diff --git a/arm/Asm.v b/arm/Asm.v index 08234975c9..85eb94c18e 100644 --- a/arm/Asm.v +++ b/arm/Asm.v @@ -199,15 +199,16 @@ Inductive instruction : Type := | Pfsts: freg -> ireg -> int -> instruction (**r float32 store *) (* Pseudo-instructions *) - | Pallocframe: Z -> ptrofs -> instruction (**r allocate new stack frame *) - | Pfreeframe: Z -> ptrofs -> instruction (**r deallocate stack frame and restore previous frame *) - | Plabel: label -> instruction (**r define a code label *) + | Pallocframe: Z -> ptrofs -> instruction (**r allocate new stack frame *) + | Pfreeframe: Z -> ptrofs -> instruction (**r deallocate stack frame and restore previous frame *) + | Plabel: label -> instruction (**r define a code label *) | Ploadsymbol: ireg -> ident -> ptrofs -> instruction (**r load the address of a symbol *) | Pmovite: testcond -> ireg -> shift_op -> shift_op -> instruction (**r integer conditional move *) | Pbtbl: ireg -> list label -> instruction (**r N-way branch through a jump table *) | Pbuiltin: external_function -> list (builtin_arg preg) -> builtin_res preg -> instruction (**r built-in function (pseudo) *) | Padc: ireg -> ireg -> shift_op -> instruction (**r add with carry *) | Pcfi_adjust: int -> instruction (**r .cfi_adjust debug directive *) + | Pcfi_rel_offset: int -> instruction (**r .cfi_rel_offset debug directive *) | Pclz: ireg -> ireg -> instruction (**r count leading zeros. *) | Pfsqrt: freg -> freg -> instruction (**r floating-point square root. *) | Prev: ireg -> ireg -> instruction (**r reverse bytes and reverse bits. *) @@ -757,6 +758,8 @@ Definition exec_instr (f: function) (i: instruction) (rs: regset) (m: mem) : out end | _ => Stuck end + | Pcfi_rel_offset ofs => + Next (nextinstr rs) m | Pbuiltin ef args res => Stuck (**r treated specially below *) (** The following instructions and directives are not generated directly by Asmgen, so we do not model them. *) diff --git a/arm/Asmexpand.ml b/arm/Asmexpand.ml index a32b0e8ba0..04b4152d04 100644 --- a/arm/Asmexpand.ml +++ b/arm/Asmexpand.ml @@ -46,18 +46,22 @@ let expand_movimm dst n = (fun n -> emit (Porr (dst,dst, SOimm n))) tl let expand_subimm dst src n = - match Asmgen.decompose_int n with - | [] -> assert false - | hd::tl -> - emit (Psub(dst,src,SOimm hd)); - List.iter (fun n -> emit (Psub (dst,dst,SOimm n))) tl + if dst <> src || n <> _0 then begin + match Asmgen.decompose_int n with + | [] -> assert false + | hd::tl -> + emit (Psub(dst,src,SOimm hd)); + List.iter (fun n -> emit (Psub (dst,dst,SOimm n))) tl + end let expand_addimm dst src n = - match Asmgen.decompose_int n with - | [] -> assert false - | hd::tl -> - emit (Padd (dst,src,SOimm hd)); - List.iter (fun n -> emit (Padd (dst,dst,SOimm n))) tl + if dst <> src || n <> _0 then begin + match Asmgen.decompose_int n with + | [] -> assert false + | hd::tl -> + emit (Padd (dst,src,SOimm hd)); + List.iter (fun n -> emit (Padd (dst,dst,SOimm n))) tl + end let expand_int64_arith conflict rl fn = if conflict then @@ -410,12 +414,22 @@ let expand_instruction instr = | Pallocframe (sz, ofs) -> emit (Pmov (IR12,SOreg IR13)); if (is_current_function_variadic ()) then begin - emit (Ppush [IR0;IR1;IR2;IR3]); - emit (Pcfi_adjust _16); - end; - expand_subimm IR13 IR13 sz; - emit (Pcfi_adjust sz); - emit (Pstr (IR12,IR13,SOimm ofs)); + emit (Ppush [IR0;IR1;IR2;IR3]); + emit (Pcfi_adjust _16); + end; + let sz' = camlint_of_coqint sz in + let