Cretonne compared to LLVM

LLVM is a collection of compiler components implemented as a set of C++ libraries. It can be used to build both JIT compilers and static compilers like Clang, and it is deservedly very popular. Chris Lattner’s chapter about LLVM in the Architecture of Open Source Applications book gives an excellent overview of the architecture and design of LLVM.

Cretonne and LLVM are superficially similar projects, so it is worth highlighting some of the differences and similarities. Both projects:

  • Use an ISA-agnostic input language in order to mostly abstract away the differences between target instruction set architectures.
  • Depend extensively on SSA form.
  • Have both textual and in-memory forms of their primary intermediate language. (LLVM also has a binary bitcode format; Cretonne doesn’t.)
  • Can target multiple ISAs.
  • Can cross-compile by default without rebuilding the code generator.

Cretonne’s scope is much smaller than that of LLVM. The classical three main parts of a compiler are:

  1. The language-dependent front end parses and type-checks the input program.
  2. Common optimizations that are independent of both the input language and the target ISA.
  3. The code generator which depends strongly on the target ISA.

LLVM provides both common optimizations and a code generator. Cretonne only provides the last part, the code generator. LLVM additionally provides infrastructure for building assemblers and disassemblers. Cretonne does not handle assembly at all—it only generates binary machine code.

Intermediate representations

LLVM uses multiple intermediate representations as it translates a program to binary machine code:

LLVM IR

This is the primary intermediate language which has textual, binary, and in-memory representations. It serves two main purposes:

  • An ISA-agnostic, stable(ish) input language that front ends can generate easily.
  • Intermediate representation for common mid-level optimizations. A large library of code analysis and transformation passes operate on LLVM IR.
SelectionDAG

A graph-based representation of the code in a single basic block is used by the instruction selector. It has both ISA-agnostic and ISA-specific opcodes. These main passes are run on the SelectionDAG representation:

  • Type legalization eliminates all value types that don’t have a representation in the target ISA registers.
  • Operation legalization eliminates all opcodes that can’t be mapped to target ISA instructions.
  • DAG-combine cleans up redundant code after the legalization passes.
  • Instruction selection translates ISA-agnostic expressions to ISA-specific instructions.

The SelectionDAG representation automatically eliminates common subexpressions and dead code.

MachineInstr

A linear representation of ISA-specific instructions that initially is in SSA form, but it can also represent non-SSA form during and after register allocation. Many low-level optimizations run on MI code. The most important passes are:

  • Scheduling.
  • Register allocation.
MC

MC serves as the output abstraction layer and is the basis for LLVM’s integrated assembler. It is used for:

  • Branch relaxation.
  • Emitting assembly or binary object code.
  • Assemblers.
  • Disassemblers.

There is an ongoing “global instruction selection” project to replace the SelectionDAG representation with ISA-agnostic opcodes on the MachineInstr representation. Some target ISAs have a fast instruction selector that can translate simple code directly to MachineInstrs, bypassing SelectionDAG when possible.

Cretonne uses a single intermediate language to cover these levels of abstraction. This is possible in part because of Cretonne’s smaller scope.

  • Cretonne does not provide assemblers and disassemblers, so it is not necessary to be able to represent every weird instruction in an ISA. Only those instructions that the code generator emits have a representation.
  • Cretonne’s opcodes are ISA-agnostic, but after legalization / instruction selection, each instruction is annotated with an ISA-specific encoding which represents a native instruction.
  • SSA form is preserved throughout. After register allocation, each SSA value is annotated with an assigned ISA register or stack slot.

The Cretonne intermediate language is similar to LLVM IR, but at a slightly lower level of abstraction.

Program structure

In LLVM IR, the largest representable unit is the module which corresponds more or less to a C translation unit. It is a collection of functions and global variables that may contain references to external symbols too.

In Cretonne IL, the largest representable unit is the function. This is so that functions can easily be compiled in parallel without worrying about references to shared data structures. Cretonne does not have any inter-procedural optimizations like inlining.

An LLVM IR function is a graph of basic blocks. A Cretonne IL function is a graph of extended basic blocks that may contain internal branch instructions. The main difference is that an LLVM conditional branch instruction has two target basic blocks—a true and a false edge. A Cretonne branch instruction only has a single target and falls through to the next instruction when its condition is false. The Cretonne representation is closer to how machine code works; LLVM’s representation is more abstract.

LLVM uses phi instructions in its SSA representation. Cretonne passes arguments to EBBs instead. The two representations are equivalent, but the EBB arguments are better suited to handle EBBs that may contain multiple branches to the same destination block with different arguments. Passing arguments to an EBB looks a lot like passing arguments to a function call, and the register allocator treats them very similarly. Arguments are assigned to registers or stack locations.

Value types

Cretonne’s type system is mostly a subset of LLVM’s type system. It is less abstract and closer to the types that common ISA registers can hold.

  • Integer types are limited to powers of two from i8 to i64. LLVM can represent integer types of arbitrary bit width.
  • Floating point types are limited to f32 and f64 which is what WebAssembly provides. It is possible that 16-bit and 128-bit types will be added in the future.
  • Addresses are represented as integers—There are no Cretonne pointer types. LLVM currently has rich pointer types that include the pointee type. It may move to a simpler ‘address’ type in the future. Cretonne may add a single address type too.
  • SIMD vector types are limited to a power-of-two number of vector lanes up to 256. LLVM allows an arbitrary number of SIMD lanes.
  • Cretonne has no aggregate types. LLVM has named and anonymous struct types as well as array types.

Cretonne has multiple boolean types, whereas LLVM simply uses i1. The sized Cretonne boolean types are used to represent SIMD vector masks like b32x4 where each lane is either all 0 or all 1 bits.

Cretonne instructions and function calls can return multiple result values. LLVM instead models this by returning a single value of an aggregate type.

Instruction set

LLVM has a small well-defined basic instruction set and a large number of intrinsics, some of which are ISA-specific. Cretonne has a larger instruction set and no intrinsics. Some Cretonne instructions are ISA-specific.

Since Cretonne instructions are used all the way until the binary machine code is emitted, there are opcodes for every native instruction that can be generated. There is a lot of overlap between different ISAs, so for example the iadd_imm instruction is used by every ISA that can add an immediate integer to a register. A simple RISC ISA like RISC-V can be defined with only shared instructions, while an Intel ISA needs a number of specific instructions to model addressing modes.

Undefined behavior

Cretonne does not generally exploit undefined behavior in its optimizations. LLVM’s mid-level optimizations do, but it should be noted that LLVM’s low-level code generator rarely needs to make use of undefined behavior either.

LLVM provides nsw and nuw flags for its arithmetic that invoke undefined behavior on overflow. Cretonne does not provide this functionality. Its arithmetic instructions either produce a value or a trap.

LLVM has an unreachable instruction which is used to indicate impossible code paths. Cretonne only has an explicit trap instruction.

Cretonne does make assumptions about aliasing. For example, it assumes that it has full control of the stack objects in a function, and that they can only be modified by function calls if their address have escaped. It is quite likely that Cretonne will admit more detailed aliasing annotations on load/store instructions in the future. When these annotations are incorrect, undefined behavior ensues.