ocaml/asmcomp/arm/emit.mlp

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(***********************************************************************)
(* *)
(* Objective Caml *)
(* *)
(* Xavier Leroy, projet Cristal, INRIA Rocquencourt *)
(* *)
(* Copyright 1998 Institut National de Recherche en Informatique et *)
(* en Automatique. All rights reserved. This file is distributed *)
(* under the terms of the Q Public License version 1.0. *)
(* *)
(***********************************************************************)
(* $Id$ *)
(* Emission of ARM assembly code *)
open Misc
open Cmm
open Arch
open Proc
open Reg
open Mach
open Linearize
open Emitaux
(* Tradeoff between code size and code speed *)
let fastcode_flag = ref true
(* Output a label *)
let emit_label lbl =
emit_string ".L"; emit_int lbl
(* Output a symbol *)
let emit_symbol s =
Emitaux.emit_symbol '$' s
(* Output a pseudo-register *)
let emit_reg r =
match r.loc with
Reg r -> emit_string (register_name r)
| _ -> fatal_error "Emit_arm.emit_reg"
(* Output the next register after the given pseudo-register *)
let emit_next_reg r =
match r.loc with
Reg r -> emit_string (register_name(r + 1))
| _ -> fatal_error "Emit_arm.emit_next_reg"
(* Layout of the stack frame *)
let stack_offset = ref 0
let frame_size () =
!stack_offset +
4 * num_stack_slots.(0) + 8 * num_stack_slots.(1) +
(if !contains_calls then 4 else 0)
let slot_offset loc cl =
match loc with
Incoming n -> frame_size() + n
| Local n ->
if cl = 0
then !stack_offset + num_stack_slots.(1) * 8 + n * 4
else !stack_offset + n * 8
| Outgoing n -> n
(* Output a stack reference *)
let emit_stack r =
match r.loc with
Stack s ->
let ofs = slot_offset s (register_class r) in `[sp, #{emit_int ofs}]`
| _ -> fatal_error "Emit_arm.emit_stack"
(* Output an addressing mode *)
let emit_addressing addr r n =
match addr with
Iindexed ofs ->
`[{emit_reg r.(n)}, #{emit_int ofs}]`
(* Record live pointers at call points *)
type frame_descr =
{ fd_lbl: int; (* Return address *)
fd_frame_size: int; (* Size of stack frame *)
fd_live_offset: int list } (* Offsets/regs of live addresses *)
let frame_descriptors = ref([] : frame_descr list)
let record_frame live =
let lbl = new_label() in
let live_offset = ref [] in
Reg.Set.iter
(function
{typ = Addr; loc = Reg r} ->
live_offset := (r lsl 1) + 1 :: !live_offset
| {typ = Addr; loc = Stack s} as reg ->
live_offset := slot_offset s (register_class reg) :: !live_offset
| _ -> ())
live;
frame_descriptors :=
{ fd_lbl = lbl;
fd_frame_size = frame_size();
fd_live_offset = !live_offset } :: !frame_descriptors;
`{emit_label lbl}:`
let emit_frame fd =
` .word {emit_label fd.fd_lbl} + 4\n`;
` .short {emit_int fd.fd_frame_size}\n`;
` .short {emit_int (List.length fd.fd_live_offset)}\n`;
List.iter
(fun n ->
` .short {emit_int n}\n`)
fd.fd_live_offset;
` .align 2\n`
(* Names of various instructions *)
let name_for_comparison = function
Isigned Ceq -> "eq" | Isigned Cne -> "ne" | Isigned Cle -> "le"
| Isigned Cge -> "ge" | Isigned Clt -> "lt" | Isigned Cgt -> "gt"
| Iunsigned Ceq -> "eq" | Iunsigned Cne -> "ne" | Iunsigned Cle -> "ls"
| Iunsigned Cge -> "cs" | Iunsigned Clt -> "cc" | Iunsigned Cgt -> "hi"
let name_for_float_comparison cmp neg =
match cmp with
Ceq -> if neg then "ne" else "eq"
| Cne -> if neg then "eq" else "ne"
| Cle -> if neg then "hi" else "ls"
| Cge -> if neg then "lt" else "ge"
| Clt -> if neg then "pl" else "mi"
| Cgt -> if neg then "le" else "gt"
let name_for_int_operation = function
Iadd -> "add"
| Isub -> "sub"
| Imul -> "mul"
| Iand -> "and"
| Ior -> "orr"
| Ixor -> "eor"
| _ -> assert false
let name_for_shift_operation = function
Ilsl -> "lsl"
| Ilsr -> "lsr"
| Iasr -> "asr"
| _ -> assert false
let name_for_shift_int_operation = function
Ishiftadd -> "add"
| Ishiftsub -> "sub"
| Ishiftsubrev -> "rsb"
let name_for_float_operation = function
Inegf -> "mnfd"
| Iabsf -> "absd"
| Iaddf -> "adfd"
| Isubf -> "sufd"
| Imulf -> "mufd"
| Idivf -> "dvfd"
| Ifloatofint -> "fltd"
| Iintoffloat -> "fixz"
| _ -> assert false
(* Recognize immediate operands *)
(* Immediate operands are 8-bit immediate values, zero-extended, and rotated
right by 0, 2, 4, ... 30 bits.
