,052 | 5,890 | -162 |
+| Lambda | 5,100 | 4,964 | -136 |
+| TyconAugmentation | 2,142 | 2,659 | +517 |
+| SynBinding | 729 | 1,144 | +415 |
+| ImportILTypeDef@712 | 674 | 983 | +309 |
+
+## Memory Leak Identified
+
+**ImportILTypeDef@712** (closures from ImportILTypeDef function) showing growth indicates a potential memory leak.
+
+### Root Cause
+In `src/Compiler/Checking/import.fs`, the `ImportILTypeDef` function was storing `tdef.CustomAttrsStored` reference in `AttributesFromIL`, which kept entire `ILTypeDef` objects alive via closure.
+
+### Fix Applied
+Modified `ImportILTypeDef` to:
+1. Added type annotation `(amap: ImportMap)` to fix type inference
+2. Check if nullness features are enabled (`amap.g.langFeatureNullness && amap.g.checkNullness`)
+3. If enabled: immediately read attrs with `tdef.CustomAttrsStored.GetCustomAttrs(tdef.MetadataIndex)` and wrap in `Given`
+4. If disabled: use empty attributes to avoid any reference
+
+This prevents the closure from keeping large `ILTypeDef` objects alive.
+
+### Validation Status
+**Pending**: After CI builds the fixed compiler, the 5000-module experiment should be repeated to verify:
+- Reduced memory growth rate
+- Stable or reduced `ImportILTypeDef@712` closure count
+- Improved CPU utilization over time
+
+## Key Findings
+
+1. **GC Heap is stable at ~300 MB** despite RSS growing to 10+ GB
+2. **ImportILTypeDef closures growing** - memory leak via CustomAttrsStored references
+3. **RSS growth not reflected in GC heap** - indicates native/unmanaged memory consumption
+4. **Val type has 27-28k instances** - F# value definitions
+5. **Entity and ILTypeDef counts grow** - IL metadata accumulation
+6. **TcEnv and NameResolutionEnv** - type checking environments (~700 instances)
+
+## Build Environment
+
+- Compiler: `/home/runner/work/fsharp/fsharp/artifacts/bin/fsc/Release/net10.0/fsc.dll`
+- Configuration: Release, ParallelCompilation=true
+- Modules: 5000
+- Platform: Linux (Ubuntu)

docs/perf-analysis/HOT_PATHS.md

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+# Hot Paths Analysis - Issue #19132
+
+## Overview
+
+This document contains hot path analysis from performance traces collected during large F# project builds.
+
+Related Issue: https://github.com/dotnet/fsharp/issues/19132
+
+## Trace Collection
+
+### Method
+```bash
+dotnet-trace collect --process-id <FSC_PID> --format speedscope --output fsc-trace --duration 00:02:00
+```
+
+### Collected Trace
+- **Process**: fsc.dll (PID 35798)
+- **Duration**: 2 minutes (captured during 5000-module build)
+- **Trace file**: `fsc-trace` (44,537 bytes)
+- **Speedscope file**: `fsc-trace.speedscope.speedscope.json` (92,797 bytes)
+
+## Trace Analysis Results
+
+### Thread Activity
+The trace captured 28 active threads:
+- Thread 35798 (main)
+- Threads 35810-35838 (worker threads)
+
+### Time Distribution
+From speedscope conversion:
+- Main thread (35798): 17.88ms - 267.12ms captured
+- High proportion of `UNMANAGED_CODE_TIME` frames
+- Many `?!?` (unresolved) stack frames
+
+### Observations
+1. **Unresolved symbols**: Many stack frames show as "?!?" indicating native code or missing debug symbols
+2. **Multi-threaded**: 28 threads active confirms ParallelCompilation is engaged
+3. **Unmanaged code**: Significant time in unmanaged code (possibly JIT, GC, or native runtime)
+
+### Limitations
+- Trace symbols not fully resolved
+- 2-minute sample may not capture all phases
+- Native/unmanaged code not fully visible
+
+## Raw Trace Data
+
+### Speedscope Frame Names (from trace)
+```
+"Process64 Process(35798) (35798) Args: "
+"(Non-Activities)"
+"Threads"
+"Thread (35798)"
+"?!?"
+"UNMANAGED_CODE_TIME"
+"Thread (35810)" ... "Thread (35838)"
+"CPU_TIME"
+```
+
+## Next Steps for Deeper Analysis
+1. Build with debug symbols for better stack resolution
+2. Use longer trace duration to capture full build
+3. Consider using PerfView or dotnet-gcdump for memory analysis