[spec/interpreter/test] Implement basic tail-call proposal

Author: rossberg

document/core/binary/instructions.rst

@@ -33,6 +33,8 @@ Control Instructions
 .. _binary-return:
 .. _binary-call:
 .. _binary-call_indirect:
+.. _binary-return_call:
+.. _binary-return_call_indirect:
 
 .. math::
    \begin{array}{llclll}
@@ -54,15 +56,17 @@ Control Instructions
        &\Rightarrow& \BRTABLE~l^\ast~l_N \\ &&|&
      \hex{0F} &\Rightarrow& \RETURN \\ &&|&
      \hex{10}~~x{:}\Bfuncidx &\Rightarrow& \CALL~x \\ &&|&
-     \hex{11}~~x{:}\Btypeidx~~\hex{00} &\Rightarrow& \CALLINDIRECT~x \\
+     \hex{11}~~x{:}\Btypeidx~~\hex{00} &\Rightarrow& \CALLINDIRECT~x \\ &&|&
+     \hex{12}~~x{:}\Bfuncidx &\Rightarrow& \RETURNCALL~x \\ &&|&
+     \hex{13}~~x{:}\Btypeidx~~\hex{00} &\Rightarrow& \RETURNCALLINDIRECT~x \\
    \end{array}
 
 .. note::
    The |ELSE| opcode :math:`\hex{05}` in the encoding of an |IF| instruction can be omitted if the following instruction sequence is empty.
 
 .. note::
    In future versions of WebAssembly, the zero byte occurring in the encoding
-   of the |CALLINDIRECT| instruction may be used to index additional tables.
+   of the |CALLINDIRECT| and |RETURNCALLINDIRECT| instructions may be used to index additional tables.
 
 .. index:: value type, polymorphism
    pair: binary format; instruction

document/core/exec/instructions.rst

@@ -934,6 +934,82 @@ Control Instructions
    \end{array}
 
 
+.. _exec-return_call:
+
+:math:`\RETURNCALL~x`
+.....................
+
+1. Let :math:`F` be the :ref:`current <exec-notation-textual>` :ref:`frame <syntax-frame>`.
+
+2. Assert: due to :ref:`validation <valid-call>`, :math:`F.\AMODULE.\MIFUNCS[x]` exists.
+
+3. Let :math:`a` be the :ref:`function address <syntax-funcaddr>` :math:`F.\AMODULE.\MIFUNCS[x]`.
+
+4. :ref:`Tail-invoke <exec-return-invoke>` the function instance at address :math:`a`.
+
+
+.. math::
+   \begin{array}{lcl@{\qquad}l}
+   (\RETURNCALL~x) &\stepto& (\RETURNINVOKE~a)
+     & (\iff \CALL~x \stepto \INVOKE~a)
+   \end{array}
+
+
+.. _exec-return_call_indirect:
+
+:math:`\RETURNCALLINDIRECT~x`
+.............................
+
+1. Let :math:`F` be the :ref:`current <exec-notation-textual>` :ref:`frame <syntax-frame>`.
+
+2. Assert: due to :ref:`validation <valid-call_indirect>`, :math:`F.\AMODULE.\MITABLES[0]` exists.
+
+3. Let :math:`\X{ta}` be the :ref:`table address <syntax-tableaddr>` :math:`F.\AMODULE.\MITABLES[0]`.
+
+4. Assert: due to :ref:`validation <valid-call_indirect>`, :math:`S.\STABLES[\X{ta}]` exists.
+
+5. Let :math:`\X{tab}` be the :ref:`table instance <syntax-tableinst>` :math:`S.\STABLES[\X{ta}]`.
+
+6. Assert: due to :ref:`validation <valid-call_indirect>`, :math:`F.\AMODULE.\MITYPES[x]` exists.
+
+7. Let :math:`\X{ft}_{\F{expect}}` be the :ref:`function type <syntax-functype>` :math:`F.\AMODULE.\MITYPES[x]`.
+
+8. Assert: due to :ref:`validation <valid-call_indirect>`, a value with :ref:`value type <syntax-valtype>` |I32| is on the top of the stack.
+
+9. Pop the value :math:`\I32.\CONST~i` from the stack.
+
+10. If :math:`i` is not smaller than the length of :math:`\X{tab}.\TIELEM`, then:
+
+    a. Trap.
+
+11. If :math:`\X{tab}.\TIELEM[i]` is uninitialized, then:
+
+    a. Trap.
+
+12. Let :math:`a` be the :ref:`function address <syntax-funcaddr>` :math:`\X{tab}.\TIELEM[i]`.
+
+13. Assert: due to :ref:`validation <valid-call_indirect>`, :math:`S.\SFUNCS[a]` exists.
+
+14. Let :math:`\X{f}` be the :ref:`function instance <syntax-funcinst>` :math:`S.\SFUNCS[a]`.
+
+15. Let :math:`\X{ft}_{\F{actual}}` be the :ref:`function type <syntax-functype>` :math:`\X{f}.\FITYPE`.
+
+16. If :math:`\X{ft}_{\F{actual}}` and :math:`\X{ft}_{\F{expect}}` differ, then:
+
+    a. Trap.
+
+17. :ref:`Tail-invoke <exec-return-invoke>` the function instance at address :math:`a`.
+
+
+.. math::
+   \begin{array}{lcl@{\qquad}l}
+   (\RETURNCALLINDIRECT~x) &\stepto& (\RETURNINVOKE~a)
+     & (\iff \CALLINDIRECT~x \stepto \INVOKE~a) \\
+   (\RETURNCALLINDIRECT~x) &\stepto& \TRAP
+     & (\iff \CALLINDIRECT~x \stepto \TRAP) \\
+   \end{array}
+
+
 .. index:: instruction, instruction sequence, block
 .. _exec-instr-seq:
 