ofs' = camlint_of_coqint ofs in + if ofs' >= 4096l && sz' >= ofs' then begin + expand_subimm IR13 IR13 (coqint_of_camlint (Int32.sub sz' (Int32.add ofs' 4l))); + emit (Ppush [IR12]); + expand_subimm IR13 IR13 ofs; + emit (Pcfi_adjust sz); + end else begin + assert (ofs' < 4096l); + expand_subimm IR13 IR13 sz; + emit (Pcfi_adjust sz); + emit (Pstr (IR12,IR13,SOimm ofs)); + end; PrintAsmaux.current_function_stacksize := camlint_of_coqint sz | Pfreeframe (sz, ofs) -> let sz = @@ -424,7 +438,13 @@ let expand_instruction instr = else sz in if Asmgen.is_immed_arith sz then emit (Padd (IR13,IR13,SOimm sz)) - else emit (Pldr (IR13,IR13,SOimm ofs)) + else begin + if camlint_of_coqint ofs >= 4096l then begin + expand_addimm IR13 IR13 ofs; + emit (Pldr (IR13,IR13,SOimm _0)) + end else + emit (Pldr (IR13,IR13,SOimm ofs)); + end | Pbuiltin (ef,args,res) -> begin match ef with | EF_builtin (name,sg) -> diff --git a/arm/Asmgen.v b/arm/Asmgen.v index e7a3b4fad9..1b96416d94 100644 --- a/arm/Asmgen.v +++ b/arm/Asmgen.v @@ -602,6 +602,22 @@ Definition storeind (src: mreg) (base: ireg) (ofs: ptrofs) (ty: typ) (k: code) : Error (msg "Asmgen.storeind") end. +(** This is a variant of [storeind] that is used to save the return address + at the beginning of a function. It uses [R12] instead of [R14] as + temporary register. *) + +Definition save_lr (ofs: ptrofs) (k: code) := + let n := Ptrofs.to_int ofs in + let n1 := mk_immed_mem_word n in + if Int.eq n n1 + then Pstr IR14 IR13 (SOimm n) :: k + else addimm IR12 IR13 (Int.sub n n1) (Pstr IR14 IR12 (SOimm n1) :: k). + +Definition save_lr_preserves_R12 (ofs: ptrofs) : bool := + let n := Ptrofs.to_int ofs in + let n1 := mk_immed_mem_word n in + Int.eq n n1. + (** Translation of memory accesses *) Definition transl_memory_access @@ -787,10 +803,12 @@ Definition transl_code' (f: Mach.function) (il: list Mach.instruction) (it1p: bo around, leading to incorrect executions. *) Definition transl_function (f: Mach.function) := - do c <- transl_code f f.(Mach.fn_code) true; + do c <- transl_code f f.(Mach.fn_code) + (save_lr_preserves_R12 f.(fn_retaddr_ofs)); OK (mkfunction f.(Mach.fn_sig) (Pallocframe f.(fn_stacksize) f.(fn_link_ofs) :: - Pstr IR14 IR13 (SOimm (Ptrofs.to_int f.(fn_retaddr_ofs))) :: c)). + save_lr f.(fn_retaddr_ofs) + (Pcfi_rel_offset (Ptrofs.to_int f.(fn_retaddr_ofs)):: c))). Definition transf_function (f: Mach.function) : res Asm.function := do tf <- transl_function f; diff --git a/arm/Asmgenproof.v b/arm/Asmgenproof.v index 09c20d5c4e..71a0e04915 100644 --- a/arm/Asmgenproof.v +++ b/arm/Asmgenproof.v @@ -241,6 +241,15 @@ Proof. destruct ty, (preg_of src); inv H; TailNoLabel. Qed. +Remark save_lr_label: + forall ofs k, tail_nolabel k (save_lr ofs k). +Proof. + unfold save_lr; intros. + destruct (Int.eq (Ptrofs.to_int ofs) (mk_immed_mem_word (Ptrofs.to_int ofs))). + TailNoLabel. + eapply tail_nolabel_trans; TailNoLabel. +Qed. + Remark transl_cond_label: forall cond args k c, transl_cond cond args k = OK c -> tail_nolabel k c. Proof. @@ -338,7 +347,7 @@ Lemma transl_find_label: end. Proof. intros. monadInv H. destruct (zlt Ptrofs.max_unsigned (list_length_z (fn_code x))); inv EQ0. - monadInv EQ. simpl. + monadInv EQ. simpl. erewrite tail_nolabel_find_label by (apply save_lr_label). simpl. eapply transl_code_label; eauto. Qed. @@ -382,7 +391,8 @@ Proof. destruct i; try (intros [A B]; apply A). intros. subst c0. repeat constructor. - intros. monadInv H0. destruct (zlt Ptrofs.max_unsigned (list_length_z (fn_code x))); inv EQ0. monadInv EQ. - exists x; exists true; split; auto. repeat constructor. + exists x; exists (save_lr_preserves_R12 (fn_retaddr_ofs f0)); split; auto. + constructor. eapply is_tail_trans. 