WebAssembly

Simplifying WebAssembly C Memory Management by Using Clang Builtin's

Following on from my
previous post
regarding passing arrays between JavaScript
and WebAssembly, I have found out how to remove the need to call back into JavaScript

Prerequisites

These examples have been tested with nodejs v12.9.1,
clang 10.0.0,
and wabt 1.0.16 on Ubuntu 18.04.

I installed clang in /opt/clang, wabt in /opt/wabt, and I use
nodenv to manage my nodejs environment.

Update your path.

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export PATH=/opt/clang/bin:/opt/wabt/bin:$PATH

What was the problem?

In order to find the size of the memory and grow it, I was having to import
functions from JavaScript. I now find there are two built in functions that
do exactly what I want:

  • __builtin_wasm_memory_size(0)
  • __builtin_wasm_memory_grow(0, blocks)

This means my growMoreMemory function now looks like this.

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#define BLKSIZ 65536

static header_t* getMoreMemory(unsigned bytes_required)
{
  // We need to add the header to the bytes required.
  bytes_required += sizeof(header_t);

  // The memory gets delivered in blocks. Ensure we get enough.
  unsigned int blocks = bytes_required / BLKSIZ;
  if (blocks * BLKSIZ < bytes_required)
    blocks += 1;
  unsigned int start_of_new_memory = __builtin_wasm_memory_size(0) * BLKSIZ;

  if (__builtin_wasm_memory_grow(0, blocks) == -1)
  	return NULL;

  long end_of_new_memory = __builtin_wasm_memory_size(0) * BLKSIZ;

  // Create the block to insert.
  header_t* block_to_insert = (header_t *) start_of_new_memory;
  block_to_insert->value.size = end_of_new_memory - start_of_new_memory - sizeof(header_t);
  block_to_insert->value.next = NULL;

  // add to the free list
  freeMemory((void *) (block_to_insert + 1));

  return free_list;
}

The code

You can find the code here.