[IR Refactor] Phase 1: IrContainer/IrStorage Composition Pattern

[mdavis36]

Summary

Refactor nvFuser's IR container classes from inheritance to composition pattern by splitting IrContainer into an interface class and IrStorage data container. This is Phase 1 of the scalar unification effort to enable Host IR JIT compilation.

Architectural Changes

Motivation

The original IrContainer was a monolithic class that mixed interface responsibilities with data storage. This refactor:

• Separates concerns: Interface (IrContainer) vs. Data (IrStorage)
• Enables composition: IrContainer owns IrStorage via unique_ptr
• Prepares for Phase 2: Foundation for shared_ptr-based IrStorage sharing
• Maintains compatibility: Fusion still inherits from IrContainer

Before: Inheritance Pattern

class IrContainer {
  // Mixed: interface methods + data storage
  std::deque<std::unique_ptr<Val>> vals_up_;
  std::deque<std::unique_ptr<Expr>> exprs_up_;
  std::unordered_set<Val*> vals_;
  std::unordered_set<Expr*> exprs_;
  // ... ~20 data members
};

class Fusion : public IrContainer {
  // Fusion-specific logic + inherits all data
};

After: Composition Pattern

// IrStorage: Pure data container
class IrStorage {
  std::deque<std::unique_ptr<Val>> vals_up_;
  std::deque<std::unique_ptr<Expr>> exprs_up_;
  std::unordered_set<Val*> vals_;
  std::unordered_set<Expr*> exprs_;
  IrContainer* parent_;  // Back-pointer to owner
};

// IrContainer: Interface that forwards to IrStorage
class IrContainer : public PolymorphicBase {
 protected:
  std::unique_ptr<IrStorage> ir_storage_;  // Owns data

  // Forwards ~20 methods to ir_storage_
  const std::unordered_set<Val*>& vals() const {
    return ir_storage_->vals();
  }
};

// Fusion: Inherits interface, data managed by IrStorage
class Fusion : public IrContainer {
  // Fusion-specific logic only
};

Key Design Decisions

1. Hybrid Pattern (Inheritance + Composition)

• Fusion inherits from IrContainer (maintains IS-A relationship)
• IrContainer owns IrStorage (composition for data)
• Preserves backward compatibility while enabling future flexibility

2. Two-Level Pointer Management

IrContainer ←─ parent_ ─── IrStorage
     ↑                         ↑
     └── ir_container_ ────────┘
              Statement

• IrStorage::parent_ - Points to owning IrContainer
• Statement::ir_container_ - Points to container (for traversal)
• Both must stay consistent during operations (swap, copy, move)

3. Method Forwarding
IrContainer acts as a facade, forwarding all operations to IrStorage:

• Keeps external API unchanged
• Encapsulates storage implementation
• Enables future IrStorage sharing without API changes

4. Passkey Access Control
Uses passkey pattern to protect internal pointer updates:

void Statement::setContainer(IrContainerPasskey, IrContainer* container);

Only IrContainer can update Statement pointers, maintaining invariants.

Changes

New Files

• csrc/ir/storage.h - IrStorage class definition (236 lines)
• csrc/ir/storage.cpp - IrStorage implementation (366 lines)

Modified Files

• csrc/ir/container.h - Converted to IrContainer interface class
• csrc/ir/container.cpp - Simplified to forwarding + swap logic
• csrc/ir/base_nodes.h - Added Statement::setContainer()
• csrc/fusion.h, csrc/fusion.cpp - Updated for composition
• csrc/ir/builder_passkey.h - Passkey updates
• CMakeLists.txt - Added new storage files

Statistics

• 9 files changed: +814 insertions, -546 deletions
• 43 commits of iterative development
• Net result: Cleaner separation of concerns

Testing

Smoke Tests: ✅ 34/34 passing (100%)

