Release 2.00, 2013-06-21 ======================== Major improvements: - Support for C types "long long" and "unsigned long long", that is, 64-bit integers. Regarding arithmetic operations on 64-bit integers, . simple operations are expanded in-line and proved correct; . more complex operations (division, modulus, conversions to/from floats) call into library functions written in assembly, heavily tested but not yet proved correct. - The register allocator was completely rewritten to use an "end-to-end" translation validation approach, using a validation algorithm described in the paper "Validating register allocation and spilling" by Silvain Rideau and Xavier Leroy, Compiler Construction 2010, http://dx.doi.org/10.1007/978-3-642-11970-5_13 This validation-based approach enables better register allocation, esp: . live-range splitting is implemented . two-address operations are treated more efficiently . no need to reserve processor registers for spilling and reloading. The improvements in quality of generated code is significant for IA32 (because of its paucity of registers) but less so for ARM and PowerPC. - Preliminary support for debugging information. The "-g" flag causes DWARF debugging information to be generated for line numbers and stack structure (Call Frame Information). With a debugger like GDB, this makes it possible to step through the code, put breakpoints by line number, and print stack backtraces. However, no information is generated yet for C type definitions nor for variables; therefore, it is not possible to print the values of variables. Improvements in ABI conformance: - For IA32 and ARM, function arguments of type "float" (single-precision FP) were incorrectly passed as "double". - For PowerPC, fixed alignment of "double" and "long long" arguments passed on stack. Improvements in code generation: - More aggressive common subexpression elimination across some builtin function calls, esp. annotations. Improvements in compiler usability: - Option -fno-taillcalls to turn off tail-call elimination. (Some static analysis tools are confused by this optimization.) - Reduced stack usage of the compiler by rewriting some key functions in tail-recursive style. - Reduced memory requirements of constant propagation pass by forgetting compile-time approximations of dead variables. - More careful elaboration of C struct and union types into CompCert C types, avoiding behaviors exponential in the nesting of structs. Bug fixing: - Fixed parsing of labeled statements inside "switch" constructs, which were causing syntax errors. - The "->" operator applied to an array type was causing a type error. - Nested conditional expressions "a ? (b ? c : d) : e" were causing a compile-time error if "c", "d" and "e" had different types. Coq development: - Adapted the memory model to the needs of the VST project at Princeton: . Memory block identifiers are now of type "positive" instead of "Z" . Strengthened invariants in the definition of memory injections and the specification of external calls. - The LTL intermediate language is now a CFG of basic blocks. - Suppressed the LTLin intermediate language, no longer used. Release 1.13, 2013-03-12 ======================== Language semantics: - Comparisons involving pointers "one past" the end of a block are now defined. (They used to be undefined behavior.) (Contributed by Robbert Krebbers). Language features: - Arguments to __builtin_annot() that are compile-time constants are now replaced by their (integer or float) value in the annotation generated in the assembly file. Improvements in performance: - ARM and PowerPC ports: more efficient access to function parameters that are passed on the call stack. - ARM port; slightly better code generated for some indirect memory accesses. Bug fixing: - Fixed a bug in the reference interpreter in -all mode causing some reductions to be incorrectly merged. - Wrong parsing of hexadecimal floating-point literals 0xMMMMpEEE. Improvements in usability: - Better error and warning messages for declarations of variables of size >= 2^32 bits. - Reference interpreter: more efficient exploration of states in -all mode. Coq development: - More efficient implementation of machine integers (module Integers) taking advantage of bitwise operations defined in ZArith in Coq 8.4. - Revised handling of return addresses in the Mach language and the Stacking and Asmgen passes. - A number of definitions that were opaque for no good reason are now properly transparent. Release 1.12.1, 2013-01-29 ========================== Ported to Coq 8.4pl1. Otherwise functionally identical to release 1.12. Release 1.12, 2013-01-11 ======================== Improvements in confidence: - Floating-point literals are now parsed and converted to IEEE-754 binary FP numbers using a provably-correct conversion function implemented on top of the Flocq library. Language semantics: - Comparison between function pointers is now correctly defined in the semantics of CompCert C (it was previously undefined behavior, by mistake). - Bit-fields of 'enum' type are now treated as either unsigned or signed, whichever is able to represent