[cfe-dev] [llvm-dev] Status regarding support for fixed point numbers (and Embedded-C) in Clang/LLVM

[Francis ANDRE via cfe-dev]

Hello

I would like to mention a deep work of Mike Cowlishaw on decimal 
arithmetic here: http://speleotrove.com/decimal/#arithmetic/ 
<http://speleotrove.com/decimal/#arithmetic/>

May be it could bring some different views and improvments from the 
fixed point numbers (and Embedded-C) in Clang/LLVM

Rgds

Le 15/03/2021 à 13:06, Björn Pettersson A via llvm-dev a écrit :
> Status regarding support for fixed point numbers (and Embedded-C) in Clang/LLVM
> ===============================================================================
>
> During the years there have been a number of questions on cfe-dev and llvm-dev
> about support for fixed point arithmetic in LLVM.
>
> In 2018 Leonard Chan wrote a proposal for adding fixed point number support in
> clang [1], accompanied by discussions about how to support the types in LLVM
> [2]. Since then a number of patches has been submitted and nowadays
> Clang/LLVM has lots of support for fixed point arithmetic, based on the
> Embedded-C draft [3].
>
>
> A major part of chapter 4 in [3] should have been implemented by now.
> Here is a list of features implemented:
>
> - Fixed point types can be enabled using the -ffixed-point clang option.
>
> - Variables can be declared using the _Fract and _Accum type specifiers (see
>    for example clang/test/Frontend/fixed_point_declarations.c for examples).
>
> - Saturated fixed point types are available by using the _Sat specifier
>    for the _Fract and _Accum types.
>
> - Support for conversion between signed/unsigned fixed-point types and
>    integral or floating point types.
>
> - Support for literals, using the following fixed-point suffixes:
>      hr: short _Fract
>      uhr: unsigned short _Fract
>      r: _Fract
>      ur: unsigned _Fract
>      lr: long _Fract
>      ulr: unsigned long _Fract
>      hk: short _Accum
>      uhk: unsigned short _Accum
>      k: _Accum
>      uk: unsigned _Accum
>      lk: long _Accum
>      ulk: unsigned long _Accum
>
> - Support for operations involving fixed-point types:
>      Unary (++, --, +, -, !)
>      Binary (+, -, *, /, <<, >>)
>      Comparison (<, <=, >=, >, ==, !=)
>      Assignment / Compound Assignment (=, +=, -=, *=, /=, <<=, >>=)
>
>
> Future improvements:
>
> - Improve user documentation (the comment that _Fract types aren't supported
>    has been removed from the UsersManual, but we haven't really added anything
>    describing that we now support fixed point types).
>
> - Support for format specifiers (r/R/k/K) for fixed-point arguments in I/O
>    functions.
>
> - Support for pragmas such as FX_FRACT_OVERFLOW and FX_ACCUM_OVERFLOW.
>    (This might be a difference compared to what they have implemented in
>    gcc, see [4].)
>
> - The Embedded-C draft [3] mentions a stdfix.h library.
>    (This has not been implemented. Also lacking in gcc according to [4].)
>
> - Implement sanitizer checks for fixed point shifts. The LLVM IR for shift has
>    UB semantics when shift count is too large. So either we need to clamp shift
>    count when lowering shift operators (if there are special rules for
>    embedded-c), or if it is UB on c level it would be nice if the sanitizer
>    catch such faults. There are some TODO:s left in the code (see for example
>    ScalarExprEmitter::EmitShl) indicating that the fixed point lowering
>    currently bypass the code for sanitizing the shift count.
>
> - Add some C++ compatibility. One thing that limits the fixed point support
>    to C is lack of name mangling rules for fixed point types (at least according
>    to the Itanium C++ ABI). One solution is to create a wrapper class, overriding
>    various operators. Another idea is to add name mangling (there is a proposal
>    for the Itanium C++ ABI here [5]).
>
>
> References:
>
> [1] - https://lists.llvm.org/pipermail/cfe-dev/2018-April/057756.html
> [2] - https://lists.llvm.org/pipermail/llvm-dev/2018-August/125433.html
> [3] - http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1169.pdf
> [4] - https://gcc.gnu.org/wiki/FixedPointArithmetic
> [5] - https://github.com/itanium-cxx-abi/cxx-abi/issues/56
>
>
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