CompCert 3.16

Code generation and optimization

• Extend neededness analysis to 64-bit integer operations, enabling more useless integer computations to be removed.
• Extend common subexpression elimination (CSE) and dead code elimination to calls to known pure built-in functions and to known pure arithmetic helper functions.
• Generation of position-independent code (PIC) for AArch64, RISC-V and x86-64 (option -fpic) (#551)
• Generation of position-independent executables (PIE) for AArch64, RISC-V and x86-64 (option -fpie) (#551)
• RISC-V: use PC-relative addressing instead of absolute addressing where possible (#550)
• PowerPC and x86/ELF: put jump tables in .rodata section instead of .text section (#508)

Bug fixes

• Reject identifiers consisting of a single $ sign. This can cause errors with some assemblers, and is not ISO C.
• Remove syntactic support for [*] array declarators, which was introduced in 3.15 (#539). They are usable only with variable-length arrays, which CompCert doesn't support.
• Check that [static <size>] declarators occur only in function prototypes and only in outermost position, as prescribed by ISO C.
• Fix corner case of designated initializers involving union with anonymous members.
• Fix wrong lexing of #pragma directives that immediately follow a // single-line comment. (Affects only non-preprocessed .i input files; preprocessed .c files are not affected.)

Usability

• Added warning dollar-in-identifier-extension for identifiers that contain $ signs. (This is not ISO C, just a popular extension.)
• Revised handling of unknown warnings: -W<unknown warning> is now a warning instead of an error; -Wno-<unknown warning> is silently ignored (#554).
• Added warning unknown-warning-option, so that warnings about unknown warnings can be ignored (-Wno-unknown-warning-option) or turned into an error (-Werror=unknown-warning-option) (#554).
• Recognize option -shared and pass it to the linker.
• Recognize options -pie and -no-pie and pass them to the linker.
• Recognize options -MD and -MMD and pass them to the preprocessor.
• On BSD platforms, use cc instead of gcc as the default preprocessor and linker.
• On BSD platforms, add -z nobtcfi linker option. (This turns IBT/BTI off on OpenBSD, until it is properly supported by CompCert.)

Rocq/Coq development

• Support Coq 8.21 (#545)
• Support Rocq 9.0 (using the coq compatibility command) (#547)
• Earliest version supported is now Coq 8.15.
• Updated vendored Flocq to version 4.2.1

CompCert 3.15

C language support

• Minimal syntactic support for _Float16 type (half-precision FP numbers). Function declarations using _Float16 are correctly parsed, but any actual use of _Float16 is rejected later during compilation. (#525)
• Support C99 array declarator syntax involving static and *. These annotations are correctly parsed, but ignored during typing and compilation. (#539)

Code generation and optimization

• Better support for _Bool type in the back-end and in the memory model.
  This avoids redundant normalizations to _Bool.
• Take types of function parameters into account during value analysis.
  This avoids redundant normalizations on parameters of small integer types.
• More conservative static analysis of pointer comparisons. (#516)
• Refined heuristic for if-conversion. Turn if-conversion off in some cases where it would prevent later optimization of conditional branches in the continuation of the if.
• CSE: remember pointer alias information in Load equations, making it possible to remove more redundant memory loads. (#518)
• More precise value analysis of integer multiplication, division, modulus.
• Constant propagation: optimize "known integer or undefined" results. For example, &x == &x, which is either 1 or undefined, is now replaced by 1.
• Optimize (x ^ cst) != 0 into x != cst.
• Avoid quadratic compile-time behavior on deeply-nested if statements. (#519)

Bug fixes

• More robust determination of symbols that may be defined in a shared object. (#538)
• Escape $NNN identifiers in generated x86 and ARM assembly code (#541).
• Wrong parameter scoping in function definitions such as int (*f(int y))(int x) { ... } (a function returning a pointer to a prototyped function).

Usability

• Mark stack as non-executable in binaries produced by ccomp.
• Check that preprocessed sources (.i files) do not contain backslash-newline sequences.
• ./configure arm-linux now selects the hard-FP ABI, since Linux distributions no longer use the soft-FP ABI.

Supporting libraries

• ARM library for 64-bit integer arithmetic: faster division, cleaned-up code.

Coq development

• Support Coq 8.20.
• Build: support TIMING and PROFILING like coq_makefile. (#512)
• Make dependency on Extraction explicit. (#515)
• Install .glob and .v files along .vo files. (#527)

CompCert 3.14

ISO C conformance

• free has well-defined semantics on blocks of size 0 (#509).

Code generation and optimization

• More simplifications of comparison operations and selection operations during the CSE pass.
• Replace selection operations with moves if both branches are equal.
• ARM 32 bits: several minor improvements to the generated code (#503 and more).