@@ -1044,6 +1120,42 @@ Invocation of :ref:`function address <syntax-funcaddr>` :math:`a`
    \end{array}
 
 
+.. _exec-return-invoke:
+
+Tail-invocation of :ref:`function address <syntax-funcaddr>` :math:`a`
+......................................................................
+
+1. Assert: due to :ref:`validation <valid-call>`, :math:`S.\SFUNCS[a]` exists.
+
+2. Let :math:`[t_1^m] \to [t_2^n]` be the :ref:`function type <syntax-functype>` :math:`S.\SFUNCS[a].\FITYPE`.
+
+3. Assert: due to :ref:`validation <valid-return_call>`, there are at least :math:`m` values on the top of the stack.
+
+4. Pop the results :math:`\val^m` from the stack.
+
+5. Assert: due to :ref:`validation <valid-return_call>`, the stack contains at least one :ref:`frame <syntax-frame>`.
+
+6. While the top of the stack is not a frame, do:
+
+   a. Pop the top element from the stack.
+
+7. Assert: the top of the stack is a frame.
+
+8. Pop the frame from the stack.
+
+9. Push :math:`\val^m` to the stack.
+
+10. :ref:`Invoke <exec-invoke>` the function instance at address :math:`a`.
+
+.. math::
+   ~\\[-1ex]
+   \begin{array}{lcl@{\qquad}l}
+    S; \FRAME_n\{F\}~B^*[\val^m~(\RETURNINVOKE~a)]~\END &\stepto&
+      \val^m~(\INVOKE~a)
+      & (\iff S.\SFUNCS[a].\FITYPE = [t_1^m] \to [t_2^n])
+   \end{array}
+
+
 .. _exec-invoke-exit:
 
 Returning from a function

document/core/exec/runtime.rst

@@ -443,6 +443,7 @@ In order to express the reduction of :ref:`traps <trap>`, :ref:`calls <syntax-ca
      \dots \\ &&|&
      \TRAP \\ &&|&
      \INVOKE~\funcaddr \\ &&|&
+     \RETURNINVOKE~\funcaddr \\ &&|&
      \INITELEM~\tableaddr~\u32~\funcidx^\ast \\ &&|&
      \INITDATA~\memaddr~\u32~\byte^\ast \\ &&|&
      \LABEL_n\{\instr^\ast\}~\instr^\ast~\END \\ &&|&
@@ -454,6 +455,7 @@ Traps are bubbled up through nested instruction sequences, ultimately reducing t
 
 The |INVOKE| instruction represents the imminent invocation of a :ref:`function instance <syntax-funcinst>`, identified by its :ref:`address <syntax-funcaddr>`.
 It unifies the handling of different forms of calls.
+Analogously, |RETURNINVOKE| represents the imminent tail invocation of a function instance.
 