We check only with 8-bit values shifted left 0 to 24 bits. *)
let rec is_immed n shift =
shift <= 24 &&
(Nativeint.logand n (Nativeint.shift (Nativeint.from 0xFF) shift) = n ||
is_immed n (shift + 2))
let is_immediate n = is_immed n 0
(* Emit a non-immediate integer constant *)
let emit_complex_intconst r n =
let i = ref n in
let shift = ref 0 in
let first = ref true in
let ninstr = ref 0 in
while Nativeint.sign !i <> 0 do
if Nativeint.to_int (Nativeint.shift !i (- !shift)) land 3 = 0 then
shift := !shift + 2
else begin
let mask = Nativeint.shift (Nativeint.from 0xFF) !shift in
let bits = Nativeint.logand !i mask in
if !first
then ` mov {emit_reg r}, #{emit_nativeint bits} @ {emit_nativeint n}\n`
else ` add {emit_reg r}, {emit_reg r}, #{emit_nativeint bits}\n`;
first := false;
shift := !shift + 8;
i := Nativeint.sub !i bits;
incr ninstr
end
done;
!ninstr
(* Name of current function *)
let function_name = ref ""
(* Entry point for tail recursive calls *)
let tailrec_entry_point = ref 0
(* Table of symbols referenced *)
let symbol_constants = (Hashtbl.create 11 : (string, int) Hashtbl.t)
(* Table of floating-point literals *)
let float_constants = (Hashtbl.create 11 : (string, int) Hashtbl.t)
(* Total space (in word) occupied by pending literals *)
let num_literals = ref 0
(* True if we've at least one pending float literal *)
let pending_float = ref false
(* Label a symbol or float constant *)
let label_constant tbl s size =
try
Hashtbl.find tbl s
with Not_found ->
let lbl = new_label() in
Hashtbl.add tbl s lbl;
num_literals := !num_literals + size;
lbl
(* Emit all pending constants *)
let emit_constants () =
Hashtbl.iter
(fun s lbl ->
`{emit_label lbl}: .word {emit_symbol s}\n`)
symbol_constants;
Hashtbl.iter
(fun s lbl ->
`{emit_label lbl}: .double {emit_string s}\n`)
float_constants;
Hashtbl.clear symbol_constants;
Hashtbl.clear float_constants;
num_literals := 0;
pending_float := false
(* Output the assembly code for an instruction *)
let emit_instr i =
match i.desc with
Lend -> 0
| Lop(Imove | Ispill | Ireload) ->
let src = i.arg.(0) and dst = i.res.(0) in
if src.loc = dst.loc then 0 else begin
match (src, dst) with
{loc = Reg rs; typ = Int|Addr}, {loc = Reg rd; typ = Int|Addr} ->
` mov {emit_reg dst}, {emit_reg src}\n`; 1
| {loc = Reg rs; typ = Float}, {loc = Reg rd; typ = Float} ->
` mvfd {emit_reg dst}, {emit_reg src}\n`; 1
| {loc = Reg rs; typ = Float}, {loc = Reg rd; typ = Int|Addr} ->
` stfd {emit_reg src}, [sp, #-8]!\n`;
` ldmfd sp!, \{{emit_reg dst}, {emit_next_reg dst}}\n`; 2
| {loc = Reg rs; typ = Int|Addr}, {loc = Stack sd} ->
` str {emit_reg src}, {emit_stack dst}\n`; 1
| {loc = Reg rs; typ = Float}, {loc = Stack sd} ->
` stfd {emit_reg src}, {emit_stack dst}\n`; 1
| {loc = Stack ss; typ = Int|Addr}, {loc = Reg rd} ->
` ldr {emit_reg dst}, {emit_stack src}\n`; 1
| {loc = Stack ss; typ = Float}, {loc = Reg rd} ->
` ldfd {emit_reg dst}, {emit_stack src}\n`; 1
| _ ->
assert false
end
| Lop(Iconst_int n) ->
let r = i.res.(0) in
let nr = Nativeint.logxor n (Nativeint.from(-1)) in
if is_immediate n then begin
` mov {emit_reg r}, #{emit_nativeint n}\n`; 1
end else if is_immediate nr then begin
` mvn {emit_reg r}, #{emit_nativeint nr}\n`; 1
end else
emit_complex_intconst r n
| Lop(Iconst_float s) ->
if float_of_string s = 0.0 then
` mvfd {emit_reg i.res.(0)}, #0.0\n`
else begin
let lbl = label_constant float_constants s 2 in
pending_float := true;
` ldfd {emit_reg i.res.(0)}, {emit_label lbl} @ {emit_string s}\n`
end;
1
| Lop(Iconst_symbol s) ->
let lbl = label_constant symbol_constants s 1 in
` ldr {emit_reg i.res.(0)}, {emit_label lbl} @ {emit_symbol s}\n`; 1
| Lop(Icall_ind) ->
` mov lr, pc\n`;
`{record_frame i.live} mov pc, {emit_reg i.arg.(0)}\n`; 2
| Lop(Icall_imm s) ->
`{record_frame i.live} bl {emit_symbol s}\n`; 1
| Lop(Itailcall_ind) ->
let n = frame_size() in
if !contains_calls then
` ldr lr, [sp, #{emit_int (n-4)}]\n`;
if n > 0 then
` add sp, sp, #{emit_int n}\n`;
` mov pc, {emit_reg i.arg.(0)}\n`; 3
| Lop(Itailcall_imm s) ->
if s = !function_name then begin
` b {emit_label !tailrec_entry_point}\n`; 1
end else begin
let n = frame_size() in
if !contains_calls then
` ldr lr, [sp, #{emit_int (n-4)}]\n`;
if n > 0 then
` add sp, sp, #{emit_int n}\n`;
` b {emit_symbol s}\n`; 3
end
| Lop(Iextcall(s, alloc)) ->
if alloc then begin
let lbl = label_constant symbol_constants s 1 in
` ldr r10, {emit_label lbl} @ {emit_symbol s}\n`;
`{record_frame i.live} bl caml_c_call\n`; 2
end else begin
` bl {emit_symbol s}\n`; 1
end
| Lop(Istackoffset n) ->
if n >= 0 then
` sub sp, sp, #{emit_int n}\n`
else
` add sp, sp, #{emit_int (-n)}\n`;
stack_offset := !stack_offset + n;
1
| Lop(Iload(size, addr)) ->
let r = i.res.(0) in
let instr =
match size with
Byte_unsigned -> "ldrb"
| Byte_signed -> "ldrsb"
| Sixteen_unsigned -> "ldrh"
| Sixteen_signed -> "ldrsh"
| Word ->
begin match r.typ with
Int | Addr -> "ldr"
| Float -> "ldfd"
end in
` {emit_string instr} {emit_reg r}, {emit_addressing addr i.arg 0}\n`;
1
| Lop(Istore(size, addr)) ->
let r = i.arg.(0) in
let instr =
match size with
Byte_unsigned | Byte_signed -> "strb"
| Sixteen_unsigned | Sixteen_signed -> "strh"
| Word ->
begin match r.typ with
Int | Addr -> "str"
| Float -> "stfd"
end in
` {emit_string instr} {emit_reg r}, {emit_addressing addr i.arg 1}\n`;
1
| Lop(Ialloc n) ->
let nn = Nativeint.from n in
if !fastcode_flag then begin
if is_immediate nn then begin
` ldr r10, [alloc_limit, #0]\n`;
` sub alloc_ptr, alloc_ptr, #{emit_int n}\n`
end else begin
ignore(emit_complex_intconst (phys_reg 8 (*r10*)) nn);
` sub alloc_ptr, alloc_ptr, r10\n`;
` ldr r10, [alloc_limit, #0]\n`
end;
` cmp alloc_ptr, r10\n`;
`{record_frame i.live} blcc caml_call_gc\n`;
` add {emit_reg i.res.(0)}, alloc_ptr, #4\n`; 5
end else if n = 8 || n = 12 || n = 16 then begin
`{record_frame i.live} bl caml_alloc{emit_int ((n-4)/4)}\n`;
` add {emit_reg i.res.(0)}, alloc_ptr, #4\n`; 2
end else begin
if is_immediate nn then
` mov r10, #{emit_int n}\n`
else
ignore(emit_complex_intconst (phys_reg 8 (*r10*)) nn);
`{record_frame i.live} bl caml_alloc\n`;
` add {emit_reg i.res.(0)}, alloc_ptr, #4\n`; 3
end
| Lop(Iintop(Ilsl | Ilsr | Iasr as op)) ->
let shift = name_for_shift_operation op in
` mov {emit_reg i.res.(0)}, {emit_reg i.arg.(0)}, {emit_string shift} {emit_reg i.arg.(1)}\n`; 1
| Lop(Iintop(Icomp cmp)) ->
let comp = name_for_comparison cmp in
` cmp {emit_reg i.arg.(0)}, {emit_reg i.arg.(1)}\n`;
` mov {emit_reg i.res.(0)}, #0\n`;
` mov{emit_string comp} {emit_reg i.res.(0)}, #1\n`; 3
| Lop(Iintop(Icheckbound)) ->
` cmp {emit_reg i.arg.(0)}, {emit_reg i.arg.(1)}\n`;
` blls caml_array_bound_error\n`; 2
| Lop(Iintop op) ->
let instr = name_for_int_operation op in
` {emit_string instr} {emit_reg i.res.(0)}, {emit_reg i.arg.(0)}, {emit_reg i.arg.(1)}\n`; 1
| Lop(Iintop_imm(Idiv, n)) -> (* n is a power of 2 *)
let l = Misc.log2 n in
let r = i.res.(0) in
` movs {emit_reg r}, {emit_reg i.arg.(0)}\n`;
if n <= 256 then
` addlt {emit_reg r}, {emit_reg r}, #{emit_int (n-1)}\n`
else begin
` addlt {emit_reg r}, {emit_reg r}, #{emit_int n}\n`;
` sublt {emit_reg r}, {emit_reg r}, #1\n`
end;
` mov {emit_reg r}, {emit_reg r}, asr #{emit_int l}\n`; 4
| Lop(Iintop_imm(Imod, n)) -> (* n is a power of 2 *)
let l = Misc.log2 n in
let a = i.arg.(0) in
let r = i.res.(0) in
let lbl = new_label() in
` cmp {emit_reg a}, #0\n`;
` mov {emit_reg r}, {emit_reg a}, lsl #{emit_int (32-l)}\n`;
` mov {emit_reg r}, {emit_reg r}, lsr #{emit_int (32-l)}\n`;
` bpl {emit_label lbl}\n`;
` cmp {emit_reg r}, #0\n`;
` subne {emit_reg r}, {emit_reg r}, #{emit_int n}\n`;
`{emit_label lbl}:\n`; 6
| Lop(Iintop_imm((Ilsl | Ilsr | Iasr as op), n)) ->
let shift = name_for_shift_operation op in
` mov {emit_reg i.res.(0)}, {emit_reg i.arg.(0)}, {emit_string shift} #{emit_int n}\n`; 1
| Lop(Iintop_imm(Icomp cmp, n)) ->
let comp = name_for_comparison cmp in
` cmp {emit_reg i.arg.(0)}, #{emit_int n}\n`;
` mov {emit_reg i.res.(0)}, #0\n`;
` mov{emit_string comp} {emit_reg i.res.(0)}, #1\n`; 3
| Lop(Iintop_imm(Icheckbound, n)) ->
` cmp {emit_reg i.arg.(0)}, #{emit_int n}\n`;
` blls caml_array_bound_error\n`; 2
| Lop(Iintop_imm(op, n)) ->
let instr = name_for_int_operation op in
` {emit_string instr} {emit_reg i.res.(0)}, {emit_reg i.arg.(0)}, #{emit_int n}\n`; 1
| Lop(Inegf | Iabsf | Ifloatofint | Iintoffloat as op) ->
let instr = name_for_float_operation op in
` {emit_string instr} {emit_reg i.res.(0)}, {emit_reg i.arg.(0)}\n`; 1
| Lop(Iaddf | Isubf | Imulf | Idivf as op) ->
let instr = name_for_float_operation op in
` {emit_string instr} {emit_reg i.res.(0)}, {emit_reg i.arg.(0)}, {emit_reg i.arg.(1)}\n`; 1
| Lop(Ispecific(Ishiftarith(op, shift))) ->
let instr = name_for_shift_int_operation op in
` {emit_string instr} {emit_reg i.res.(0)}, {emit_reg i.arg.(0)}, {emit_reg i.arg.(1)}`;
if shift >= 0
then `, lsl #{emit_int shift}\n`
else `, asr #{emit_int (-shift)}\n`;
1
| Lop(Ispecific(Ishiftcheckbound shift)) ->
` cmp {emit_reg i.arg.(1)}, {emit_reg i.arg.(0)}, lsr #{emit_int shift}\n`;
` blcs caml_array_bound_error\n`; 2
| Lop(Ispecific(Irevsubimm n)) ->
` rsb {emit_reg i.res.(0)}, {emit_reg i.arg.(0)}, #{emit_int n}\n`; 1
| Lreloadretaddr ->
let n = frame_size() in
` ldr lr, [sp, #{emit_int(n-4)}]\n`; 1
| Lreturn ->
let n = frame_size() in
if n > 0 then
` add sp, sp, #{emit_int n}\n`;
` mov pc, lr\n`; 2
| Llabel lbl ->
`{emit_label lbl}:\n`; 0
| Lbranch lbl ->
` b {emit_label lbl}\n`; 1
| Lcondbranch(tst, lbl) ->
begin match tst with
Itruetest ->
` cmp {emit_reg i.arg.(0)}, #0\n`;
` bne {emit_label lbl}\n`
| Ifalsetest ->
` cmp {emit_reg i.arg.(0)}, #0\n`;
` beq {emit_label lbl}\n`
| Iinttest cmp ->
` cmp {emit_reg i.arg.(0)}, {emit_reg i.arg.(1)}\n`;
let comp = name_for_comparison cmp in
` b{emit_string comp} {emit_label lbl}\n`
| Iinttest_imm(cmp, n) ->
` cmp {emit_reg i.arg.(0)}, #{emit_int n}\n`;
let comp = name_for_comparison cmp in
` b{emit_string comp} {emit_label lbl}\n`
| Ifloattest(cmp, neg) ->
begin match cmp with
Ceq | Cne ->
` cmf {emit_reg i.arg.(0)}, {emit_reg i.arg.(1)}\n`
| _ ->
` cmfe {emit_reg i.arg.(0)}, {emit_reg i.arg.(1)}\n`
end;
let comp = name_for_float_comparison cmp neg in
` b{emit_string comp} {emit_label lbl}\n`
| Ioddtest ->
` tst {emit_reg i.arg.(0)}, #1\n`;
` bne {emit_label lbl}\n`
| Ieventest ->
` tst {emit_reg i.arg.(0)}, #1\n`;
` beq {emit_label lbl}\n`
end;
2
| Lcondbranch3(lbl0, lbl1, lbl2) ->
` cmp {emit_reg i.arg.(0)}, #1\n`;
begin match lbl0 with
None -> ()
| Some lbl -> ` blt {emit_label lbl}\n`
end;
begin match lbl1 with
None -> ()
| Some lbl -> ` beq {emit_label lbl}\n`
end;
begin match lbl2 with
None -> ()
| Some lbl -> ` bgt {emit_label lbl}\n`
end;
4
| Lswitch jumptbl ->
` ldr pc, [pc, {emit_reg i.arg.(0)}, lsl #2]\n`;
` mov r0, r0\n`; (* nop *)
for i = 0 to Array.length jumptbl - 1 do
` .word {emit_label jumptbl.(i)}\n`
done;
2 + Array.length jumptbl
| Lsetuptrap lbl ->
` bl {emit_label lbl}\n`; 1
| Lpushtrap ->
stack_offset := !stack_offset + 8;
` stmfd sp!, \{trap_ptr, lr}\n`;
` mov trap_ptr, sp\n`; 2
| Lpoptrap ->
` ldmfd sp!, \{trap_ptr, lr}\n`;
stack_offset := !stack_offset - 8; 1
| Lraise ->
` mov sp, trap_ptr\n`;
` ldmfd sp!, \{trap_ptr, pc}\n`; 2
(* Emission of an instruction sequence *)
let no_fallthrough = function
Lop(Itailcall_ind | Itailcall_imm _) -> true
| Lreturn -> true
| Lbranch _ -> true
| Lswitch _ -> true
| Lraise -> true
| _ -> false
let rec emit_all ninstr i =
if i.desc = Lend then () else begin
let n = emit_instr i in
let ninstr' = ninstr + n in
let limit = (if !pending_float then 127 else 511) - !num_literals in
if ninstr' >= limit - 32 && no_fallthrough i.desc then begin
emit_constants();
emit_all 0 i.next
end else
if ninstr' >= limit then begin
let lbl = new_label() in
` b {emit_label lbl}\n`;
emit_constants();
`{emit_label lbl}:\n`;
emit_all 0 i.next
end else
emit_all ninstr' i.next
end
(* Emission of a function declaration *)
let fundecl fundecl =
function_name := fundecl.fun_name;
fastcode_flag := fundecl.fun_fast;
tailrec_entry_point := new_label();
stack_offset := 0;
Hashtbl.clear symbol_constants;
Hashtbl.clear float_constants;
` .text\n`;
` .align 0\n`;
` .global {emit_symbol fundecl.fun_name}\n`;
`{emit_symbol fundecl.fun_name}:\n`;
let n = frame_size() in
if n > 0 then
` sub sp, sp, #{emit_int n}\n`;
if !contains_calls then
` str lr, [sp, #{emit_int(n - 4)}]\n`;
`{emit_label !tailrec_entry_point}:\n`;
emit_all 0 fundecl.fun_body;
emit_constants()
(* Emission of data *)
let emit_item = function
Cdefine_symbol s ->
` .global {emit_symbol s}\n`;
`{emit_symbol s}:\n`
| Cdefine_label lbl ->
`{emit_label (10000 + lbl)}:\n`
| Cint8 n ->
` .byte {emit_int n}\n`
| Cint16 n ->
` .short {emit_int n}\n`
| Cint n ->
` .word {emit_nativeint n}\n`
| Cfloat f ->
` .align 0\n`;
` .double {emit_string f}\n`
| Csymbol_address s ->
` .word {emit_symbol s}\n`
| Clabel_address lbl ->
` .word {emit_label (10000 + lbl)}\n`
| Cstring s ->
emit_string_directive " .ascii " s
| Cskip n ->
if n > 0 then ` .space {emit_int n}\n`
| Calign n ->
` .align {emit_int(Misc.log2 n)}\n`
let data l =
` .data\n`;
List.iter emit_item l
(* Beginning / end of an assembly file *)
let begin_assembly() =
`trap_ptr .req r11\n`;
`alloc_ptr .req r8\n`;
`alloc_limit .req r9\n`;
`sp .req r13\n`;
`lr .req r14\n`;
`pc .req r15\n`;
let lbl_begin = Compilenv.current_unit_name() ^ "_data_begin" in
` .data\n`;
` .global {emit_symbol lbl_begin}\n`;
`{emit_symbol lbl_begin}:\n`;
let lbl_begin = Compilenv.current_unit_name() ^ "_code_begin" in
` .text\n`;
` .global {emit_symbol lbl_begin}\n`;
`{emit_symbol lbl_begin}:\n`
let end_assembly () =
let lbl_end = Compilenv.current_unit_name() ^ "_code_end" in
` .text\n`;
` .global {emit_symbol lbl_end}\n`;
`{emit_symbol lbl_end}:\n`;
let lbl_end = Compilenv.current_unit_name() ^ "_data_end" in
` .data\n`;
` .global {emit_symbol lbl_end}\n`;
`{emit_symbol lbl_end}:\n`;
` .word 0\n`;
let lbl = Compilenv.current_unit_name() ^ "_frametable" in
` .data\n`;
` .global {emit_symbol lbl}\n`;
`{emit_symbol lbl}:\n`;
` .word {emit_int (List.length !frame_descriptors)}\n`;
List.iter emit_frame !frame_descriptors;
frame_descriptors := []