Bug fixes

• x86 under Windows: the wrong sub instruction was generated for Pallocframe.
• ARM: fix PC displacement overflow involving floating-point constants.
• ARM: fix error on printing of "s17" register.
• RISC-V: do not use 64-bit FP registers for memcpy if option -fno-fpu is given.

Usability

• Added generation of CFI debugging directives for AArch64 and RISC-V.
• Removed the command-line option -fstruct-return, deprecated since release 2.6.

Formal semantics

• The big-step semantics for Clight now supports the two models for function arguments (either as stack-allocated variables or as register-like temporaries).

Coq development

• Support Coq 8.17, 8.18, and 8.19.
• Revised most uses of the intuition tactic (#496 and more).
• Address most other deprecation warnings from Coq 8.18 and 8.19.
• Updated local copy of MenhirLib to version 20231231.
• Updated local copy of Flocq to version 4.1.4.

Distribution

• The small test suite was moved to a separate Git repository. Use git submodule init && git submodule update to download it.

CompCert 3.13

Code generation and optimization:

• Slightly more precise value analysis, with a better distinction between "any integer value" and "any integer or pointer value". (#490)
• Recognize a few more nested conditional expressions that can be if-converted into a conditional move.
• Register allocation: better support for "preferred" registers, e.g. registers R0-R3 on ARM Thumb, which enable more compact instruction encodings.
• ARM Thumb: use more instructions that can be encoded in 16 bits, producing more compact code.
• x86: use a shorter instruction encoding for loading unsigned 32-bit constants. (#487)

Bug fixes:

• _Noreturn on function definitions (but not declarations) was ignored sometimes.
• The argument of __builtin_va_end was discarded, is now evaluated for its side effects.
• Tail call optimization must be disabled in variadic functions, otherwise a va_list structure can get out of scope.
• Nested compound initializers were elaborated in the wrong order, causing compile-time failures later in the compilation pipeline.
• The signedness of a bit field was determined differently when its type was an enum type and when it was a typedef to an enum type.

ABI conformance:

• Define wchar_t exactly like the ABI says. On most platforms, CompCert was defining wchar_t as signed int, but it must be unsigned int for AArch64-ELF and unsigned long for PowerPC-EABI.

Supported platforms:

• Removed support for Cygwin 32 bits, which has reached end of life. Cygwin 64 bits remains supported.

Coq development:

• Support Coq 8.16.0 and 8.16.1, addressing most of the new warnings in 8.16.
• When installing the Coq development, also install coq-native generated files.
• Tweak some proof scripts for compatibility with future Coq versions. (#465, #470, #491, #492)
• Updated the vendored copy of Flocq to version 4.1.1.

CompCert 3.12

New features:

• Support unstructured switch statements such as Duff's device. This is achieved via an optional, unverified code rewrite, activated by option -funstructured-switch. (#459)
• Support C11 Unicode string literals and character constants, such as u8"été" or u32'❎'.

Usability:

• Support the -std=c99, -std=c11 and -std=c18 option. These options are passed to the preprocessor, helping it select the
  correct version of the standard header files. It also controls CompCert's warning for uses of C11 features. (#456)
• The source character set of CompCert is now officially Unicode with UTF-8 encoding, A new warning invalid-utf8 is triggered if byte sequences that are not valid UTF-8 are found outside of comments. Other source character sets and stricter validation can be supported via the -finput-charset option, see next.
• If the GNU preprocessor is used, the source character set can be selected with the -finput-charset= option. In particular, -finput-charset=utf8 checks that the source file is correctly UTF-8 encoded, and -finput-charset=ascii that it contains no Unicode characters at all.
• Support mergeable sections for string literals and for numerical constants.
• AArch64, ARM, RISC-V and x86 ELF targets: use .data.rel.ro section for const data whose initializers contain relocatable addresses. This is required by the LLVM linker and simplifies the work of the GNU linker.
• configure script: add option -sharedir to specify where to put the compcert.ini configuration file (#450, #460)
• ARM 32 bits: emit appropriate Tag_ABI_VFP attribute (#461)

Optimizations:

• Recognize more if-else statements that can be if-converted into a conditional move. In particular, debug statements generated in -g mode no longer prevent this conversion.

Bug fixes:

• Revised simplification of nested conditional, ||, && expressions to make sure the generated Clight code is well typed and is not rejected later by ccomp (#458).
• In -g mode, when running under Windows, the ccomp executable could fail on an uncaught exception while inserting lines of the source C file as comments in the generated assembly file.
• Reintroduced DWARF debug information for bit fields in structs (it was missing since 3.10).

Coq development:

• RTLgen: use the state and error monad for reserving goto labels (#371) (by Pierre Nigron)
• Add Declare Scope statements where appropriate, and re-enable the undeclared-scope warning.