all values of the enum. (Previously: always signed.) - The "&&" and "||" operators are now primitive in CompCert C and are given explicit semantic rules, instead of being expressed in terms of "_ ? _ : _" as in previous CompCert releases. - Added a "Ebuiltin" expression form (invocation of built-in function) to CompCert C, and a "Sbuiltin" statement form to Clight. Used it to simplify the encoding of annotations, memcpy, and volatile memory accesses. Performance improvements: - Better code generated for "&&" and "||" operators. - More aggressive elimination of conditional branches during constant propagation, taking better advantage of inferred constants. Language features: - By popular demand, "asm" statements for inline assembly are now supported if the flag -finline-asm is set. Use with extreme caution, as the semantic preservation proof assumes these statements have no effect on the processor state. Internal simplifications and reorganization: - Clight, Csharpminor, Cminor: suppressed the "Econdition" conditional expressions, no longer useful. - Clight: a single loop form, the three C loops are derived forms. - Clight: volatile memory accesses are materialized as builtin operations. - Clight: removed dependencies on CompCert C syntax and semantics. - New pass SimplLocals over Clight that replaces local scalar variables whose address is never taken by temporary, nonadressable variables. (This used to be done in Cminorgen.) - Csharpminor: simplified semantics. - Cminor: suppressed the "Oboolval" and "Onotbool" operators, which can be expressed in terms of "Ocmpu" at no performance costs. - All languages: programs are now presented as a list of global definitions (of functions or variables) instead of two lists, one for functions and the other for variables. Other changes: - For compatibility with other C compilers, output files are now generated in the current directory, by default; output file name can be controlled with the -o option, somewhat like with GCC. - Reference interpreter: better handling of volatile memory accesses. - IA32/MacOS X: now supports referencing global variables defined in shared libraries; old hack for stdio is no longer needed. - PowerPC/MacOS X: this port was removed, as recent version of MacOS X no longer support PowerPC. Release 1.11, 2012-07-13 ======================== Improvements in confidence: - Floating-point numbers and arithmetic operations, previously axiomatized, are now implemented and proved correct in Coq, using the Flocq library of S. Boldo and G. Melquiond. Language semantics: - In accordance with ISO C standards, the signed division min_int / -1 and the signed remainder min_int % -1 (where min_int is the smallest representable signed integer) now have undefined semantics and are treated as "going wrong" behaviors. (Previously, they were defined with results min_int and 0 respectively, but this behavior requires unnatural code to be generated on IA32 and PowerPC.) Performance improvements: - Function inlining is now implemented. The functions that are inlined are those declared "inline" in the C source, provided they are not recursive. - Constant propagation is now able to propagate the initial values of "const" global variables. - Added option -ffloat-const-prop to control the propagation of floating-point constants; see user's manual for documentation. - Common subexpression elimination can now eliminate memory loads following a memory store at the same location. - ARM: make use of the "fcmpzd" and "fmdrr" instructions. New tool: - The "cchecklink" tool performs a posteriori validation of the assembling and linking phases. It is available for PowerPC-EABI only. It takes as inputs an ELF-PowerPC executable as produced by the linker, as well as .sdump files (abstract assembly) as produced by "ccomp -sdump", and checks that the executable contains properly-assembled and linked code and data corresponding to those produced by CompCert. Other changes: - Elimination of "static" functions and "static" global variables that are not referenced in the generated code. - The memory model was enriched with "max" permissions in addition to "current" permissions, to better reason over "const" blocks and already-deallocated blocks. - More efficient implementation of the memory model, resulting in faster interpretation of source files by "ccomp -interp". - Added option "-falign-functions" to control alignment of function code. Release 1.10, 2012-03-13 ======================== Improvements in confidence: - CompCert C now natively supports volatile types. Its semantics fully specifies the meaning of volatile memory accesses. The translation of volatile accesses to built-in function invocations is now proved correct. - CompCert C now natively supports assignment between composite types (structs or unions), passing composite types by value as function parameters, and other instances of using composites as r-values, with the exception of returning composites by value from a function. (The latter remains emulated, using the -fstruct-return option.) - PowerPC: removed the -fmadd option, not semantically-preserving in the strict