Boost.Predef should provide a way to detect WASM

[jcelerier]

Relevant macros gathered from echo | em++ -dM -E -x c++ -:

#define EMSCRIPTEN 1
#define __EMSCRIPTEN__ 1
#define __wasm 1
#define __wasm32 1
#define __wasm32__ 1
#define __wasm__ 1

[jcelerier]

Also it's already detected in some way by boost.config: https://www.boost.org/doc/libs/1_82_0/boost/config/detail/select_platform_config.hpp

[sleeptightAnsiC]

This would require adding targets for wasm32 and wasm64, and then adding Emscripten as a compiler (or as platform) and ensuring it does not clash with Clang/GCC. [EDIT] Also SIMD detection which seems a bit tricky: https://emscripten.org/docs/porting/simd.html

boost.config doesn't seem to do anything specific with WASM, it just treats it as POSIX environment.

I wonder if there are any C/C++ compilers capable of targeting WASM other than Clang and its derivatives?

[sleeptightAnsiC]

Also what kind of PLAT and OS should be assigned for this? boost.config just ignores this and does not assign anything specific. I've seen some projects simply referring to this as "HTML5" (or "WEB") and it could be probably fine as PLAT but Emscripten seems to be referring to itself as a platform which complicates everything a bit. This check is fine to skip as boost.predef allows for this but I wonder what user of WASM/Emscripten would expect? When running emcc with predef it already detects OS as unix but I'm not sure if this is correct.

[comparison: emcc -dumpmachine]

$ emcc -dumpmachine
wasm32-unknown-emscripten

$ emcc -dumpmachine --target=wasm64
wasm64-unknown-emscripten

$ clang -dumpmachine --target=wasm64
wasm64

$ x86_64-w64-mingw32-gcc -dumpmachine
x86_64-w64-mingw32

[comparison: emcc AVAILABLE macros]

$ git clone https://github.com/boostorg/predef.git

$ echo '#include "boost/predef.h"' > main.c
$ echo 'int main(void) {return 0;}' >> main.c

$ clang -dM -E main.c -I./predef/include | grep BOOST | grep AVAILABLE | grep -v NOT_AVAILABLE
#define BOOST_ARCH_WORD_BITS_32 BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ARCH_WORD_BITS_64 BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ARCH_X86 BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ARCH_X86_64 BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ARCH_X86_64_AVAILABLE 
#define BOOST_ARCH_X86_AVAILABLE 
#define BOOST_COMP_CLANG_AVAILABLE 
#define BOOST_COMP_GNUC_AVAILABLE 
#define BOOST_COMP_LLVM_AVAILABLE 
#define BOOST_COMP_LLVM_DETECTION BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ENDIAN_LITTLE_BYTE BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ENDIAN_LITTLE_BYTE_AVAILABLE 
#define BOOST_HW_SIMD_AVAILABLE 
#define BOOST_HW_SIMD_X86_AVAILABLE 
#define BOOST_LANG_STDC_AVAILABLE 
#define BOOST_LIB_C_GNU_AVAILABLE 
#define BOOST_OS_LINUX BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_OS_LINUX_AVAILABLE 
#define BOOST_OS_UNIX BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_OS_UNIX_AVAILABLE 
#define BOOST_PREDEF_TESTED_AT(symbol,major,minor,patch) ( (symbol) >= BOOST_VERSION_NUMBER_AVAILABLE )
#define BOOST_VERSION_NUMBER_AVAILABLE BOOST_VERSION_NUMBER_MIN