2: apply tail_nolabel_is_tail; apply save_lr_label. repeat constructor. - exact transf_function_no_overflow. Qed. @@ -854,7 +864,10 @@ Opaque loadind. generalize EQ; intros EQ'. monadInv EQ'. destruct (zlt Ptrofs.max_unsigned (list_length_z (fn_code x0))); inversion EQ1. clear EQ1. subst x0. monadInv EQ0. - set (tfbody := Pallocframe (fn_stacksize f) (fn_link_ofs f) :: Pstr IR14 IR13 (SOimm (Ptrofs.to_int (fn_retaddr_ofs f))) :: x0) in *. + set (ra_ofs := fn_retaddr_ofs f) in *. + set (ra_ofs' := Ptrofs.to_int ra_ofs) in *. + set (tfbody := Pallocframe (fn_stacksize f) (fn_link_ofs f) :: + save_lr ra_ofs (Pcfi_rel_offset ra_ofs' :: x0)) in *. set (tf := {| fn_sig := Mach.fn_sig f; fn_code := tfbody |}) in *. unfold store_stack in *. exploit Mem.alloc_extends. eauto. eauto. apply Zle_refl. apply Zle_refl. @@ -865,32 +878,40 @@ Opaque loadind. intros [m3' [P Q]]. (* Execution of function prologue *) set (rs2 := nextinstr (rs0#IR12 <- (parent_sp s) #IR13 <- (Vptr stk Ptrofs.zero))). - set (rs3 := nextinstr rs2). + edestruct (save_lr_correct tge tf ra_ofs (Pcfi_rel_offset ra_ofs' :: x0) rs2) as (rs3 & X & Y & Z). + change (rs2 IR13) with sp. change (rs2 IR14) with (rs0 IR14). rewrite ATLR. eexact P. + set (rs4 := nextinstr rs3). assert (EXEC_PROLOGUE: exec_straight tge tf (fn_code tf) rs0 m' - x0 rs3 m3'). + x0 rs4 m3'). + { change (fn_code tf) with tfbody; unfold tfbody. - apply exec_straight_two with rs2 m2'. + eapply exec_straight_trans with (rs2 := rs2) (m2 := m2'). + apply exec_straight_one. unfold exec_instr. rewrite C. fold sp. rewrite <- (sp_val _ _ _ AG). unfold Tptr, chunk_of_type, Archi.ptr64 in F. rewrite F. auto. - simpl. auto. - simpl. unfold exec_store. change (rs2 IR14) with (rs0 IR14). - rewrite Ptrofs.add_zero_l. simpl. unfold Tptr, chunk_of_type, Archi.ptr64 in P. simpl in P. - rewrite Ptrofs.add_zero_l in P. rewrite ATLR. rewrite Ptrofs.of_int_to_int by auto. - rewrite P. auto. auto. auto. - left; exists (State rs3 m3'); split. + auto. + eapply exec_straight_trans with (rs2 := rs3) (m2 := m3'). + eexact X. + apply exec_straight_one. + simpl; reflexivity. reflexivity. + } + (* After the function prologue is the code for the function body *) + exploit exec_straight_steps_2; eauto using functions_transl. omega. constructor. + intros (ofsbody & U & V). + (* Conclusions *) + left; exists (State rs4 m3'); split. eapply exec_straight_steps_1; eauto. omega. constructor. - econstructor; eauto. - change (rs3 PC) with (Val.offset_ptr (Val.offset_ptr (rs0 PC) Ptrofs.one) Ptrofs.one). - rewrite ATPC. simpl. constructor; eauto. - eapply code_tail_next_int. omega. - eapply code_tail_next_int. omega. constructor. - unfold rs3, rs2. - apply agree_nextinstr. apply agree_nextinstr. + econstructor; eauto. rewrite U. econstructor; eauto. + apply agree_nextinstr. + apply agree_undef_regs2 with rs2. + apply agree_nextinstr. eapply agree_change_sp. apply agree_undef_regs with rs0; eauto. - intros. Simpl. congruence. + intros; Simpl. + congruence. + intros; apply Y; eauto with asmgen. - (* external function *) exploit functions_translated; eauto. diff --git a/arm/Asmgenproof1.v b/arm/Asmgenproof1.v