CompCert 3.11

New features

• Support _Generic expressions from ISO C11.

ISO C conformance

• Enumeration types are compatible only with int but not with other integer types.
• Fixed confusion between unprototyped function pointer types T (*)() and prototyped, zero-argument function pointer types T (*)(void) in type casts (#431).

Usability

• Improved control-flow analysis of calls to "noreturn" functions, resulting in more accurate warnings.
• More detailed warning for unprototyped function definitions, now shows the prototyped type that is given to the function.
• Extended the warning above to definitions of the form T f() { ... }, i.e. unprototyped but with no parameters. (Before, the warning would trigger only if parameters were declared.)
• Check (and warn if requested) for arguments of struct/union types passed to a variable-argument function.

Bug fixes

• RISC-V: fixed an error in the modeling of float32 <-> float64 conversions when the argument is a NaN (#428).
• x86: changed the compilation of __builtin_fmin and __builtin_fmax so that their NaN behavior is the one documented in the manual.
• Improved reproducibility of register allocation. (Before, compiling CompCert with two different OCaml versions could have resulted in correct but different allocations.)
• Hardened the configure script against Cygwin installations that produce \r\n for end-of-lines (#434).
• RISC-V: tail calls to far-away functions were causing link-time errors (#436, #437).

Coq development

• Updated the Flocq library to version 4.1.
• Support for Coq 8.14.1, 8.15.0, 8.15.1, 8.15.2.
• Minimal Coq version supported is now 8.12.0.

CompCert 3.10

Major improvement

• Bit fields in structs and unions are now handled in the formally-verified part of CompCert. (Before, they were being implemented through an unverified source-to-source translation.)
  The CompCert C and Clight languages provide abstract syntax for bit-field declarations and formal semantics for bit-field accesses.
  The translation of bit-field accesses to bit manipulations is performed in the Cshmgen pass and proved correct.

Usability

• The layout of bit-fields in memory now follows the ELF ABI algorithm, improving ABI compatibility for the CompCert target platforms that use ELF.
• Handle the # 0 line directive generated by some C preprocessors (#398).
• Un-define the __SIZEOF_INT128__ macro that is predefined by some C preprocessors (#419).
• macOS assembler: use ## instead of # to delimit comments (#399).

ISO C conformance

• Stricter checking of multi-character constants 'abc'.
  Multi-wide-character constants L'ab' are now rejected, like GCC and Clang do.
• Ignore (but still warn about) unnamed plain members of structs and unions (#411).
• Ignore unnamed bit fields when initializing unions.

Bug fixing

• x86 64 bits: overflow in offset of leaq instructions (#407).
• AArch64, ARM, PowerPC, RISC-V: wrong expansion of __builtin_memcpy_aligned in cases involving arguments that are stack addresses (#410, #412)
• PowerPC 64 bits: wrong selection of 64-bit rotate-and-mask instructions (rldic, rldicl, rldicr), resulting in assertion failures later in the compiler.
• RISC-V: update the Asm semantics to reflect the fact that register X1 is destroyed by some builtins.

Compiler internals

• The "PTree" data structure (binary tries) was reimplemented, using a canonical representation that guarantees extensionality and improves performance.
• Add the ability to give formal semantics to numerical builtins with small integer return types.
• PowerPC: share code for memory accesses between Asmgen and Asmexpand.
• Declare __compcert_i64* helper runtime functions during the C2C pass, so that they are not visible during source elaboration.

The clightgen tool

• Add support for producing Csyntax abstract syntax instead of Clight abstract syntax (option -csyntax to clightgen) (contributed by Bart Jacobs; #404, #413).

Coq development

• Added support for Coq 8.14 (#415).
• Added support for OCaml 4.13.
• Updated the Flocq library to version 3.4.2.
• Replaced Global Set Asymmetric Patterns by more local settings (#408).

CompCert 3.9

New features

• New port: AArch64 (ARM 64 bits, "Apple silicon") under macOS.
• Support bitfields of types other than int, provided they are no larger than 32 bits (#387)
• Support __builtin_unreachable and __builtin_expect (#394) (but these builtins are not used for optimization yet)

Optimizations

• Improved branch tunneling: optimized conditional branches can introduce further opportunities for tunneling, which are now taken into account.

Usability

• Pragmas within functions are now ignored (with a warning) instead of being lifted just before the function like in earlier versions.
• configure script: add -mandir option (#382)

Compiler internals

• Finer control of variable initialization in sections. Now we can put variables initialized with symbol addresses that need relocation in specific sections (e.g. const_data on macOS).
• Support re-normalization of function parameters at function entry, as required by the AArch64/ELF ABI.
• PowerPC 64 bits: remove Pfcfi, Pfcfiu, Pfctiu pseudo-instructions, expanding the corresponding int<->FP conversions during the selection pass instead.

