wasm-link: Implement indirect function table

The function table contains all function pointers that are called
by using call_indirect. During codegen, we create a relocation
where the linker will resolve the correct index into the table and stores
this value within the data section at the location of the pointer.

3 files changed, 39 insertions, 4 deletions

diff --git a/src/arch/wasm/CodeGen.zig b/src/arch/wasm/CodeGen.zig
index 3fac50ca7f..cbda80aee3 100644
--- a/src/arch/wasm/CodeGen.zig
+++ b/src/arch/wasm/CodeGen.zig
@@ -1065,9 +1065,16 @@ fn airCall(self: *Self, inst: Air.Inst.Index) InnerError!WValue {
     const pl_op = self.air.instructions.items(.data)[inst].pl_op;
     const extra = self.air.extraData(Air.Call, pl_op.payload);
     const args = self.air.extra[extra.end..][0..extra.data.args_len];
+    const ty = self.air.typeOf(pl_op.operand);
 
-    const target: *Decl = blk: {
-        const func_val = self.air.value(pl_op.operand).?;
+    const fn_ty = switch (ty.zigTypeTag()) {
+        .Fn => ty,
+        .Pointer => ty.childType(),
+        else => unreachable,
+    };
+
+    const target: ?*Decl = blk: {
+        const func_val = self.air.value(pl_op.operand) orelse break :blk null;
 
         if (func_val.castTag(.function)) |func| {
             break :blk func.data.owner_decl;
@@ -1082,9 +1089,24 @@ fn airCall(self: *Self, inst: Air.Inst.Index) InnerError!WValue {
         try self.emitWValue(arg_val);
     }
 
-    try self.addLabel(.call, target.link.wasm.sym_index);
+    if (target) |direct| {
+        try self.addLabel(.call, direct.link.wasm.sym_index);
+    } else {
+        // in this case we call a function pointer
+        // so load its value onto the stack
+        std.debug.assert(ty.zigTypeTag() == .Pointer);
+        const operand = self.resolveInst(pl_op.operand);
+        const result = try self.load(operand, fn_ty, operand.local_with_offset.offset);
+        try self.addLabel(.local_get, result.local);
+
+        var fn_type = try self.genFunctype(fn_ty);
+        defer fn_type.deinit(self.gpa);
+
+        const fn_type_index = try self.bin_file.putOrGetFuncType(fn_type);
+        try self.addLabel(.call_indirect, fn_type_index);
+    }
 
-    const ret_ty = target.ty.fnReturnType();
+    const ret_ty = fn_ty.fnReturnType();
     switch (ret_ty.zigTypeTag()) {
         .Void, .NoReturn => return WValue.none,
         else => {
diff --git a/src/arch/wasm/Emit.zig b/src/arch/wasm/Emit.zig
index d01f319b91..c4368ab943 100644
--- a/src/arch/wasm/Emit.zig
+++ b/src/arch/wasm/Emit.zig
@@ -47,6 +47,7 @@ pub fn emitMir(emit: *Emit) InnerError!void {
 
             // relocatables
             .call => try emit.emitCall(inst),
+            .call_indirect => try emit.emitCallIndirect(inst),
             .global_get => try emit.emitGlobal(tag, inst),
             .global_set => try emit.emitGlobal(tag, inst),
             .memory_address => try emit.emitMemAddress(inst),
@@ -276,6 +277,13 @@ fn emitCall(emit: *Emit, inst: Mir.Inst.Index) !void {
     });
 }
 
+fn emitCallIndirect(emit: *Emit, inst: Mir.Inst.Index) !void {
+    const label = emit.mir.instructions.items(.data)[inst].label;
+    try emit.code.append(std.wasm.opcode(.call_indirect));
+    try leb128.writeULEB128(emit.code.writer(), @as(u32, 0)); // TODO: Emit relocation for table index
+    try leb128.writeULEB128(emit.code.writer(), label);
+}
+
 fn emitMemAddress(emit: *Emit, inst: Mir.Inst.Index) !void {
     const symbol_index = emit.mir.instructions.items(.data)[inst].label;
     try emit.code.append(std.wasm.opcode(.i32_const));
diff --git a/src/arch/wasm/Mir.zig b/src/arch/wasm/Mir.zig
index 97d8875984..5891893f33 100644
--- a/src/arch/wasm/Mir.zig
+++ b/src/arch/wasm/Mir.zig
@@ -69,6 +69,11 @@ pub const Inst = struct {
         ///
         /// Uses `label`
         call = 0x10,
+        /// Calls a function pointer by its function signature
+        /// and index into the function table.
+        ///
+        /// Uses `label`
+        call_indirect = 0x11,
         /// Loads a local at given index onto the stack.
         ///
         /// Uses `label`