add makefile platform for emscripten

Makefile.libretro

@@ -41,7 +41,17 @@ else ifeq ($(platform), android)
    fpic := -fPIC
    SHARED :=  -Wl,--fix-cortex-a8 -llog -lz -shared -Wl,--version-script=$(LIBRETRO_DIR)/link.T -Wl,--no-undefined
    PLATFLAGS := -DAND -DLSB_FIRST -DALIGN_DWORD
-
+else ifeq ($(platform), emscripten)
+   pthread=0
+   TARGET := $(TARGET_NAME)_libretro_$(platform).bc
+   EXTERNAL_ZLIB=1
+   STATIC_LINKING=1
+   STATIC_LINKING_LINK=1
+   ifneq ($(pthread),0)
+   SHARED := -lpthread
+   CFLAGS += -pthread
+   endif
+
 # Classic Platforms ####################
 # Platform affix = classic_<ISA>_<µARCH>
 # Help at https://modmyclassic.com/comp

libretro/libretro-common/libco/emscripten_fiber.c

@@ -0,0 +1,64 @@
+/*
+  libco.emscripten (2020-02-27)
+  authors: Toad King
+  license: public domain
+*/
+
+#define LIBCO_C
+#include <libco.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <malloc.h>
+#include <emscripten/fiber.h>
+
+#define ASYNCIFY_STACK_SIZE (12582912)
+
+static thread_local emscripten_fiber_t *co_active_;
+
+static void co_thunk(void *coentry)
+{
+   ((void (*)(void))coentry)();
+}
+
+static void co_init(void)
+{
+   if (!co_active_)
+   {
+      emscripten_fiber_t *co_primary = calloc(1, sizeof(emscripten_fiber_t));
+      void *asyncify_stack = malloc(ASYNCIFY_STACK_SIZE);
+
+      emscripten_fiber_init_from_current_context(co_primary, asyncify_stack, ASYNCIFY_STACK_SIZE);
+      co_active_ = co_primary;
+   }
+}
+
+cothread_t co_active(void)
+{
+   co_init();
+   return co_active_;
+}
+
+cothread_t co_create(unsigned int stacksize, void (*coentry)(void))
+{
+   co_init();
+
+   emscripten_fiber_t *fiber = calloc(1, sizeof(emscripten_fiber_t));
+   void *asyncify_stack = malloc(ASYNCIFY_STACK_SIZE);
+   void *c_stack = memalign(16, stacksize);
+   emscripten_fiber_init(fiber, co_thunk, coentry, c_stack, stacksize, asyncify_stack, ASYNCIFY_STACK_SIZE);
+
+   return (cothread_t)fiber;
+}
+
+void co_delete(cothread_t cothread)
+{
+   free(cothread);
+}
+
+void co_switch(cothread_t cothread)
+{
+   emscripten_fiber_t *old_fiber = co_active_;
+   co_active_ = (emscripten_fiber_t *)cothread;
+
+   emscripten_fiber_swap(old_fiber, co_active_);
+}

libretro/libretro-common/libco/libco.c

@@ -9,7 +9,9 @@ void *genode_alloc_secondary_stack(unsigned long stack_size);
 void genode_free_secondary_stack(void *stack);
 #endif
 
-#if defined _MSC_VER
+#ifdef EMSCRIPTEN
+    #include "emscripten_fiber.c"
+#elif defined _MSC_VER
   #include <Windows.h>
   #if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP)
     #include "fiber.c"