Bug fixing

• PowerPC 64 bits: incorrect ld and std instructions were generated and rejected by the assembler.
• PowerPC: some variadic functions had the wrong position for their first variadic parameter.
• RISC-V: fix calling convention in the case of floating-point arguments that are passed in integer registers.
• AArch64: the default function alignment was incorrect, causing a warning from the LLVM assembler.
• Pick the correct archiver to build .a library archives (#380).
• x86 32 bits: make sure functions returning structs and unions return the correct pointer in register EAX (#377).
• PowerPC, ARM, AArch64: updated the registers destroyed by asm pseudo-instructions and built-in functions.
• Remove spurious error on initialization of a local struct containing a flexible array member.
• Fixed bug in emulation of assignment to a volatile bit-field (#395).

The clightgen tool

• Move the $ notation for Clight identifiers to scope clight_scope and submodule ClightNotations, to avoid clashes with Ltac2's use of $ (#392).

Coq development

• Compatibility with Coq 8.12.2, 8.13.0, 8.13.1, 8.13.2.
• Compatibility with Menhir 20210419 and up.
• Oldest Coq version supported is now 8.9.0.
• Use the lia tactic instead of omega.
• Updated the Flocq library to version 3.4.0.

Licensing and distribution

• Dual-licensed source files are now distributed under the LGPL version 2.1 (plus the Inria non-commercial license) instead of the GPL version 2 (plus the Inria non-commercial license).

CompCert 3.8

New features

• Support _Static_assert from ISO C11.
• Support __builtin_constant_p from GCC and Clang.
• New port: x86 64 bits Windows with the Cygwin 64 environment. (configure with target x86_64-cygwin).
• The following built-in functions are now available for all ports: __builtin_sqrt, __builtin_fabsf, and all variants of __builtin_clz and __builtin_ctz.
• Added __builtin_fmin and __builtin_fmax for AArch64.

Removed features

• The x86 32 bits port is no longer supported under macOS.

Compiler internals

• Simpler translation of CompCert C casts used for their effects but not for their values.
• Known builtins whose results are unused are eliminated earlier.
• Improved error reporting for ++ and -- applied to pointers to incomplete types.
• Improved error reporting for redefinitions and implicit definitions of built-in functions.
• Added formal semantics for some PowerPC built-ins.

The clightgen tool

• New -canonical-idents mode, selected by default, to change the way C identifiers are encoded as CompCert idents (positive numbers). In -canonical-idents mode, a fixed one-to-one encoding is used so that the same identifier occurring in different compilation units encodes to the same number.
• The -short-idents flag restores the previous encoding where C identifiers are consecutively numbered in order of appearance, causing the same identifier to have different numbers in different compilation units.
• Removed the automatic translation of annotation builtins to Coq logical assertions, which was never used and possibly confusing.

Coq and OCaml development

• Compatibility with Coq 8.12.1, 8.12.0, 8.11.2, 8.11.1.
• Can use already-installed Flocq and MenhirLib libraries instead of their local copies (options -use-external-Flocq and -use-external-MenhirLib to the configure script).
• Automatically build to OCaml bytecode on platforms where OCaml native-code compilation is not available.
• Install the compcert.config summary of configuration choices in the same directory as the Coq development.
• Updated the list of dual-licensed source files.

CompCert 3.7

ISO C conformance:

• Functions declared extern then implemented inline remain extern
• The type of a wide char constant is wchar_t, not int
• Support vertical tabs and treat them as whitespace
• Define the semantics of free(NULL)

Bug fixing:

• Take sign into account for conversions from 32-bit integers to 64-bit pointers
• PowerPC: more precise determination of small data accesses
• AArch64: when addressing global variables, check for correct alignment
• PowerPC, ARM: double rounding error in int64->float32 conversions

ABI conformance:

• x86, AArch64: re-normalize values of small integer types returned by function calls
• PowerPC: float arguments passed on stack are passed in 64-bit format
• RISC-V: use the new ELF psABI instead of the old ABI from ISA 2.1

Usability and diagnostics:

• Unknown builtin functions trigger a specific error message
• Improved error messages

Coq formalization:

• Revised modeling of the PowerPC/EREF isel instruction
• Weaker ec_readonly condition over external calls
  (permissions can be dropped on read-only locations)

Coq and OCaml development:

• Compatibility with Coq version 8.10.1, 8.10.2, 8.11.0
• Compatibility with OCaml 4.10 and up
• Compatibility with Menhir 20200123 and up
• Coq versions prior to 8.8.0 are no longer supported
• OCaml versions prior to 4.05.0 are no longer supported