• AbstractTensorTest: 28/28 ✅ - Statement registration
• FusionHash: 3/3 ✅ - Fusion hashing
• FusionClear: 1/1 ✅ - Container lifecycle
• FusionMove: 1/1 ✅ - Move semantics
• FusionCopy: 1/1 ✅ - Copy semantics

Validation: 15 test runs, 64 total executions, no flakiness

Backward Compatibility

✅ Fully backward compatible:

• Fusion still inherits from IrContainer (IS-A relationship preserved)
• All existing APIs work unchanged (forwarding maintains signatures)
• Polymorphic usage patterns continue to work
• No breaking changes to external code

Migration Path

This PR is Phase 1 of a multi-phase refactor:

Phase 1 (This PR): IrContainer owns IrStorage via unique_ptr

• ✅ Achieves composition pattern
• ✅ Maintains backward compatibility
• ✅ Foundation established

Phase 2 (Future): Convert to shared_ptr

class IrContainer {
  std::shared_ptr<IrStorage> ir_storage_;  // Enable sharing
};

• Multiple containers can share same IrStorage
• Enables scalar value unification

Phase 3 (Future): Scalar Unification

• Share scalar values across Fusion boundaries
• Enable Host IR JIT compilation
• Complete original goal

[greptile-apps]

Greptile Summary

Refactors IrContainer from monolithic inheritance to composition pattern by extracting data storage into new IrStorage class, establishing foundation for Phase 2 shared pointer-based storage sharing.

Key Changes

• New IrStorage class (storage.h/cpp): Pure data container managing vals/exprs with deleted copy/move constructors, accessed only through unique_ptr in IrContainer
• IrContainer becomes forwarding interface: Reduced from ~400 lines to ~60 lines, all ~20 public methods now forward to owned IrStorage instance
• Pointer consistency maintained: Two-level pointer system (IrStorage::parent_ and Statement::ir_container_) kept synchronized through careful swap/copy operations
• Full backward compatibility: Fusion continues inheriting from IrContainer, all external APIs unchanged

Architecture Improvements

The refactor successfully separates concerns while maintaining the existing API surface. The IrContainer::swap() implementation (container.cpp:22-49) correctly handles the critical pointer fixup after swapping storage contents, updating both parent pointers and all Statement container pointers. The IrStorage::copy() method (storage.cpp:88-121) properly initializes IrCloner with to->parent(), avoiding the nullptr issue from the previous thread.

All 34 smoke tests passing with no flakiness demonstrates the refactor maintains existing semantics correctly.

Confidence Score: 5/5

• This PR is safe to merge - well-structured refactoring with comprehensive testing and no breaking changes
• The refactoring demonstrates excellent software engineering: clean separation of concerns, proper encapsulation (deleted copy/move on IrStorage), careful pointer management in swap/copy operations, and complete backward compatibility. The 34/34 passing smoke tests with 64 executions and no flakiness provide strong validation. Code reduction (400+ lines to 60 in container.cpp) indicates successful simplification without loss of functionality
• No files require special attention - all changes are well-structured and properly integrated

Important Files Changed

Filename	Overview
csrc/ir/storage.h	New file defining IrStorage data container class with proper encapsulation - copy/move deleted, parent pointer properly managed
csrc/ir/storage.cpp	Implementation of IrStorage with copy/swap semantics and proper parent pointer maintenance throughout operations
csrc/ir/container.h	Refactored to forwarding interface pattern - all data moved to IrStorage, proper passkey access control maintained
csrc/ir/container.cpp	Reduced from 400+ lines to ~60 lines - now handles pointer fixup in swap and delegates to IrStorage for operations
csrc/fusion.cpp	Updated to call IrContainer::swap instead of direct member swap, maintains consistent pointer management

Sequence Diagram

sequenceDiagram
    participant User
    participant Fusion
    participant IrContainer
    participant IrStorage
    participant Statement
    
    Note over Fusion,IrStorage: Construction Pattern
    User->>Fusion: new Fusion()
    Fusion->>IrContainer: IrContainer()
    IrContainer->>IrStorage: new IrStorage()
    IrContainer->>IrStorage: parent_ = this
    
