compilers/mixins/emscripten: Implement thread support

Emscripten has pthread support (as well as C++ threads), but we don't
currently implement them. This fixes that by adding the necessary code.
The one thing I'm not sure about is setting the pool size. The docs
suggest that you really want to do this to ensure that your code works
correctly, but the number should really be configurable, not sure how to
set that.

Fixes #6684

2 files changed, 26 insertions, 14 deletions

diff --git a/docs/markdown/Builtin-options.md b/docs/markdown/Builtin-options.md
index 067966f..ac40263 100644
--- a/docs/markdown/Builtin-options.md
+++ b/docs/markdown/Builtin-options.md
@@ -134,20 +134,22 @@ The following options are available. Note that both the options themselves and
 the possible values they can take will depend on the target platform or
 compiler being used:
 
-| Option       | Default value | Possible values                          | Description |
-| ------       | ------------- | ---------------                          | ----------- |
-| c_args       |               | free-form comma-separated list           | C compile arguments to use |
-| c_link_args  |               | free-form comma-separated list           | C link arguments to use |
-| c_std        | none          | none, c89, c99, c11, c17, c18, gnu89, gnu99, gnu11, gnu17, gnu18 | C language standard to use |
-| c_winlibs    | see below     | free-form comma-separated list           | Standard Windows libs to link against |
-| cpp_args     |               | free-form comma-separated list           | C++ compile arguments to use |
-| cpp_link_args|               | free-form comma-separated list           | C++ link arguments to use |
-| cpp_std      | none          | none, c++98, c++03, c++11, c++14, c++17, <br/>c++1z, gnu++03, gnu++11, gnu++14, gnu++17, gnu++1z, <br/> vc++14, vc++17, vc++latest | C++ language standard to use |
-| cpp_debugstl | false         | true, false                              | C++ STL debug mode |
-| cpp_eh       | default       | none, default, a, s, sc                  | C++ exception handling type |
-| cpp_rtti     | true          | true, false                              | Whether to enable RTTI (runtime type identification) |
-| cpp_winlibs  | see below     | free-form comma-separated list           | Standard Windows libs to link against |
-| fortran_std  | none          | [none, legacy, f95, f2003, f2008, f2018] | Fortran language standard to use |
+| Option           | Default value | Possible values                          | Description |
+| ------           | ------------- | ---------------                          | ----------- |
+| c_args           |               | free-form comma-separated list           | C compile arguments to use |
+| c_link_args      |               | free-form comma-separated list           | C link arguments to use |
+| c_std            | none          | none, c89, c99, c11, c17, c18, gnu89, gnu99, gnu11, gnu17, gnu18 | C language standard to use |
+| c_winlibs        | see below     | free-form comma-separated list           | Standard Windows libs to link against |
+| c_thread_count   | 4             | integer value ≥ 0                        | Number of threads to use with emcc when using threads |
+| cpp_args         |               | free-form comma-separated list           | C++ compile arguments to use |
+| cpp_link_args    |               | free-form comma-separated list           | C++ link arguments to use |
+| cpp_std          | none          | none, c++98, c++03, c++11, c++14, c++17, <br/>c++1z, gnu++03, gnu++11, gnu++14, gnu++17, gnu++1z, <br/> vc++14, vc++17, vc++latest | C++ language standard to use |
+| cpp_debugstl     | false         | true, false                              | C++ STL debug mode |
+| cpp_eh           | default       | none, default, a, s, sc                  | C++ exception handling type |
+| cpp_rtti         | true          | true, false                              | Whether to enable RTTI (runtime type identification) |
+| cpp_thread_count | 4             | integer value ≥ 0                        | Number of threads to use with emcc when using threads |
+| cpp_winlibs      | see below     | free-form comma-separated list           | Standard Windows libs to link against |
+| fortran_std      | none          | [none, legacy, f95, f2003, f2008, f2018] | Fortran language standard to use |
 
 The default values of `c_winlibs` and `cpp_winlibs` are in compiler-specific
 argument forms, but the libraries are: kernel32, user32, gdi32, winspool,
@@ -162,3 +164,7 @@ while the `cpp_eh=[value]` will result in `/EH[value]`.
 Since *0.51.0* `cpp_eh=default` will result in `/EHsc` on MSVC. When using
 gcc-style compilers, nothing is passed (allowing exceptions to work), while
 `cpp_eh=none` passes `-fno-exceptions`.
+
+Since *0.54.0* The `<lang>_thread_count` option can be used to control the
+value passed to `-s PTHREAD_POOL_SIZE` when using emcc. No other c/c++
+compiler supports this option.
diff --git a/docs/markdown/snippets/emscripten_threads.md b/docs/markdown/snippets/emscripten_threads.md
new file mode 100644
index 0000000..bdf808e
--- /dev/null
+++ b/docs/markdown/snippets/emscripten_threads.md
@@ -0,0 +1,6 @@
+## Emscripten (emcc) now supports threads
+
+In addition to properly setting the compile and linker arguments, a new meson
+builtin has been added to control the PTHREAD_POOL_SIZE option,
+`-D<lang>_thread_count`, which may be set to any integer value greater than 0.
+If it set to 0 then the PTHREAD_POOL_SIZE option will not be passed.