Implement the GC in Rust (#1750)

This implements the current garbage collector in Rust. No changes were made to
the GC design -- it's just ports the one implemented in code generator to Rust.

The goals are:

- Evaluate Rust for Motoko's RTS implementation
- Make the collector easier to read, understand, modify, and extend.

rts/Makefile

@@ -177,6 +177,7 @@ mo-rts.wasm: $(RTS_RUST_WASM_O) $(RTS_WASM_O) $(TOMMATH_WASM_O) $(MUSL_WASM_O)
 	$(WASM_LD) -o $@ \
 		--import-memory --shared --no-entry --gc-sections \
 		--export=__wasm_call_ctors \
+		--export=memcpy \
 		--whole-archive \
 		$+
 

rts/bigint.c

@@ -50,7 +50,7 @@ export void* mp_realloc(void *ptr, size_t old_size, size_t new_size) {
   if (new_size > FIELD(r, 1)) {
     void *newptr = mp_alloc(new_size);
     if (old_size != FIELD(r, 1)) bigint_trap();
-    as_memcpy(newptr, ptr, old_size);
+    memcpy(newptr, ptr, old_size);
     return newptr;
   } else if (new_size == FIELD(r, 1)) {
     // No need to grow

rts/motoko-rts/Cargo.toml

@@ -7,9 +7,19 @@ edition = "2018"
 [dependencies.libc]
 version = "0.2.73"
 
-# added here so that it ends up in Cargo.lock, so that nix will pre-fetch it
+# Added here so that it ends up in Cargo.lock, so that nix will pre-fetch it
 [dependencies.compiler_builtins]
 version = "0.1.32"
+# Without this feature we get dozens of duplicate symbol errors when generating
+# the final shared .wasm file:
+#
+#     wasm-ld: error: duplicate symbol: __multi3
+#     >>> defined in _build/wasm/libmotoko_rts.a(compiler_builtins-d709bd899857aa61.compiler_builtins.3abndchk-cgu.0.rcgu.o)
+#     >>> defined in _build/wasm/libmotoko_rts.a(compiler_builtins-06d1ead628e1f468.compiler_builtins.6moz1ltd-cgu.0.rcgu.o)
+#
+# It seems like we're linking multiple versions of compiler_builtins in the same
+# shared library, which we should fix at some point. TODO
+features = ["mangled-names"]
 
 [lib]
 crate-type = ["staticlib"]

rts/motoko-rts/Xargo.toml

@@ -3,5 +3,4 @@ stage = 0
 
 [dependencies.compiler_builtins]
 stage = 1
-features = [ "mem" ]
 version = "0.1.32"

rts/motoko-rts/src/alloc.rs

@@ -0,0 +1,41 @@
+//! Implements allocation routines used by the generated code and the GC.
+
+use core::arch::wasm32;
+
+use crate::gc;
+use crate::rts_trap_with;
+use crate::types::{skew, Bytes, SkewedPtr, Words};
+
+#[no_mangle]
+unsafe extern "C" fn alloc_bytes(n: Bytes<u32>) -> SkewedPtr {
+    alloc_words(n.to_words())
+}
+
+#[no_mangle]
+unsafe extern "C" fn alloc_words(n: Words<u32>) -> SkewedPtr {
+    let bytes = n.to_bytes();
+    // Update ALLOCATED
+    gc::ALLOCATED += Bytes(bytes.0 as u64);
+
+    // Update heap pointer
+    let old_hp = gc::HP;
+    let new_hp = old_hp + bytes.0;
+    gc::HP = new_hp;
+
+    // Grow memory if needed
+    grow_memory(new_hp as usize);
+
+    skew(old_hp as usize)
+}
+
+/// Page allocation. Ensures that the memory up to the given pointer is allocated.
+pub(crate) unsafe fn grow_memory(ptr: usize) {
+    let total_pages_needed = ((ptr / 65536) + 1) as i32;
+    let current_pages = wasm32::memory_size(0) as i32;
+    let new_pages_needed = total_pages_needed - current_pages;
+    if new_pages_needed > 0 {
+        if wasm32::memory_grow(0, new_pages_needed as usize) == core::usize::MAX {
+            rts_trap_with("Cannot grow memory\0".as_ptr());
+        }
+    }
+}