    Note over Fusion,Statement: Registration Flow
    User->>Fusion: registerVal(val)
    Fusion->>IrContainer: registerVal(val)
    IrContainer->>IrStorage: registerVal(val)
    IrStorage->>IrStorage: vals_up_.emplace_back(val)
    IrStorage->>IrStorage: vals_.insert(val)
    IrStorage->>Statement: val->setName()
    
    Note over Fusion,IrStorage: Copy Operation
    User->>Fusion: Fusion(other)
    Fusion->>IrContainer: copy(from, to)
    IrContainer->>IrStorage: IrStorage::copy(from, to)
    IrStorage->>IrStorage: to->clear()
    IrStorage->>IrStorage: IrCloner(to->parent())
    IrStorage-->>IrContainer: return ir_cloner
    
    Note over Fusion,Statement: Swap Operation (Critical)
    User->>Fusion: swap(a, b)
    Fusion->>IrContainer: IrContainer::swap(a, b)
    IrContainer->>IrStorage: IrStorage::swap(*a.ir_storage(), *b.ir_storage())
    Note over IrStorage: Swaps data members including parent_
    IrContainer->>IrStorage: a.ir_storage()->parent_ = &a
    IrContainer->>Statement: val->ir_container_ = &a (for all vals)
    IrContainer->>Statement: expr->ir_container_ = &a (for all exprs)
    IrContainer->>IrStorage: b.ir_storage()->parent_ = &b
    IrContainer->>Statement: val->ir_container_ = &b (for all vals)
    IrContainer->>Statement: expr->ir_container_ = &b (for all exprs)

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[github-actions]

Review updated until commit ee2e2a9

Description

• Refactor IrContainer from inheritance to composition pattern with IrStorage data container

• Split monolithic IrContainer into interface class (IrContainer) and data container (IrStorage)

• Move all data members and implementation logic from IrContainer to new IrStorage class

• Update Fusion and build system to work with new composition-based architecture

Changes walkthrough

	Relevant files
Enhancement	7 files storage.h Define IrStorage data container class                                        +198/-0  storage.cpp Implement IrStorage data container methods                              +391/-0  container.h Refactor IrContainer as composition interface                        +162/-162 container.cpp Update IrContainer to forward to IrStorage                              +32/-370 fusion.cpp Update Fusion for composition pattern                                        +17/-16  fusion.h Update Fusion swap function declaration                                    +1/-1      base_nodes.h Add IrStorage friend declarations                                                +3/-0
Configuration changes	1 files CMakeLists.txt Add storage.cpp to build system                                                    +1/-0
Additional files	1 files builder_passkey.h +0/-2

PR Reviewer Guide

Here are some key observations to aid the review process:

🧪 PR contains tests

⚡ Recommended focus areas for review

Parent Pointer Management The swap function has complex logic to update parent pointers and Statement::ir_container_ pointers after swapping. This is critical for maintaining correct container relationships. The logic looks correct but should be carefully reviewed to ensure all edge cases are handled, especially when one or both containers might be in an invalid state. void IrContainer::swap(IrContainer& a, IrContainer& b) noexcept { // We need to be careful to call IrStorage swap not unique_ptr swap, which // will only swap the ptrs NOT the contents. IrStorage::swap(*(a.ir_storage()), *(b.ir_storage())); // Fix parent pointers after swapping containers // After swap, each IrContainer owns a different IrStorage, so we must update // the parent backpointers in those containers to point to their new owners if (a.ir_storage_) { a.ir_storage()->parent_ = &a; // Also update all Statement ir_container_ pointers to point to new owner for (auto val : a.vals()) { val->ir_container_ = &a; } for (auto expr : a.deterministic_exprs()) { expr->ir_container_ = &a; } } if (b.ir_storage_) { b.ir_storage()->parent_ = &b; // Also update all Statement ir_container_ pointers to point to new owner for (auto val : b.vals()) { val->ir_container_ = &b; } for (auto expr : b.deterministic_exprs()) { expr->ir_container_ = &b; } } } Clear Method Behavior Change The clear() method now calls ir_storage()->clear() instead of IrContainer::clear(). This changes the behavior from clearing the entire container to just clearing the storage contents. The comment explains this preserves the container object to avoid dangling Statement pointers, but this behavioral change should be validated to ensure it doesn't break existing functionality. void Fusion::clear() noexcept { // Perf scope isn't safe here as this function could be called by // the Fusion destructor and the scope initializer could call the // constructor of Trace, which could throw an exception. // FUSER_PERF_SCOPE("Fusion clear"); // Clear container contents instead of destroying it // This preserves the container object so Statement pointers don't become // dangling ir_storage()->clear(); Parent Pointer Validation The parent_ pointer is used extensively throughout IrStorage methods (like in inContainer(), zeroVal(), etc.) but there's an NVF_ERROR check only in the parent() getter method. Consider if parent_ pointer validity should be checked more broadly, especially in methods that could be called before the parent is properly set. IrContainer* parent() const { NVF_ERROR( parent_ != nullptr, "Call to IrContainer::parent() holds nullptr.") return parent_; } private: // Parent IrInterface that owns this container (for pure composition pattern) // Used by Statement::fusion() to navigate back to owning Fusion IrContainer* parent_ = nullptr; };

Test failures

• (High, 1) nvFuser internal assert crash in Thunder grad consistency test (H100 runner)

  Test Name	H100	Source
  thunder.tests.test_grad.test_phantom_grad_vs_torch_consistency_outer_nvfuser_cuda_thunder.dtypes.float64	❌

• (Medium, 1) Thunder NVFuser GPT autograd produces zero output (thunder.tests.test_networks)

  Test Name	H100	Source
  thunder.tests.test_networks.test_nanogpt_complete_autograd_nvfuser_cuda_thunder.dtypes.float32	❌

[mdavis36]

!test

[mdavis36]

!test

[wujingyue]

Why introduce IrStorage as an extra level of indirection? Is it mainly for migration convenience so we can still remove it after updating all callsites to ir_container_->foo()?

[mdavis36]

Why introduce IrStorage as an extra level of indirection? Is it mainly for migration convenience so we can still remove it after updating all callsites to ir_container_->foo()?

@wujingyue essentially yes, the semantics of how Fusion type containers are copied and moved is a little cumbersome as they largely rely on copy/swap functions that call nested copy/swap implementation. This should probably be revisited in the near future to bake that behavior directly into the class constructors and operators see comment. Having IrContainer work as a thin interface in the interim allows:

• More clarity about when containers are copied/swapped.
• Dedicated place for updating the backpointers properly.
• Keeps the actual implementation of IrStorage (formerly IrContainer) essentially the same.

My hope is when we move to a shared_ptr implementation, having the interface layer will help with clearly defining the semantics.

[greptile-apps]

_{8 files reviewed, 2 comments}

_{Edit Code Review Agent Settings | Greptile}

[mdavis36]

!test

[mdavis36]

!test

[wujingyue]

Is this temporary? Later when we make IrStorage a shared pointer there can be multiple parents.

[mdavis36]

Probably, a lot of functionality here requires referencing the owning Fusion type to propagate it through IrCloner and Statements. I'm not sure how this will look yet with shared_ptr. I imagine much of the functionality in here may have to be moved up to the Fusion/IrContainer level or the relationship of Statement -> IrContainer will need to be changed to Statement -> IrStorage.

[wujingyue]

Suggested change

	std::unique_ptr<std::vector<Val*>> axioms_;
	std::vector<Val*> axioms_;

unique_ptr can usually be avoided for STL containers which have good move operators.

[mdavis36]

I'll address this in a follow up PR, this will be modifying implementation too, which I'm hoping to minimize for this.

[mdavis36]

!test