sense. Language features: - Support for _Bool type from ISO C99. - Support for _Alignof(ty) operator from ISO C 2011 and __alignof__(ty), __alignof__(expr) from GCC. Performance improvements: - Improvements in instruction selection, especially for integer casts and their combinations with bitwise operations. - Shorter, more efficient code generated for accessing volatile global variables. - Better code generated for the && and || operators. - More aggressive common subexpression elimination (CSE) of memory loads. - Improved register allocation for invocations of built-ins, especially for annotations. - In Cminor and down, make safe operators non-strict: they return Vundef instead of getting stuck. This enables more optimizations. - Cast optimization is no longer performed by a separate pass over RTL, but equivalent optimization is done during Cminor generation and during instruction selection. Other improvements: - PowerPC/EABI: uninitialized global variables now go in common (bss) section. - PowerPC: work around limited excursion of conditional branch instructions. - PowerPC: added __builtin_fnmadd() and __builtin_fnmsub(). - Reference interpreter: better printing of pointer values and locations. - Added command-line options -Wp, -Wa, -Wl, to pass specific options to the preprocessor, assembler, or linker, respectively. - More complete Cminor parser and printer (contributed by Andrew Tolmach). Release 1.9.1, 2011-11-28 ========================= Bug fixes: - Initialization of a char array by a short string literal was wrongly rejected - Incorrect handling of volatile arrays. - IA32 code generator: make sure that min_int / -1 does not cause a machine trap. Improvements: - Added language option -flongdouble to treat "long double" like "double". - The reference interpreter (ccomp -interp) now supports 2-argument main functions (int main(int, char **)). - Improved but still very experimental emulation of packed structs (-fpacked-structs) - Coq->Caml extraction: extract Coq pairs to Caml pairs and Coq characters to Caml "char" type. Release 1.9, 2011-08-22 ======================= - The reduction semantics of CompCert C was made executable and turned into a reference interpreter for CompCert C, enabling animation of the semantics. (Thanks to Brian Campbell for suggesting this approach.) Usage is: ccomp -interp [options] source.c Options include: -trace to print a detailed trace of reduction steps -random to randomize execution order -all to explore all possible execution orders in parallel - Revised and strengthened the top-level statements of semantic preservation. In particular, we now show: . backward simulation for the whole compiler without assuming a deterministic external world; . if the source program goes wrong after performing some I/O, the compiled code performs at least these I/O before continuing with an arbitrary behavior. - Fixed two omissions in the semantics of CompCert C (reported by Brian Campbell): . Functions calls through a function pointer had undefined semantics. . Conditional expressions "e1 ? e2 : e3" where e2 and e3 have different types were missing a cast to their common type. - Support for "read-modify-write" operations over volatiles (such as e++ or --e or e |= 1 where e has volatile type) through a new presimplification (flag -fvolatile-rmw, "on" by default). - New optimization pass: Redundant Reload Elimination, which fixes up inefficiencies introduced during the Reload pass. On x86, it increases performance by up to 10%. On PowerPC and ARM, the effect is negligible. - Revised handling of annotation statements. Now they come in two forms: 1. __builtin_annot("format", x1, ..., xN) (arbitrarily many arguments; no code generated, even if some of the xi's were spilled; no return value) 2. __builtin_annot_intval("format", x1) (one integer argument, reloaded in a register if needed, returned as result). - Related clean-ups in the handling of external functions and compiler built-ins. In particular, __builtin_memcpy is now fully specified. - ARM code generator was ported to the new ABI (EABI in ARM parlance, armel in Debian parlance), using VFD instructions for floating-point. (Successfully tested on a Trimslice platform running Ubuntu 11.04.) - IA32 code generator: . Added -fno-sse option to prevent generation of SSE instructions for memory copy operations. . More realistic modeling of the ST0 (top-of-FP-stack) register and of floating-point compare and branch. - PowerPC code generator: more efficient instruction sequences generated for insertion in a bit field and for some comparisons against 0. Release 1.8.2, 2011-05-24 ========================= - Support for "aligned" and "section" attributes on global variables, e.g. __attribute__((aligned(16))) int x; - Experimental emulation of packed structs (flag -fpacked-structs). - Pointer comparisons now treated as unsigned comparisons (previously: signed). This fixes an issue with arrays straddling the 0x8000_0000 boundary. Consequently, the "ofs" part of pointer values "Vptr b ofs" is now treated as unsigned (previously: signed). - Elimination of unreferenced labels now performed by a separate pass (backend/CleanupLabels.v) and proved correct. - Stacking pass revised: supports more flexible layout of the stack frame; two-step proof (Stackingproof + Machabstr2concr) merged into one single proof (Stackingproof). - The requirement that pointers be valid in pointer comparisons was pushed through all intermediate languages of the back-end (previously: requirement present only up to Csharpminor). - Emulation of assignment between structs and between unions was simplified and made more efficient, thanks to a better implementation of __builtin_memcpy. - Improvements to the compiler driver: . -E option now prints preprocessed result to standard output instead of saving it in a .i file . support for .s (assembly) and .S (assembly to be preprocessed) input files Release 1.8.1, 2011-03-14 ========================= - Adapted to Coq 8.3pl1. - Reduced compilation times through several algorithmic improvements (contributed by A. Pilkiewicz). - In the various semantics, allow float-to-int conversions to fail (if the float argument is outside the range of representable ints). - Initialization of global C variables made more robust and proved correct. - ABI conformance improved: . the "char" type is now signed for x86, remains unsigned for PowerPC and ARM . placement of bit-fields now follows SVR4 conventions (affects PowerPC) - Bug fixes in the C pre-simplifier: . nontermination with some recursive struct types . issues with zero-width bit fields . elimination of struct assignments duplicating some volatile accesses Release 1.8, 2010-09-21 ======================= - The input language to the proved part of the compiler is no longer Clight but CompCert C: a larger subset of the C language supporting in particular side-effects within expressions. The transformations that pull side effects out of expressions and materialize implicit casts, formerly performed by untrusted Caml code, are now fully proved in Coq. - New port targeting Intel/AMD x86 processors. Generates 32-bit x86 code using SSE2 extensions for floating-point arithmetic. Works under Linux, MacOS X, and the Cygwin environment for Windows. CompCert's compilation strategy is not a very good match for the x86 architecture, therefore the performance of the generated code is not as good as for the PowerPC port, but still usable. (About 75% of the performance of gcc -O1 for x86, compared with > 90% for PowerPC.) - More faithful semantics for volatile accesses: . volatile reads and writes from a volatile global variable are treated like input and output system calls (respectively), bypassing the memory model entirely; . volatile reads and writes from other locations are treated like regular loads and stores. - Introduced __builtin_memcpy() and __builtin_memcpy_words(), use them instead of memcpy() to compile struct and union assignments. - Introduced __builtin_annotation() to transmit assertions from the source program all the way to the generated assembly code. - Elimination of some useless casts around "&", "|" and "^" bitwise operators. - Produce fewer "moves" during RTL generation. This speeds up the rest of compilation and slightly improves the result of register allocation when register pressure is high. - Improvements in register allocation: . Implemented a spilling heuristic during register allocation. This heuristic reduces significantly the amount of spill code generated when register pressure is high. . More coalescing between low-pressure and high-pressure variables. . Aggressive coalescing between pairs of spilled variables. - Fixed some bugs in the emulation of bit fields. Release 1.7.1, 2010-04-13 ========================= Bug fixes in the new C pre-simplifier: - Missing cast on return value for some functions - Incorrect simplification of some uses of || and && - Nontermination in the presence of a bit field of size exactly 32 bits. - Global initializers for structs containing bit fields. - Wrong type in volatile reads from variables of type 'unsigned int'. Small improvements to the PowerPC port: - Added __builtin_trap() built-in function. - Support for '#pragma reserve_register' (EABI) - Less aggressive alignment of global variables. - Generate '.type' and '.size' directives (EABI). Release 1.7, 2010-03-31 ======================= - New implementation of the C type-checker, simplifier, and translation to Clight. Compared with the previous CIL-based solution, the new implementation is more modular and supports more optional simplifications. - More features of the C language are handled by expansion during translation to Clight: . assignment between structs and unions (option -fstruct-assign) . passing structs and union by value (option -fstruct-passing) . bit-fields in structs (option -fbitfields) - The "volatile" modifier is now honored. Volatile accesses are represented in Clight by calls to built-in functions, which are preserved throughout the compilation chain, then turned into processor loads and stores at the end. - Generic support for C built-in functions. These predefined external functions give access to special instructions of the processor. See powerpc/CBuiltins.ml for the