Fix FOH for SOS (#76586)

The issue that FOH reported sizeof(ObjHeader)-bytes less than needed in UpdateFrozenSegment. It used to report the actual end of the last object while it's expected to be actual end + sizeof(ObjHeader) (for next object)

src/coreclr/vm/frozenobjectheap.cpp

@@ -126,7 +126,7 @@ FrozenObjectSegment::FrozenObjectSegment():
     }
 
     m_pStart = static_cast<uint8_t*>(committedAlloc);
-    m_pCurrent = m_pStart;
+    m_pCurrent = m_pStart + sizeof(ObjHeader);
     m_SizeCommitted = si.ibCommit;
     INDEBUG(m_ObjectsCount = 0);
     return;
@@ -136,18 +136,27 @@ Object* FrozenObjectSegment::TryAllocateObject(PTR_MethodTable type, size_t obje
 {
     _ASSERT(m_pStart != nullptr && FOH_SEGMENT_SIZE > 0 && m_SegmentHandle != nullptr); // Expected to be inited
     _ASSERT(IS_ALIGNED(m_pCurrent, DATA_ALIGNMENT));
+    _ASSERT(objectSize <= FOH_COMMIT_SIZE);
+    _ASSERT(m_pCurrent >= m_pStart + sizeof(ObjHeader));
 
-    uint8_t* obj = m_pCurrent;
-    if (reinterpret_cast<size_t>(m_pStart + FOH_SEGMENT_SIZE) < reinterpret_cast<size_t>(obj + objectSize))
+    const size_t spaceUsed = (size_t)(m_pCurrent - m_pStart);
+    const size_t spaceLeft = FOH_SEGMENT_SIZE - spaceUsed;
+
+    _ASSERT(spaceUsed >= sizeof(ObjHeader));
+    _ASSERT(spaceLeft >= sizeof(ObjHeader));
+
+    // Test if we have a room for the given object (including extra sizeof(ObjHeader) for next object)
+    if (spaceLeft - sizeof(ObjHeader) < objectSize)
     {
-        // Segment is full
         return nullptr;
     }
 
     // Check if we need to commit a new chunk
-    if (reinterpret_cast<size_t>(m_pStart + m_SizeCommitted) < reinterpret_cast<size_t>(obj + objectSize))
+    if (spaceUsed + objectSize + sizeof(ObjHeader) > m_SizeCommitted)
     {
+        // Make sure we don't go out of bounds during this commit
         _ASSERT(m_SizeCommitted + FOH_COMMIT_SIZE <= FOH_SEGMENT_SIZE);
+
         if (ClrVirtualAlloc(m_pStart + m_SizeCommitted, FOH_COMMIT_SIZE, MEM_COMMIT, PAGE_READWRITE) == nullptr)
         {
             ClrVirtualFree(m_pStart, 0, MEM_RELEASE);
@@ -158,11 +167,11 @@ Object* FrozenObjectSegment::TryAllocateObject(PTR_MethodTable type, size_t obje
 
     INDEBUG(m_ObjectsCount++);
 
-    m_pCurrent = obj + objectSize;
-
-    Object* object = reinterpret_cast<Object*>(obj + sizeof(ObjHeader));
+    Object* object = reinterpret_cast<Object*>(m_pCurrent);
     object->SetMethodTable(type);
 
+    m_pCurrent += objectSize;
+
     // Notify GC that we bumped the pointer and, probably, committed more memory in the reserved part
     GCHeapUtilities::GetGCHeap()->UpdateFrozenSegment(m_SegmentHandle, m_pCurrent, m_pStart + m_SizeCommitted);
 

src/coreclr/vm/frozenobjectheap.h

@@ -43,9 +43,20 @@ class FrozenObjectSegment
     Object* TryAllocateObject(PTR_MethodTable type, size_t objectSize);
 
 private:
+    // Start of the reserved memory, the first object starts at "m_pStart + sizeof(ObjHeader)" (its pMT)
     uint8_t* m_pStart;
+
+    // Pointer to the end of the current segment, ready to be used as a pMT for a new object
+    // meaning that "m_pCurrent - sizeof(ObjHeader)" is the actual start of the new object (header).
+    //
+    // m_pCurrent <= m_SizeCommitted
     uint8_t* m_pCurrent;
+
+    // Memory committed in the current segment
+    //
+    // m_SizeCommitted <= m_pStart + FOH_SIZE_RESERVED
     size_t m_SizeCommitted;
+
     segment_handle m_SegmentHandle;
     INDEBUG(size_t m_ObjectsCount);
 };