 The |INITELEM| and |INITDATA| instructions perform initialization of :ref:`element <syntax-elem>` and :ref:`data <syntax-data>` segments during module :ref:`instantiation <exec-instantiation>`.
 

document/core/syntax/instructions.rst

@@ -301,7 +301,9 @@ Instructions in this group affect the flow of control.
      \BRTABLE~\vec(\labelidx)~\labelidx \\&&|&
      \RETURN \\&&|&
      \CALL~\funcidx \\&&|&
-     \CALLINDIRECT~\typeidx \\
+     \CALLINDIRECT~\typeidx \\&&|&
+     \RETURNCALL~\funcidx \\&&|&
+     \RETURNCALLINDIRECT~\typeidx \\
    \end{array}
 
 The |NOP| instruction does nothing.
@@ -344,9 +346,13 @@ The |CALLINDIRECT| instruction calls a function indirectly through an operand in
 Since tables may contain function elements of heterogeneous type |ANYFUNC|,
 the callee is dynamically checked against the :ref:`function type <syntax-functype>` indexed by the instruction's immediate, and the call aborted with a :ref:`trap <trap>` if it does not match.
 
+The |RETURNCALL| and |RETURNCALLINDIRECT| instructions are *tail-call* variants of the previous ones.
+That is, they first return from the current function before actually performing the respective call.
+It is guaranteed that no sequence of nested calls using only these instructions can cause resource exhaustion due to hitting an :ref:`implementation's limit <impl-exec>` on the number of active calls.
+
 .. note::
    In the current version of WebAssembly,
-   |CALLINDIRECT| implicitly operates on :ref:`table <syntax-table>` :ref:`index <syntax-tableidx>` :math:`0`.
+   |CALLINDIRECT| and |RETURNCALLINDIRECT| implicitly operate on :ref:`table <syntax-table>` :ref:`index <syntax-tableidx>` :math:`0`.
    This restriction may be lifted in future versions.
 
 

document/core/text/instructions.rst

@@ -83,6 +83,8 @@ The same label identifier may optionally be repeated after the corresponding :ma
 .. _text-return:
 .. _text-call:
 .. _text-call_indirect:
+.. _text-return_call:
+.. _text-return_call_indirect:
 
 All other control instruction are represented verbatim.
 
@@ -98,6 +100,9 @@ All other control instruction are represented verbatim.
      \text{return} &\Rightarrow& \RETURN \\ &&|&
      \text{call}~~x{:}\Tfuncidx_I &\Rightarrow& \CALL~x \\ &&|&
      \text{call\_indirect}~~x,I'{:}\Ttypeuse_I &\Rightarrow& \CALLINDIRECT~x
+       & (\iff I' = \{\}) \\ &&|&
+     \text{return\_call}~~x{:}\Tfuncidx_I &\Rightarrow& \RETURNCALL~x \\ &&|&
+     \text{return\_call\_indirect}~~x,I'{:}\Ttypeuse_I &\Rightarrow& \RETURNCALLINDIRECT~x
        & (\iff I' = \{\}) \\
    \end{array}
 

document/core/util/macros.def

@@ -313,6 +313,8 @@
 .. |RETURN| mathdef:: \xref{syntax/instructions}{syntax-instr-control}{\K{return}}
 .. |CALL| mathdef:: \xref{syntax/instructions}{syntax-instr-control}{\K{call}}
 .. |CALLINDIRECT| mathdef:: \xref{syntax/instructions}{syntax-instr-control}{\K{call\_indirect}}
+.. |RETURNCALL| mathdef:: \xref{syntax/instructions}{syntax-instr-control}{\K{return\_call}}
+.. |RETURNCALLINDIRECT| mathdef:: \xref{syntax/instructions}{syntax-instr-control}{\K{return\_call\_indirect}}
 
 .. |DROP| mathdef:: \xref{syntax/instructions}{syntax-instr-parametric}{\K{drop}}
 .. |SELECT| mathdef:: \xref{syntax/instructions}{syntax-instr-parametric}{\K{select}}
@@ -865,6 +867,7 @@
 