list of PowerPC built-in functions. - The memory model now exposes the bit-level in-memory representation of integers and floats. This strengthens the semantic preservation theorem: we now prove that C code that directly manipulates these bit-level representations (e.g. via a union between floats and integers) is correctly compiled. - The memory model now supports fine-grained access control to individual bytes of a memory block. This feature is currently unused in the compiler proofs, but will facilitate connections with separation logics later. - External functions are now allowed to read and modify memory. The semantic preservation proofs were strengthened accordingly. In particular, this enables the malloc() and free() C library functions to be modeled as external functions in a provably correct manner. - Minor improvements in the handling of global environments and the construction of the initial memory state. - Bug fixes in the handling of '#pragma section' and '#pragma set_section'. - The C test suite was enriched and restructured. Release 1.6, 2010-01-13 ======================= - Support Clight initializers of the form "int * x = &y;". - Fixed spurious compile-time error on Clight initializers of the form "const enum E x[2] = { E_1, E_2 };". - Produce informative error message if a 'return' without argument occurs in a non-void function, or if a 'return' with an argument occurs in a void function. - Preliminary support for '#pragma section' and '#pragma set_section'. - Preliminary support for small data areas in PowerPC code generator. - Back-end: added support for jump tables; used them to compile dense 'switch' statements. - PowerPC code generator: force conversion to single precision before doing a "store single float" instruction. Release 1.5, 2009-08-28 ======================= - Support for "goto" in the source language Clight. - Added small-step semantics for Clight. - Traces for diverging executions are now uniquely defined; tightened semantic preservation results accordingly. - Emulated assignments between structures (during the C to Clight initial translation). - Fixed spurious compile-time error on Clight statements of the form "x = f(...);" where x is a global variable. - Fixed spurious compile-time error on Clight initializers where the initial value is the result of a floating-point computation (e.g. "double x = 3.14159 / 2;"). - Simplified the interface of the generic dataflow solver. - Reduced running time and memory requirements for the constant propagation pass. - Improved the implementation of George and Appel's graph coloring heuristic: runs faster, produces better results. - Revised the implementation of branch tunneling. - Improved modularization between processor-dependent and processor-independent parts. Release 1.4.1, 2009-06-05 ========================= - Adapted to Coq 8.2-1. No changes in functionality. Release 1.4, 2009-04-20 ======================= - Modularized the processor dependencies in the back-end. - Three target architectures are now supported: PowerPC / MacOS X (most mature) PowerPC / EABI & Linux (getting stable) ARM / Linux EABI (still experimental) - Added alignment constraints to the memory model. - Clight: added support for conditional expressions (a ? b : c); removed support for array accesses a[i], now a derived form. - C front-end: honor "static" modifiers on globals. - New optimization over RTL: turning calls into tail calls when possible. - Instruction selection pass: elimination of redundant casts following a memory load of a "small" memory quantity. - Linearization pass: improved the linearization heuristic. - Reloading pass: more economical use of temporaries. - Back-end: removed "alloc heap" instruction; removed pointer validity checks in pointer comparisons. Release 1.3, 2008-08-11 ======================= - Added "goto" and labeled statements to Cminor. Extended RTLgen and its proof accordingly. - Introduced small-step transition semantics for Cminor; used it in proof of RTLgen pass; proved consistency of Cminor big-step semantics w.r.t. transition semantics. - Revised division of labor between the Allocation pass and the Reload pass. The semantics of LTL and LTLin no longer need to anticipate the passing of arguments through the conventional locations. - Cleaned up Stacking pass: the positions of the back link and of the return address in the stack frame are no longer hard-wired in the Mach semantics. - Added operator to convert from float to unsigned int; used it in C front-end - Added flag -fmadd to control recognition of fused multiply-add and -sub - Semantics of pointer-pointer comparison in Clight was incomplete: pointers within different blocks can now be compared using == or != - Addition integer + pointer is now supported in Clight. - Improved instruction selection for complex conditions involving || and &&. - Improved translation of Cminor "switch" statements to RTL decision trees. - Fixed error in C parser and simplifier related to "for" loops with complex expressions as condition. - More benchmark programs in test/ Release 1.2, 2008-04-03 ======================= - First public release