The CompCert verified compiler

Commented Coq development

Version 3.7, 2020-03-31

PATCHED for the Kalray MPPA-KVX VLIW CORE

Introduction

This web page is a patched version of the table of contents of the official CompCert documentation, as given on the CompCert Web site. The unmodified parts of this table appear in gray.

A high-level view of this backend of CompCert is provided by this HAL preprint of Six, Boulmé and Monniaux (2019):

Certified Compiler Backends for VLIW Processors (Highly Modular Postpass-Scheduling in the CompCert Certified Compiler)

Our source code is available on our GitLab public repository (see conditions in the LICENSE file).

General-purpose libraries, data structures and algorithms

Coqlib: addendum to the Coq standard library.
Maps: finite maps.
Integers: machine integers.
Floats: machine floating-point numbers.
Iteration: various forms of "while" loops.
Ordered: construction of ordered types.
Lattice: construction of semi-lattices.
Kildall: resolution of dataflow inequations by fixpoint iteration.
UnionFind: a persistent union-find data structure.
Postorder: postorder numbering of a directed graph.

The `abstractbb` library, introduced for KVX core

AbstractBasicBlocksDef: an IR for verifying some semantic properties on basic-blocks.
Parallelizability: verifying that sequential and parallel semantics are equivalent for a given abstract basic-block.
ImpSimuTest: verifying that a given abstract basic-block is simulated by another one for sequential semantics. This module refines SeqSimuTheory with hash-consing and uses the Impure library to reason on physical equality and handling of imperative code in Coq.

Definitions and theorems used in many parts of the development

Errors: the Error monad.
AST: identifiers, whole programs and other common elements of abstract syntaxes.
Linking: generic framework to define syntactic linking over the CompCert languages.
Values: run-time values.
Events: observable events and traces.
Memory: memory model.
See also: Memdata (in-memory representation of data).
Globalenvs: global execution environments.
Smallstep: tools for small-step semantics.
Behaviors: from small-step semantics to observable behaviors of programs.
Determinism: determinism properties of small-step semantics.
Op: operators, addressing modes and their semantics.
Unityping: a solver for atomic unification constraints.

Source, intermediate and target languages: syntax and semantics

The CompCert C source language: syntax and semantics and determinized semantics and type system.
See also: type expressions and operators (syntax and semantics).
See also: reference interpreter.
Clight: a simpler version of CompCert C where expressions contain no side-effects.
Csharpminor: low-level structured language.
Cminor: low-level structured language, with explicit stack allocation of certain local variables.
CminorSel: like Cminor, with machine-specific operators and addressing modes.
RTL: register transfer language (3-address code, control-flow graph, infinitely many pseudo-registers).
See also: Registers (representation of pseudo-registers).
LTL: location transfer language (3-address code, control-flow graph of basic blocks, finitely many physical registers, infinitely many stack slots).
See also: Locations (representation of locations) and Machregs (description of processor registers).
Linear: like LTL, but the CFG is replaced by a linear list of instructions with explicit branches and labels.
Mach: like Linear, with a more concrete view of the activation record.

Languages introduced for KVX core

Machblock: a variant of Mach, with a syntax for basic-blocks, and a block-step semantics (execute one basic-block in one step). This IR is generic over the processor, even if currently, only used for KVX.
Asmvliw: abstract syntax and semantics for KVX VLIW assembly: atomic instructions are grouped by "bundles". These bundles are executed sequentially, but execution is parallel within bundles.
Asmblock: a variant of Asmvliw, with a sequential semantics within bundles, which make them corresponds here to usual basic-blocks. This IR is an intermediate step between Machblock and Asmvliw.
Asm: a variant of Asmvliw with a flat syntax for bundles, instead of a structured one (bundle termination is encoded as a pseudo-instruction). This IR is mainly a wrapper of Asmvliw for a smooth integration in CompCert (and an easier pretty-printing of the abstract syntax).

Compiler passes

Pass	Source & target	Compiler code	Correctness proof
Pulling side-effects out of expressions; fixing an evaluation order	CompCert C to Clight	SimplExpr	SimplExprspec SimplExprproof
Pulling non-adressable scalar local variables out of memory	Clight to Clight	SimplLocals	SimplLocalsproof
Simplification of control structures; explication of type-dependent computations	Clight to Csharpminor	Cshmgen	Cshmgenproof
Stack allocation of local variables whose address is taken; simplification of switch statements	Csharpminor to Cminor	Cminorgen	Cminorgenproof
Recognition of operators and addressing modes	Cminor to CminorSel	Selection SelectOp SelectLong SelectDiv SplitLong	Selectionproof SelectOpproof SelectLongproof SelectDivproof SplitLongproof
Construction of the CFG, 3-address code generation	CminorSel to RTL	RTLgen	RTLgenspec RTLgenproof
Recognition of tail calls	RTL to RTL	Tailcall	Tailcallproof
Function inlining	RTL to RTL	Inlining	Inliningspec Inliningproof
Postorder renumbering of the CFG	RTL to RTL	Renumber	Renumberproof
Constant propagation	RTL to RTL	Constprop ConstpropOp	Constpropproof ConstproppOproof
Common subexpression elimination	RTL to RTL	CSE CombineOp	CSEproof CombineOpproof
Redundancy elimination	RTL to RTL	Deadcode	Deadcodeproof
Removal of unused static globals	RTL to RTL	Unusedglob	Unusedglobproof
Register allocation (validation a posteriori)	RTL to LTL	Allocation	Allocproof
Branch tunneling	LTL to LTL	Tunneling	Tunnelingproof
Linearization of the CFG	LTL to Linear	Linearize	Linearizeproof
Removal of unreferenced labels	Linear to Linear	CleanupLabels	CleanupLabelsproof
Synthesis of debugging information	Linear to Linear	Debugvar	Debugvarproof
Laying out the activation records	Linear to Mach	Stacking Bounds Stacklayout	Stackingproof Separation

Compilation passes introduced for KVX VLIW

Reconstruction of basic-blocks at Mach level	Mach to Machblock	Machblockgen	ForwardSimulationBlock Machblockgenproof
Emission of purely sequential assembly code	Machblock to Asmblock	Asmblockgen	Asmblockgenproof0 Asmblockgenproof1 Asmblockgenproof
Bundling (and basic-block scheduling)	Asmblock to Asmvliw	PostpassScheduling using Asmblockdeps and the `abstractbb` library	PostpassSchedulingproof
Flattening bundles (only a bureaucratic operation)	Asmvliw to Asm	Asmgen	Asmgenproof (whole simulation proof from `Mach` to `Asm`)

All together

Compiler: composing the passes together; whole-compiler semantic preservation theorems.
Complements: interesting consequences of the semantic preservation theorems.

Static analyses

The following static analyses are performed over the RTL intermediate representation to support optimizations such as constant propagation, CSE, and dead code elimination.

Liveness: liveness analysis.
ValueAnalysis: value and alias analysis
See also: ValueDomain: the abstract domain for value analysis.
See also: ValueAOp: processor-dependent parts of value analysis.
Deadcode: neededness analysis
See also: NeedDomain: the abstract domain for neededness analysis.
See also: NeedOp: processor-dependent parts of neededness analysis.

Type systems

The type system of CompCert C is fully formalized. For some intermediate languages of the back-end, simpler type systems are used to statically capture well-formedness conditions.

Ctyping: typing for CompCert C + type-checking functions.
RTLtyping: typing for RTL + type reconstruction.
Lineartyping: typing for Linear.