wasm2c: serialize types at wasm2c-time (#2120)

This makes wasm2c serialize each function type, rather than registering
function types at module-initialization time. The serialized function
type is the SHA-256 of the mangled param and result types (with a space
between params and results).

At runtime in call_indirect, a known (immediate) function type is
compared against the function type stored in a funcref structure. For
call_indirects to functions local to the module, or for any
call_indirect when the toolchain merges string constants across
compilation units (generally, GCC and clang), this can be done by
comparing the pointers to each function type. Otherwise, the actual
32-byte values are compared.

The function type IDs can be looked up at runtime with
`Z_[modname]_get_func_type`, which matches the API from
`wasm_rt_register_func_type`. A new `callback` example demos this.

wasm2c does the SHA-256 either by linking against libcrypto or, if not
available or if requested via `cmake -DUSE_INTERNAL_SHA256=ON`, by using
a vendored (header-only) PicoSHA2. There is no runtime dependency on
SHA-256 in the wasm2c runtime or generated modules.

This eliminates the last of the per-module state, so this commit also removes
the [modname]_init_module() function and the s_module_initialized bool.

1 files changed, 39 insertions, 0 deletions

diff --git a/wasm2c/examples/callback/main.c b/wasm2c/examples/callback/main.c
new file mode 100644
index 00000000..b0b37d46
--- /dev/null
+++ b/wasm2c/examples/callback/main.c
@@ -0,0 +1,39 @@
+#include <stdio.h>
+
+#include "callback.h"
+
+/*
+ * The callback function. Prints the null-terminated string at the given
+ * location in the instance's exported memory.
+ */
+void print(Z_callback_instance_t* instance, uint32_t ptr) {
+  puts(Z_callbackZ_memory(instance)->data + ptr);
+}
+
+int main(int argc, char** argv) {
+  /* Initialize the Wasm runtime. */
+  wasm_rt_init();
+
+  /* Instantiate the callback module. */
+  Z_callback_instance_t inst;
+  Z_callback_instantiate(&inst);
+
+  /*
+   * Call the module's "set_print_function" function, which takes a funcref to
+   * the callback. A funcref has three members: the function type (which can be
+   * looked up with "Z_callback_get_func_type"), a pointer to the function, and
+   * a module instance pointer that will be passed to the function when called.
+   */
+  wasm_rt_func_type_t fn_type = Z_callback_get_func_type(1, 0, WASM_RT_I32);
+  wasm_rt_funcref_t fn_ref = {fn_type, (wasm_rt_function_ptr_t)print, &inst};
+  Z_callbackZ_set_print_function(&inst, fn_ref);
+
+  /* "say_hello" uses the previously installed callback. */
+  Z_callbackZ_say_hello(&inst);
+
+  /* Free the module instance and the Wasm runtime state. */
+  Z_callback_free(&inst);
+  wasm_rt_free();
+
+  return 0;
+}