 .. |TRAP| mathdef:: \xref{exec/runtime}{syntax-trap}{\K{trap}}
 .. |INVOKE| mathdef:: \xref{exec/runtime}{syntax-invoke}{\K{invoke}}
+.. |RETURNINVOKE| mathdef:: \xref{exec/runtime}{syntax-return_invoke}{\K{return\_invoke}}
 .. |INITELEM| mathdef:: \xref{exec/runtime}{syntax-init_elem}{\K{init\_elem}}
 .. |INITDATA| mathdef:: \xref{exec/runtime}{syntax-init_data}{\K{init\_data}}
 

document/core/valid/instructions.rst

@@ -681,6 +681,69 @@ Control Instructions
    }
 
 
+.. _valid-return_call:
+
+:math:`\RETURNCALL~x`
+.....................
+
+* The return type :math:`C.\CRETURN` must not be empty in the context.
+
+* The function :math:`C.\CFUNCS[x]` must be defined in the context.
+
+* Let :math:`[t_1^\ast] \to [t_2^?]` be the :ref:`function type <syntax-functype>` :math:`C.\CFUNCS[x]`.
+
+* The :ref:`result type <syntax-resulttype>` must be the same as :math:`C.\CRETURN`.
+
+* Then the instruction is valid with type :math:`[t_3^\ast~t_1^\ast] \to [t_4^\ast]`, for any sequences of :ref:`value types <syntax-valtype>` :math:`t_3^\ast` and :math:`t_4^\ast`.
+
+.. math::
+   \frac{
+     C.\CFUNCS[x] = [t_1^\ast] \to [t_2^?]
+     \qquad
+     C.\CRETURN = [t_2^?]
+   }{
+     C \vdashinstr \CALL~x : [t_3^\ast~t_1^\ast] \to [t_4^\ast]
+   }
+
+.. note::
+   The |RETURNCALL| instruction is :ref:`stack-polymorphic <polymorphism>`.
+
+
+.. _valid-return_call_indirect:
+
+:math:`\RETURNCALLINDIRECT~x`
+.............................
+
+* The return type :math:`C.\CRETURN` must not be empty in the context.
+
+* The table :math:`C.\CTABLES[0]` must be defined in the context.
+
+* Let :math:`\limits~\elemtype` be the :ref:`table type <syntax-tabletype>` :math:`C.\CTABLES[0]`.
+
+* The :ref:`element type <syntax-elemtype>` :math:`\elemtype` must be |ANYFUNC|.
+
+* The type :math:`C.\CTYPES[x]` must be defined in the context.
+
+* Let :math:`[t_1^\ast] \to [t_2^?]` be the :ref:`function type <syntax-functype>` :math:`C.\CTYPES[x]`.
+
+* Then the instruction is valid with type :math:`[t_3^\ast~t_1^\ast~\I32] \to [t_4^\ast]`, for any sequences of :ref:`value types <syntax-valtype>` :math:`t_3^\ast` and :math:`t_4^\ast`.
+
+
+.. math::
+   \frac{
+     C.\CTABLES[0] = \limits~\ANYFUNC
+     \qquad
+     C.\CTYPES[x] = [t_1^\ast] \to [t_2^?]
+     \qquad
+     C.\CRETURN = [t_2^?]
+   }{
+     C \vdashinstr \CALLINDIRECT~x : [t_3^\ast~t_1^\ast~\I32] \to [t_4^\ast]
+   }
+
+.. note::
+   The |RETURNCALLINDIRECT| instruction is :ref:`stack-polymorphic <polymorphism>`.
+
+
 .. index:: instruction, instruction sequence
 .. _valid-instr-seq:
 

interpreter/README.md

@@ -214,6 +214,8 @@ op:
   br_if <var>
   br_table <var>+
   return
+  return_call <var>
+  return_call_indirect <func_type>
   call <var>
   call_indirect <func_type>
   drop

interpreter/binary/decode.ml

@@ -246,8 +246,13 @@ let rec instr s =
     let x = at var s in
     expect 0x00 s "zero flag expected";
     call_indirect x
+  | 0x12 -> return_call (at var s)
+  | 0x13 ->
+    let x = at var s in
+    expect 0x00 s "zero flag expected";
+    return_call_indirect x
 
-  | 0x12 | 0x13 | 0x14 | 0x15 | 0x16 | 0x17 | 0x18 | 0x19 as b -> illegal s pos b
+  | 0x14 | 0x15 | 0x16 | 0x17 | 0x18 | 0x19 as b -> illegal s pos b
 
   | 0x1a -> drop
   | 0x1b -> select

interpreter/binary/encode.ml

@@ -157,6 +157,8 @@ let encode m =
       | Return -> op 0x0f
       | Call x -> op 0x10; var x
       | CallIndirect x -> op 0x11; var x; u8 0x00
+      | ReturnCall x -> op 0x12; var x
+      | ReturnCallIndirect x -> op 0x13; var x; u8 0x00
 
       | Drop -> op 0x1a
       | Select -> op 0x1b