C++ 17 parallel algorithms

[afraidofdark]

I have checked from the library support "C++17 library features / Parallel algorithms and execution policies" are not implemented for Clang libc++ but GCC libstdc++ has them. However addign "-stdlib=libstdc++" flag to the compiler yields more errors. This is what I want to ask, can we switch to gcc libstdc++ with emscripten ? Is there a work around that enables me to use parrallel algorithms ?

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.29 (4307484)
clang version 16.0.0 (https://github.com/llvm/llvm-project 947d529e4194e0567cfbbea99127066f76c87269)
Target: wasm32-unknown-emscripten
Thread model: posix

Failing command line in full:
C:\Users\afrai\Documents\ToolKit\Web>emcmake cmake -DTK_CXX_EXTRA:STRING="-O3 -g" -S .. -G Ninja && ninja & pause
configure: cmake "-DTK_CXX_EXTRA:STRING=-O3 -g" -S .. -G Ninja -DCMAKE_TOOLCHAIN_FILE=C:\Users\afrai\OneDrive\Desktop\emscripten\emsdk\upstream\emscripten\cmake\Modules\Platform\Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR=C:/Users/afrai/OneDrive/Desktop/emscripten/emsdk/node/14.18.2_64bit/bin/node.exe;--experimental-wasm-bulk-memory;--experimental-wasm-threads
-- Configuring done
-- Generating done
-- Build files have been written to: C:/Users/afrai/Documents/ToolKit/Web
[1/2] Building CXX object ToolKit/CMakeFiles/ToolKit.dir/MathUtil.cpp.o
FAILED: ToolKit/CMakeFiles/ToolKit.dir/MathUtil.cpp.o
C:\Users\afrai\OneDrive\Desktop\emscripten\emsdk\upstream\emscripten\em++.bat -IC:/Users/afrai/Documents/ToolKit/ToolKit -IC:/Users/afrai/Documents/ToolKit/Dependency -IC:/Users/afrai/Documents/ToolKit/Dependency/OpenAL/include -IC:/Users/afrai/Documents/ToolKit/Dependency/RapidXml -IC:/Users/afrai/Documents/ToolKit/Dependency/stb -IC:/Users/afrai/Documents/ToolKit/Dependency/zlib-1.2.12/dist/include -std=c++17 -O3 -g -include stdafx.h -MD -MT ToolKit/CMakeFiles/ToolKit.dir/MathUtil.cpp.o -MF ToolKit\CMakeFiles\ToolKit.dir\MathUtil.cpp.o.d -o ToolKit/CMakeFiles/ToolKit.dir/MathUtil.cpp.o -c C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:426:16: error: no member named 'execution' in namespace 'std'; did you mean 'exception'?
std::execution::par_unseq,
~~~~~^~~~~~~~~
exception
C:\Users\afrai\OneDrive\Desktop\emscripten\emsdk\upstream\emscripten\cache\sysroot/include/c++/v1\exception:100:29: note: 'exception' declared here
class _LIBCPP_EXCEPTION_ABI exception
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:426:27: error: no member named 'par_unseq' in 'std::exception'
std::execution::par_unseq,
~~~~~~~~~~~~~~~~^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:438:45: error: variable 'vertexIndx' cannot be implicitly captured in a lambda with no capture-default specified
for (uint32_t vertexIndx = 0; vertexIndx < 3; vertexIndx++)
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:438:29: note: 'vertexIndx' declared here
for (uint32_t vertexIndx = 0; vertexIndx < 3; vertexIndx++)
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'vertexIndx' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, vertexIndx
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'vertexIndx' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &vertexIndx
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:438:61: error: variable 'vertexIndx' cannot be implicitly captured in a lambda with no capture-default specified
for (uint32_t vertexIndx = 0; vertexIndx < 3; vertexIndx++)
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:438:29: note: 'vertexIndx' declared here
for (uint32_t vertexIndx = 0; vertexIndx < 3; vertexIndx++)
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'vertexIndx' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, vertexIndx
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'vertexIndx' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &vertexIndx
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:440:17: error: variable 'positions' cannot be implicitly captured in a lambda with no capture-default specified
positions[vertexIndx] =
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:432:18: note: 'positions' declared here
Vec3 positions[3] = {face.vertices[0]->pos,
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'positions' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, positions
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'positions' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &positions
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:440:27: error: variable 'vertexIndx' cannot be implicitly captured in a lambda with no capture-default specified
positions[vertexIndx] =
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:438:29: note: 'vertexIndx' declared here
for (uint32_t vertexIndx = 0; vertexIndx < 3; vertexIndx++)
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'vertexIndx' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, vertexIndx
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'vertexIndx' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &vertexIndx
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:441:61: error: variable 'vertexIndx' cannot be implicitly captured in a lambda with no capture-default specified
CPUSkinning((SkinVertex*) face.vertices[vertexIndx],
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:438:29: note: 'vertexIndx' declared here
for (uint32_t vertexIndx = 0; vertexIndx < 3; vertexIndx++)
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'vertexIndx' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, vertexIndx
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'vertexIndx' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &vertexIndx
