Add wasm2c tool (#710)

Add `wasm2c`, a new tool that reads a `.wasm` file and generates a C
source file and its accompanying header file. The C output currently
only supports gcc/clang compilers, since it uses builtins for some
functionality.

The resulting C code is not standalone; there are runtime functions that
must be provided, as well as pointers to all imports.

The C runtime symbols that must be provided are as follows:

* `void wasm_rt_trap(wasm_rt_trap_t code)`:
  Called when the WebAssembly code traps. This function must not return.
* `u32 wasm_rt_register_func_type(u32 param_count, u32 result_count, ...)`:
  Register a function type with the given signature. This function must
  check whether this signature has already been registered and return
  the original index.
* `void wasm_rt_allocate_memory(wasm_rt_memory_t*, u32 initial, u32 max)`:
  Allocate the memory buffer for the given memory object, given the
  number of pages. The memory must be zeroed before returning.
* `u32 wasm_rt_grow_memory(wasm_rt_memory_t*, u32 delta)`:
  Grow memory by the given number of pages. If allocation fails, or the
  new pages size is larger than the maximum, return -1. Otherwise return
  the previous number of pages. The newly allocated memory must be
  zeroed.
* `void wasm_rt_allocate_table(wasm_rt_table_t*, u32 initial, u32 max)`:
  Allocate the buffer for the given table object. The buffer must be
  zeroed before returning.
* `u32 wasm_rt_call_stack_depth`:
  A symbol that tracks the current call stack depth. If this value
  exceeds `WASM_RT_MAX_CALL_STACK_DEPTH` then a trap occurs. This value
  defaults to 500, but can redefined.

An example implementation can be found in `spec-wasm2c-prefix.c`.

All functionality from the WebAssembly MVP is supported, and the
generated code passes all of the core spec tests. There is a new test
tool called `run-spec-wasm2c.py` which runs the following:

* `wast2json` to convert the spec test to json and wasm files
* `wasm2c` to convert the wasm to C source and headers
* a C compiler (default `cc`) to compile and link all C source files,
  including a C test runner (`spec-wasm2c-prefix.c`)
* Finally, the resulting executable to produce output

1 files changed, 5 insertions, 1 deletions

diff --git a/test/find_exe.py b/test/find_exe.py
index 5bc919a6..8bc19d41 100644
--- a/test/find_exe.py
+++ b/test/find_exe.py
@@ -26,7 +26,7 @@ REPO_ROOT_DIR = os.path.dirname(SCRIPT_DIR)
 EXECUTABLES = [
     'wat2wasm', 'wast2json', 'wasm2wat', 'wasm-objdump', 'wasm-interp',
     'wasm-opcodecnt', 'wat-desugar', 'wasm-link', 'spectest-interp',
-    'wasm-validate',
+    'wasm-validate', 'wasm2c',
 ]
 
 
@@ -103,3 +103,7 @@ def GetWatDesugarExecutable(override=None):
 
 def GetWasmValidateExecutable(override=None):
   return FindExecutable('wasm-validate', override)
+
+
+def GetWasm2CExecutable(override=None):
+  return FindExecutable('wasm2c', override)