examples/buffer.html

@@ -38,6 +38,8 @@ WebAssembly.instantiateStreaming(fetch('buffer.wasm'), importObject)
     let offset = stdlib_types___alloc_bytes__(size)
     var i8arr = new Uint8Array(app.instance.exports.memory.buffer, offset + 4, size);
 
+    log('len(i8arr) = ' + app.instance.exports.length(offset));
+
     //Fill it
     for (var i = 0; i < size; i++) {
       i8arr[i] = i + 5;

examples/buffer.py

@@ -1,3 +1,7 @@
 @exported
 def index(inp: bytes, idx: i32) -> i32:
     return inp[idx]
+
+@exported
+def length(inp: bytes) -> i32:
+    return len(inp)

phasm/compiler.py

@@ -231,7 +231,7 @@ def expression(inp: ourlang.Expression) -> Statements:
 
         yield from expression(inp.varref)
         yield from expression(inp.index)
-        yield wasm.Statement('call', '$___access_bytes_index___')
+        yield wasm.Statement('call', '$stdlib.types.__subscript_bytes__')
         return
 
     if isinstance(inp, ourlang.AccessStructMember):
@@ -381,7 +381,7 @@ def module(inp: ourlang.Module) -> wasm.Module:
         stdlib_alloc.__find_free_block__,
         stdlib_alloc.__alloc__,
         stdlib_types.__alloc_bytes__,
-        _generate____access_bytes_index___(inp),
+        stdlib_types.__subscript_bytes__,
     ] + [
         function(x)
         for x in inp.functions.values()
@@ -413,38 +413,6 @@ def _generate____new_reference___(mod: ourlang.Module) -> wasm.Function:
         ],
     )
 
-def _generate____access_bytes_index___(mod: ourlang.Module) -> wasm.Function:
-    return wasm.Function(
-        '___access_bytes_index___',
-        None,
-        [
-            ('byt', type_(mod.types['i32']), ),
-            ('ofs', type_(mod.types['i32']), ),
-        ],
-        [
-        ],
-        type_(mod.types['i32']),
-        [
-            wasm.Statement('local.get', '$ofs'),
-            wasm.Statement('local.get', '$byt'),
-            i32.load(),
-            wasm.Statement('i32.lt_u'),
-
-            wasm.Statement('if', comment='$ofs < len($byt)'),
-            wasm.Statement('local.get', '$byt'),
-            wasm.Statement('i32.const', '4', comment='Leading size field'),
-            wasm.Statement('i32.add'),
-            wasm.Statement('local.get', '$ofs'),
-            wasm.Statement('i32.add'),
-            wasm.Statement('i32.load8_u', comment='Within bounds'),
-            wasm.Statement('return'),
-            wasm.Statement('end'),
-
-            wasm.Statement('i32.const', str(0), comment='Out of bounds'),
-            wasm.Statement('return'),
-        ],
-    )
-
 def _generate_tuple_constructor(inp: ourlang.TupleConstructor) -> Statements:
     yield wasm.Statement('i32.const', str(inp.tuple.alloc_size()))
     yield wasm.Statement('call', '$___new_reference___')

phasm/stdlib/alloc.py

@@ -113,7 +113,7 @@ def __alloc__(g: Generator, alloc_size: i32) -> i32:
     g.local.get(alloc_size)
     g.i32.store()
 
-    # Return address of the allocated bytes
+    # Return address of the allocated memory, skipping the allocator header
     g.local.get(result)
     g.i32.const(4) # Header size
     g.i32.add()

phasm/stdlib/types.py

@@ -30,3 +30,37 @@ def __alloc_bytes__(g: Generator, length: i32) -> i32:
     g.local.get(result)
 
     return i32('return') # To satisfy mypy
+
+@func_wrapper()
+def __subscript_bytes__(g: Generator, adr: i32, ofs: i32) -> i32:
+    """
+    Returns an index from a bytes value
+
+    If ofs is more than the length of the bytes, this
+    function returns 0, following the 'no undefined behaviour'
+    philosophy.
+
+    adr   i32    The pointer for the allocated bytes
+    ofs   i32    The offset within the allocated bytes
+    """
+    g.local.get(ofs)
+    g.local.get(adr)
+    g.i32.load()
+    g.i32.lt_u()
+
+    with g.if_():
+        # The offset is less than the length
+
+        g.local.get(adr)
+        g.i32.const(4) # Bytes header
+        g.i32.add()
+        g.local.get(ofs)
+        g.i32.add()
+        g.i32.load8_u()
+        g.return_()
+
+    # The offset is outside the allocated bytes
+    g.i32.const(0)
+    g.return_()
+
+    return i32('return') # To satisfy mypy

phasm/wasmgenerator.py

@@ -30,9 +30,11 @@ class Generator_i32:
         # irelop
         self.eq = functools.partial(self.generator.add, 'i32.eq')
         self.ne = functools.partial(self.generator.add, 'i32.ne')
+        self.lt_u = functools.partial(self.generator.add, 'i32.lt_u')
 
         # 2.4.4. Memory Instructions
         self.load = functools.partial(self.generator.add, 'i32.load')
+        self.load8_u = functools.partial(self.generator.add, 'i32.load8_u')
         self.store = functools.partial(self.generator.add, 'i32.store')
 
     def const(self, value: int, comment: Optional[str] = None) -> None: