wasm: Implement `@wasmMemorySize()` builtin

This implements the `wasmMemorySize` builtin, in Sema and the Wasm backend.
The Stage2 implementation differs from stage1 in the way that `index` must be a comptime value.
The stage1 variant is incorrect, as the index is part of the instruction encoding, and therefore,
cannot be a runtime value.

3 files changed, 15 insertions, 3 deletions

diff --git a/src/arch/wasm/CodeGen.zig b/src/arch/wasm/CodeGen.zig
index 4f19aeca6b..0ea0418d7f 100644
--- a/src/arch/wasm/CodeGen.zig
+++ b/src/arch/wasm/CodeGen.zig
@@ -1671,6 +1671,8 @@ fn genInst(self: *Self, inst: Air.Inst.Index) !WValue {
         .wrap_errunion_payload => self.airWrapErrUnionPayload(inst),
         .wrap_errunion_err => self.airWrapErrUnionErr(inst),
 
+        .wasm_memory_size => self.airWasmMemorySize(inst),
+
         .add_sat,
         .sub_sat,
         .mul_sat,
@@ -3426,6 +3428,16 @@ fn airPrefetch(self: *Self, inst: Air.Inst.Index) InnerError!WValue {
     return WValue{ .none = {} };
 }
 
+fn airWasmMemorySize(self: *Self, inst: Air.Inst.Index) !WValue {
+    const ty_pl = self.air.instructions.items(.data)[inst].ty_pl;
+    const extra = self.air.extraData(Air.WasmMemoryIndex, ty_pl.payload).data;
+
+    const result = try self.allocLocal(Type.usize);
+    try self.addLabel(.memory_size, extra.index);
+    try self.addLabel(.local_set, result.local);
+    return result;
+}
+
 fn cmpOptionals(self: *Self, lhs: WValue, rhs: WValue, operand_ty: Type, op: std.math.CompareOperator) InnerError!WValue {
     assert(operand_ty.hasRuntimeBits());
     assert(op == .eq or op == .neq);
diff --git a/src/arch/wasm/Emit.zig b/src/arch/wasm/Emit.zig
index d787c46774..4432e19638 100644
--- a/src/arch/wasm/Emit.zig
+++ b/src/arch/wasm/Emit.zig
@@ -89,10 +89,10 @@ pub fn emitMir(emit: *Emit) InnerError!void {
             .local_set => try emit.emitLabel(tag, inst),
             .local_tee => try emit.emitLabel(tag, inst),
             .memory_grow => try emit.emitLabel(tag, inst),
+            .memory_size => try emit.emitLabel(tag, inst),
 
             // no-ops
             .end => try emit.emitTag(tag),
-            .memory_size => try emit.emitTag(tag),
             .@"return" => try emit.emitTag(tag),
             .@"unreachable" => try emit.emitTag(tag),
 
diff --git a/src/arch/wasm/Mir.zig b/src/arch/wasm/Mir.zig
index ed0867e583..04edb09dca 100644
--- a/src/arch/wasm/Mir.zig
+++ b/src/arch/wasm/Mir.zig
@@ -226,9 +226,9 @@ pub const Inst = struct {
         i64_store32 = 0x3E,
         /// Returns the memory size in amount of pages.
         ///
-        /// Uses `nop`
+        /// Uses `label`
         memory_size = 0x3F,
-        /// Increases the memory at by given number of pages.
+        /// Increases the memory by given number of pages.
         ///
         /// Uses `label`
         memory_grow = 0x40,