build : enable atomics for wasm-emscripten

When targetting wasm32-unknown-emscripten, we need to have the -pthread option during
compilation for all the objects that use atomic instructions.

When -pthread option is used on a object that has atomic instructions, a section is added
to the binary notifying the linker that it is safe to use shared linear memory because
atomic instructions were not stripped (default behavior if the option is not set).

cf. https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md#target-features-section

Before this patch, lib.c and thread_task.c were using atomic instructions but since -pthread was not
passed during compilation a section was added to the binary, disallowing shared linear memory
and preventing linking dav1d with a multi threaded wasm application.

It also renames stdatomic_dependency to stdatomic_dependencies, for clarity because it is now
an array of dependency objects.

cf. https://mesonbuild.com/Reference-manual.html#dependency-object

1 files changed, 8 insertions, 3 deletions

diff --git a/meson.build b/meson.build
index 334d8c2..8a9d4c6 100644
--- a/meson.build
+++ b/meson.build
@@ -173,16 +173,16 @@ libm_dependency = cc.find_library('m', required: false)
 
 # Header checks
 
-stdatomic_dependency = []
+stdatomic_dependencies = []
 if not cc.check_header('stdatomic.h')
     if cc.get_id() == 'msvc'
         # we have a custom replacement for MSVC
-        stdatomic_dependency = declare_dependency(
+        stdatomic_dependencies += declare_dependency(
             include_directories : include_directories('include/compat/msvc'),
         )
     elif cc.compiles('''int main() { int v = 0; return __atomic_fetch_add(&v, 1, __ATOMIC_SEQ_CST); }''',
                      name : 'GCC-style atomics', args : test_args)
-        stdatomic_dependency = declare_dependency(
+        stdatomic_dependencies += declare_dependency(
             include_directories : include_directories('include/compat/gcc'),
         )
     else
@@ -190,6 +190,11 @@ if not cc.check_header('stdatomic.h')
     endif
 endif
 
+if host_machine.cpu_family().startswith('wasm')
+    # enable atomics + bulk-memory features
+    stdatomic_dependencies += thread_dependency.partial_dependency(compile_args: true)
+endif
+
 if cc.check_header('unistd.h')
     cdata.set('HAVE_UNISTD_H', 1)
 endif