$ clang --target=wasm32 -dM -E main.c -I./predef/include | grep BOOST | grep AVAILABLE | grep -v NOT_AVAILABLE
In file included from main.c:1:
In file included from ./include/boost/predef.h:16:
In file included from ./include/boost/predef/library.h:13:
In file included from ./include/boost/predef/library/c.h:13:
In file included from ./include/boost/predef/library/c/_prefix.h:11:
./include/boost/predef/detail/_cassert.h:14:10: fatal error: 'assert.h' file not found
   14 | #include <assert.h>
      |          ^~~~~~~~~~
1 error generated.                                                                                                                                                                                                                
#define BOOST_ARCH_WORD_BITS_32 BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_COMP_CLANG_AVAILABLE 
#define BOOST_COMP_GNUC_AVAILABLE 
#define BOOST_COMP_LLVM_AVAILABLE 
#define BOOST_COMP_LLVM_DETECTION BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ENDIAN_LITTLE_BYTE BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ENDIAN_LITTLE_BYTE_AVAILABLE 
#define BOOST_LANG_STDC_AVAILABLE 
#define BOOST_PREDEF_TESTED_AT(symbol,major,minor,patch) ( (symbol) >= BOOST_VERSION_NUMBER_AVAILABLE )
#define BOOST_VERSION_NUMBER_AVAILABLE BOOST_VERSION_NUMBER_MIN

$ emcc -dM -E main.c -I./predef/include | grep BOOST | grep AVAILABLE | grep -v NOT_AVAILABLE
#define BOOST_ARCH_WORD_BITS_32 BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_COMP_CLANG_AVAILABLE 
#define BOOST_COMP_GNUC_AVAILABLE 
#define BOOST_COMP_LLVM_AVAILABLE 
#define BOOST_COMP_LLVM_DETECTION BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ENDIAN_LITTLE_BYTE BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_ENDIAN_LITTLE_BYTE_AVAILABLE 
#define BOOST_LANG_STDC_AVAILABLE 
#define BOOST_OS_UNIX BOOST_VERSION_NUMBER_AVAILABLE
#define BOOST_OS_UNIX_AVAILABLE 
#define BOOST_PREDEF_TESTED_AT(symbol,major,minor,patch) ( (symbol) >= BOOST_VERSION_NUMBER_AVAILABLE )
#define BOOST_VERSION_NUMBER_AVAILABLE BOOST_VERSION_NUMBER_MIN

[rainerdeyke]

Can somebody make an executive decision here?

My opinion, FWIW:

wasm32 and wasm64 are architectures. The correct macros should be BOOST_ARCH_WASM32 and BOOST_ARCH_WASM64.

Emscripten is analogous to mingw: an emulation layer that emulates POSIX on a different platform (a web browser and Windows respectively), plus a compiler that targets that emulation layer. Since mingw is treated as a platform, so should Emscripten be. So the correct macro would be BOOST_PLAT_EMSCRIPTEN. Emscripten is based on clang, so BOOST_COMP_CLANG should also be detected.

Architecture and platform are orthogonal here: non-Emscripten clang can target the wasm architecture, and Emscripten can (or could) output Javascript instead of wasm.

If we're running in a web browser is the third not-quite-orthogonal concern, analogous to the OS. Emscripten can target node.js and standalone wasm runtimes in addition to targeting web browsers. However, this may not be possible to detect at compile time, because the same wasm files can run both in web browsers and standalone.

[grafikrobot]

@rainerdeyke I like that suggestion.

[grafikrobot]

@rainerdeyke although.. wondering if there's a difference between WASM32 and WASM64 that isn't covered by WASM, BOOST_ARCH_WORD_BITS_32, and BOOST_ARCH_WORD_BITS_64?

[rainerdeyke]

I'm mostly going by the precedent of having both BOOST_ARCH_X86_32 and BOOST_ARCH_X86_64. That said, there doesn't seem to be a clear policy here.

• For x86 there's the generic BOOST_ARCH_X86, plus BOOST_ARCH_X86_32 and BOOST_ARCH_X86_64.
• For PowerPC there's the generic BOOST_ARCH_PPC plus BOOST_ARCH_PPC_64, but no BOOST_ARCH_PPC_32.
• For Arm there's just BOOST_ARCH_ARM.

So this could go either way.