docs: add a WebAssembly entry

The doc briefly describes the state of WASM support
for user-defined functions.

docs/design-notes/wasm.md

@@ -0,0 +1,209 @@
+# WASM support for user-defined functions
+
+This document describes the details of WASM language support in user-defined functions (UDF). `wasm` is one of the possible languages to implement these functions in, aside `lua`.
+
+## Experimental status
+
+Before the design of WebAssembly integration and ABI is finalized, it's only available in experimental mode.
+User-defined functions are already experimental at the time of this writing, but in order to be ready
+for backward incompatible changes, the language accepted by CQL is currently named "xwasm".
+Once the ABI is set in stone, it should be changed to "wasm".
+
+## Supported types
+
+Due to the limitations imposed by WebAssembly specification, the following types can be natively supported with CQL:
+ - int
+ - bigint
+ - smallint
+ - tinyint
+ - bool
+ - float
+ - double
+
+The rest of CQL types (text, date, timestamp, etc.) are implemented by putting their serialized representation into wasm module memory
+and passing each parameter as a pointer to a struct of the form:
+```c
+{
+    int32_t size;
+    char buf[0];
+}
+```
+
+## Support for NULL values
+
+Native WebAssembly types can only be represented directly if the function does not operate on NULL values. Fortunately, user-defined functions
+explicitly specify whether they accept NULL or not.
+
+If the function is specified not to accept NULL, all parameters are represented
+as in the description above.
+
+If the function is specified to accept NULL, each parameter should be represented in WebAssembly as a struct,
+which starts with its size, followed by a serialized form explained in the paragraph above, i.e.
+```c
+{
+    int32_t size;
+    char buf[0];
+}
+```
+
+the important distinction is that size equal to -1 (minus one)indicates that the value is NULL and should not be parsed.
+
+## Return values
+
+NOTE: ABI for return values is experimental and subject to change. It can (and should) be redesigned
+after implementing helper libraries for a few popular languages (including C++, C, Rust).
+
+Natively supported types are returned as is. All the other types are returned via memory, similarly to the way
+they are passed as parameters: the wasm function should return the serialized form of the returned value,
+preceded by its size:
+```c
+{
+    int32_t size;
+    char buf[0];
+}
+```
+
+Currently, returning NULL values is possible only for functions declared to be `CALLED ON NULL INPUT`.
+For such functions, the return value is always expected to be presented in the serialized form (which
+allows representing nulls), even for types natively supported by WebAssembly.
+The decision is experimental and it was done in order to allow returning some values as native WebAssembly types
+without having to allocate memory for them and serialize them first.
+Alternative ways of expressing whether a function can **return** null should be considered - perhaps
+as CQL syntax extension.
+
+## How to generate a correct wasm UDF source code
+
+Scylla accepts UDF's source code in WebAssembly text format - also known as `wat`. The source can use and define whatever's needed for execution, including multiple helper functions and symbols. The only requirement for it to be accepted as correct UDF source is that the WebAssembly module exports a symbol with the same name as the function, and this symbol's type is indeed a function with correct signature.
+
+UDF's source code can be, naturally, simply coded by hand in wat. It is not often very convenient to program directly in assembly, so here are a few tips.
+
+### Compiling from Rust to wasm
+
+#### C
+
+Clang is capable of compiling C source code to wasm and it also supports useful built-ins
+for using wasm-specific interfaces, like `__builtin_wasm_memory_size` and `__builtin_wasm_memory_grow`
+for memory management.
+
+Example source code:
+
+```c
+struct __attribute__((packed)) nullable_bigint { 
+    int size;
+    long long v;
+};
+
+static long long swap_int64(long long val) { 
+    val = ((val << 8) & 0xFF00FF00FF00FF00ULL ) | ((val >> 8) & 0x00FF00FF00FF00FFULL );
+    val = ((val << 16) & 0xFFFF0000FFFF0000ULL ) | ((val >> 16) & 0x0000FFFF0000FFFFULL );
+    return (val << 32) | ((val >> 32) & 0xFFFFFFFFULL);
+}
+
+long long fib_aux(long long n) { 
+    if (n < 2) { 
+        return n;
+    } 
+    return fib_aux(n-1) + fib_aux(n-2);
+}
+
+struct nullable_bigint* fib(struct nullable_bigint* p) { 
+    // Initialize memory for the return struct
+    struct nullable_bigint* ret = (struct nullable_bigint*)__builtin_wasm_memory_size(0);
+    __builtin_wasm_memory_grow(0, sizeof(struct nullable_bigint));
+
+    ret->size = sizeof(long long);
+    if (p->size == -1) { 
+        ret->v = swap_int64(42);
+    } else { 
+        ret->v = swap_int64(fib_aux(swap_int64(p->v)));
+    } 
+    return ret;
+}
+```
+
+Compilation instructions:
+```bash
+ clang -O2 --target=wasm32 --no-standard-libraries -Wl,--export-all -Wl,--no-entry fibnull.c -o fibnull.wasm
+ wasm2wat fibnull.wasm > fibnull.wat
+```
+
+#### Rust
+
+Rust ecosystem exposes a rather convenient way of generating WebAssembly, with the help of `cargo wasi` and `wasm_bindgen`.
+
+As a short example, here's a sample Rust code which can be compiled to WebAssembly:
+```rust
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+pub fn fib(n: i32) -> i32 {
+    if n < 2 {
+        n
+    } else {
+        fib(n - 1) + fib(n - 2)
+    }
+}
+```
+
+A more detailed guide and examples can be found here:
+https://bytecodealliance.github.io/cargo-wasi/hello-world.html
+https://rustwasm.github.io/wasm-bindgen/
+
+### Generating wat from wasm
+
+For those who want to use precompiled WASM modules, it's enough to translate WASM bytecode to `wat` representation. On Linux, it can be achieved by a `wasm2wat` tool, available in most distributions in the `wabt` package.
+
+## Example
+
+Here's how a `wasm` function can be declared:
+
+```cql
+CREATE FUNCTION ks.fib (input bigint) RETURNS NULL ON NULL INPUT RETURNS bigint LANGUAGE xwasm
+AS '(module
+  (func $fib (param $n i64) (result i64)
+    (if
+      (i64.lt_s (local.get $n) (i64.const 2))
+      (return (local.get $n))
+    )
+    (i64.add
+      (call $fib (i64.sub (local.get $n) (i64.const 1)))
+      (call $fib (i64.sub (local.get $n) (i64.const 2)))
+    )
+  )
+  (export "fib" (func $fib))
+)'
+```
+
+and it can be invoked just like a regular UDF:
+```cql
+scylla@cqlsh:ks> CREATE TABLE t(id int, n bigint, PRIMARY KEY(id,n));
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 0);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 1);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 2);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 3);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 4);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 5);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 6);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 7);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 8);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 9);
+scylla@cqlsh:ks> INSERT INTO t(id, n) VALUES (0, 10);
+scylla@cqlsh:ks> SELECT n, ks.fib(n) FROM t;
+
+ n  | ks.fib(n)
+----+-----------
+  0 |         0
+  1 |         1
+  2 |         1
+  3 |         2
+  4 |         3
+  5 |         5
+  6 |         8
+  7 |        13
+  8 |        21
+  9 |        34
+ 10 |        55
+
+(11 rows)
+```
+