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:442:33: error: variable 'skinMesh' cannot be implicitly captured in a lambda with no capture-default specified
skinMesh->m_skeleton.get(),
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:437:25: note: 'skinMesh' declared here
SkinMesh* skinMesh = (SkinMesh*) mesh;
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'skinMesh' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, skinMesh
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'skinMesh' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &skinMesh
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:448:41: error: variable 'positions' cannot be implicitly captured in a lambda with no capture-default specified
positions[0],
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:432:18: note: 'positions' declared here
Vec3 positions[3] = {face.vertices[0]->pos,
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'positions' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, positions
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'positions' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &positions
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:449:41: error: variable 'positions' cannot be implicitly captured in a lambda with no capture-default specified
positions[1],
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:432:18: note: 'positions' declared here
Vec3 positions[3] = {face.vertices[0]->pos,
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'positions' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, positions
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'positions' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &positions
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:450:41: error: variable 'positions' cannot be implicitly captured in a lambda with no capture-default specified
positions[2],
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:432:18: note: 'positions' declared here
Vec3 positions[3] = {face.vertices[0]->pos,
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'positions' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, positions
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'positions' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &positions
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:451:41: error: variable 'dist' cannot be implicitly captured in a lambda with no capture-default specified
dist))
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:446:19: note: 'dist' declared here
float dist = FLT_MAX;
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'dist' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, dist
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'dist' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &dist
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:454:19: error: variable 'dist' cannot be implicitly captured in a lambda with no capture-default specified
if (dist < closestPickedDistance && t >= 0.0f)
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:446:19: note: 'dist' declared here
float dist = FLT_MAX;
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'dist' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, dist
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'dist' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &dist
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:456:41: error: variable 'dist' cannot be implicitly captured in a lambda with no capture-default specified
t = dist;
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:446:19: note: 'dist' declared here
float dist = FLT_MAX;
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'dist' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, dist
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'dist' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &dist
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:457:41: error: variable 'dist' cannot be implicitly captured in a lambda with no capture-default specified
closestPickedDistance = dist;
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:446:19: note: 'dist' declared here
float dist = FLT_MAX;
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:11: note: lambda expression begins here
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'dist' by value
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, dist
C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp:429:78: note: capture 'dist' by reference
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
^
, &dist
15 errors generated.
em++: error: 'C:/Users/afrai/OneDrive/Desktop/emscripten/emsdk/upstream/bin\clang++.exe -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -DEMSCRIPTEN -IC:\Users\afrai\OneDrive\Desktop\emscripten\emsdk\upstream\emscripten\cache\sysroot\include\SDL --sysroot=C:\Users\afrai\OneDrive\Desktop\emscripten\emsdk\upstream\emscripten\cache\sysroot -Xclang -iwithsysroot/include\compat -IC:/Users/afrai/Documents/ToolKit/ToolKit -IC:/Users/afrai/Documents/ToolKit/Dependency -IC:/Users/afrai/Documents/ToolKit/Dependency/OpenAL/include -IC:/Users/afrai/Documents/ToolKit/Dependency/RapidXml -IC:/Users/afrai/Documents/ToolKit/Dependency/stb -IC:/Users/afrai/Documents/ToolKit/Dependency/zlib-1.2.12/dist/include -std=c++17 -O3 -g3 -include stdafx.h -MD -MT ToolKit/CMakeFiles/ToolKit.dir/MathUtil.cpp.o -MF ToolKit\CMakeFiles\ToolKit.dir\MathUtil.cpp.o.d -c C:/Users/afrai/Documents/ToolKit/ToolKit/MathUtil.cpp -o ToolKit/CMakeFiles/ToolKit.dir/MathUtil.cpp.o' failed (returned 1)
ninja: build stopped: subcommand failed.

Full link command and output with -v appended:
Even for runtime issues it helps a lot if you can include the full link command.
Adding -v to the link command will show all of the sub-commands run which
can help us diagnose your issue.

[sbc100]

Could you share small sample of the code that fails to compile? It looks like std::execution::par_unseq not being defined but I can't see what includes were included. A small snippet would be useful.

it could be that std::execution::par_unseq is not defined when pthreads are disabled. Are you building with -pthread (or equivalently -sUSE_PTHREADS)?

[afraidofdark]

`
#include "MathUtil.h"

#include "Camera.h"
#include "Mesh.h"
#include "Node.h"
#include "ResourceComponent.h"
#include "Skeleton.h"

#include
#include
#include
#include

#include "DebugNew.h"

// ...... snipped

bool RayMeshIntersection(const Mesh* const mesh,
const Ray& ray,
float& t,
const SkeletonComponent* skelComp)
{
float closestPickedDistance = FLT_MAX;
bool hit = false;

// Intentionaly undefined for emscripten, but this is the part that fails without #ifndef
#ifndef EMSCRIPTEN
std::mutex updateHit;
std::for_each(
std::execution::par_unseq,
mesh->m_faces.begin(),
mesh->m_faces.end(),
[&updateHit, &t, &closestPickedDistance, &ray, &hit, skelComp, mesh](
const Face& face)
{
Vec3 positions[3] = {face.vertices[0]->pos,
face.vertices[1]->pos,
face.vertices[2]->pos};
if (skelComp != nullptr && mesh->IsSkinned())
{
SkinMesh* skinMesh = (SkinMesh*) mesh;
for (uint32_t vertexIndx = 0; vertexIndx < 3; vertexIndx++)
{
positions[vertexIndx] =
CPUSkinning((SkinVertex*) face.vertices[vertexIndx],
skinMesh->m_skeleton.get(),
skelComp->m_map);
}
}
float dist = FLT_MAX;
if (RayTriangleIntersection(ray,
positions[0],
positions[1],
positions[2],
dist))
{
std::lock_guardstd::mutex guard(updateHit);
if (dist < closestPickedDistance && t >= 0.0f)
{
t = dist;
closestPickedDistance = dist;
hit = true;
}
}
});
#else
for (const Face& face : mesh->m_faces)
{
float dist = FLT_MAX;
if (RayTriangleIntersection(ray,
face.vertices[0]->pos,
face.vertices[1]->pos,
face.vertices[2]->pos,
dist))
{
if (dist < closestPickedDistance && t >= 0.0f)
{
t = dist;
closestPickedDistance = dist;
hit = true;
}
}
}
#endif

return hit;

}
`

here is the full source file
https://github.com/Oyun-Teknolojileri/ToolKit/blob/master/ToolKit/MathUtil.cpp

I think, parallel algorithms are implemented using intel's thread building blocks (tbb) but I'll try the flag and post the results -sUSE_PTHREADS

[afraidofdark]

Using -pthread in the compiler flag also yielded the same results.
Can you share a small cpp file where parallel algorithms are used and compiles to emscripten ? Because I don't think its possible with Clang libc++

checkout https://en.cppreference.com/w/cpp/compiler_support/17
"Parallel algorithms and execution policies" section.

[sbc100]

Not quite quire I understand what you are saying but If something doesn't work with clang and libc++ it almost certainly won't work with emscripten, since emscripten is based on clang and libc++.

[pca006132]

just to note that intel tbb now supports emscripten, so we can probably compile parallel stl now

[afraidofdark]

I have checked the compiler support page again and saw that parallel execution policies are now partially supported. I have checked the code below to see if it compiles with clang v18 ( which is the verrsion my emscripten is using ) and it compiles fine. But em++ does not recognize the policies in the code.

Online compiler
Clang Support page Cpp 17

Compile command
em++ -std=c++17 -s WASM=1 -O3 -o output.js parallel.cpp

Code

#include <execution>
#include <algorithm>

int main()
{
    int a[] = {2,1};
    std::sort(std::execution::par_unseq, std::begin(a), std::end(a) );
    return a[0];
}

Error

parallel.cpp:7:20: error: no member named 'execution' in namespace 'std'; did you mean 'exception'?
    7 |     std::sort(std::execution::par_unseq, std::begin(a), std::end(a) );
      |               ~~~~~^~~~~~~~~
      |                    exception
C:\Users\Cihan\Documents\emsdk\upstream\emscripten\cache\sysroot/include/c++/v1\exception:153:29: note: 'exception'
      declared here
  153 | class _LIBCPP_EXCEPTION_ABI exception {
      |                             ^
parallel.cpp:7:31: error: no member named 'par_unseq' in 'std::exception'
    7 |     std::sort(std::execution::par_unseq, std::begin(a), std::end(a) );
      |               ~~~~~~~~~~~~~~~~^
2 errors generated.

Emscripten Version

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.46 (19607820c447a13fd8d0b7680c56148427d6e1b8)
clang version 18.0.0 (https://github.com/llvm/llvm-project 75501f53624de92aafce2f1da698b249a7293dc7)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: C:\Users\Cihan\Documents\emsdk\upstream\bin
PS C:\Users\Cihan\Desktop\xxx>

[sbc100]

This seems like an issue with libc++ rather than emscripten. If you add -stdlib=libc++ to the godbolt link it will fail with clang 17 too: https://gcc.godbolt.org/z/jf3fv9qdM

If you think this is bug then perhaps file a bug against libc++ in the llvm-project tracker.

[pca006132]

clang works if you use -fexperimental-library https://gcc.godbolt.org/z/W3cnbWrsz

[sbc100]

Adding -fexperimental-library also seems to fix the above example under emscripten. Which makes sense because emscripten and clang are both uses the same libc++.

[sbc100]

Given that emcc seems to have the same behaviour as clang here I think this issue can probably be closed?

[afraidofdark]

@sbc100 whats the command that builds the above code with emscripten ? The example code fails with the flag -fexperimental-library. And if the command works on your installment are you using the main branch ? Which version of emscripten compiles it ?

em++ -std=c++17 -fexperimental-library -s WASM=1 -O3 -o output.js parallel.cpp

parallel.cpp:7:20: error: no member named 'execution' in namespace 'std'; did you mean 'exception'?
    7 |     std::sort(std::execution::par_unseq, std::begin(a), std::end(a) );
      |               ~~~~~^~~~~~~~~
      |                    exception
C:\Users\Cihan\Documents\emsdk\upstream\emscripten\cache\sysroot/include/c++/v1\exception:153:29: note: 'exception'
      declared here
  153 | class _LIBCPP_EXCEPTION_ABI exception {
      |                             ^
parallel.cpp:7:31: error: no member named 'par_unseq' in 'std::exception'
    7 |     std::sort(std::execution::par_unseq, std::begin(a), std::end(a) );
      |               ~~~~~~~~~~~~~~~~^
2 errors generated.
em++: error: 'C:/Users/Cihan/Documents/emsdk/upstream/bin\clang++.exe -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -DEMSCRIPTEN --sysroot=C:\Users\Cihan\Documents\emsdk\upstream\emscripten\cache\sysroot -Xclang -iwithsysroot/include\fakesdl -Xclang -iwithsysroot/include\compat -std=c++17 -fexperimental-library -O3 parallel.cpp -c -o C:\Users\Cihan\AppData\Local\Temp\emscripten_temp_1nru5zo4\parallel_0.o' failed (returned 1)
PS C:\Users\Cihan\Desktop\xxx>

[afraidofdark]

I fallow the documentation libc++ and based on the information I have found there, I have updated my compile command like below but it still fails with emscripten.

em++ -stdlib=libc++ -std=c++17 -fexperimental-library -D_LIBCPP_ENABLE_EXPERIMENTAL -lc++experimental -s WASM=1 -O3 -o output.js parallel.cpp

I think headers are also different, neither of the below worked for me.

#include <experimental/xxx>
#include <experimental/execution>
#include <experimental/execution_policy>

If it suppose to compile, why not it compiles the example code with emscripten ? I'd like to keep digging and as a result we may update the documentation or its a configuration that needs to be added to emscripten.

[sbc100]

For me it works find with just ./em++ parallel.cc -c -fexperimental-library.

No need for any other flags. What version of emscripten are you using?

[sbc100]

We did recently update our libc++ version in #20707. That change currently only exists in the 3.1.50..

and I can confirm that with 3.1.49 or older that code doesn't compiler so I guess #20707 was the fix for this?

[afraidofdark]

Builds just fine with version 3.1.50 this ticket can be closed.