index eec531dc50..7084336e71 100644 --- a/arm/Asmgenproof1.v +++ b/arm/Asmgenproof1.v @@ -616,6 +616,37 @@ Proof. intros; Simpl. Qed. +(** Saving the link register *) + +Lemma save_lr_correct: + forall ofs k (rs: regset) m m', + Mem.storev Mint32 m (Val.offset_ptr rs#IR13 ofs) (rs#IR14) = Some m' -> + exists rs', + exec_straight ge fn (save_lr ofs k) rs m k rs' m' + /\ (forall r, if_preg r = true -> r <> IR12 -> rs'#r = rs#r) + /\ (save_lr_preserves_R12 ofs = true -> rs'#IR12 = rs#IR12). +Proof. + intros; unfold save_lr, save_lr_preserves_R12. + set (n := Ptrofs.to_int ofs). set (n1 := mk_immed_mem_word n). + assert (EQ: Val.offset_ptr rs#IR13 ofs = Val.add rs#IR13 (Vint n)). + { destruct rs#IR13; try discriminate. simpl. f_equal; f_equal. unfold n; symmetry; auto with ptrofs. } + destruct (Int.eq n n1). +- econstructor; split. apply exec_straight_one. simpl; unfold exec_store. rewrite <- EQ, H; reflexivity. auto. + split. intros; Simpl. intros; Simpl. +- destruct (addimm_correct IR12 IR13 (Int.sub n n1) (Pstr IR14 IR12 (SOimm n1) :: k) rs m) + as (rs1 & A & B & C). + econstructor; split. + eapply exec_straight_trans. eexact A. + apply exec_straight_one. simpl; unfold exec_store. + rewrite B. rewrite Val.add_assoc. simpl. + rewrite Int.sub_add_opp. rewrite Int.add_assoc. + rewrite (Int.add_commut (Int.neg n1)). + rewrite Int.add_neg_zero. rewrite Int.add_zero. + rewrite <- EQ. rewrite C by eauto with asmgen. rewrite H. reflexivity. + auto. + split. intros; Simpl. congruence. +Qed. + (** Translation of shift immediates *) Lemma transl_shift_correct: diff --git a/arm/Stacklayout.v b/arm/Stacklayout.v index c867ba5966..043393bfd4 100644 --- a/arm/Stacklayout.v +++ b/arm/Stacklayout.v @@ -19,11 +19,10 @@ Require Import Bounds. (** The general shape of activation records is as follows, from bottom (lowest offsets) to top: - Space for outgoing arguments to function calls. -- Local stack slots. -- Saved values of integer callee-save registers used by the function. -- Saved values of float callee-save registers used by the function. -- Saved return address into caller. - Pointer to activation record of the caller. +- Saved return address into caller. +- Local stack slots. +- Saved values of callee-save registers used by the function. - Space for the stack-allocated data declared in Cminor. The [frame_env] compilation environment records the positions of @@ -36,11 +35,11 @@ Definition fe_ofs_arg := 0. function. *) Definition make_env (b: bounds) := - let ol := align (4 * b.(bound_outgoing)) 8 in (* locals *) + let olink := 4 * b.(bound_outgoing) in (* back link*) + let ora := olink + 4 in (* return address *) + let ol := align (ora + 4) 8 in (* locals *) let ocs := ol + 4 * b.(bound_local) in (* callee-saves *) - let ora := align (size_callee_save_area b ocs) 4 in (* retaddr *) - let olink := ora + 4 in (* back link *) - let ostkdata := align (olink + 4) 8 in (* stack data *) + let ostkdata := align (size_callee_save_area b ocs) 8 in (* retaddr *) let sz := align (ostkdata + b.(bound_stack_data)) 8 in {| fe_size := sz; fe_ofs_link := olink; @@ -67,33 +66,32 @@ Lemma frame_env_separated: Proof. Local Opaque Z.add Z.mul sepconj range. intros; simpl. - set (ol := align (4 * b.(bound_outgoing)) 8); + set (olink := 4 * b.(bound_outgoing)); + set (ora := olink + 4); + set (ol := align (ora + 4) 8); set (ocs := ol + 4 * b.(bound_local)); - set (ora := align (size_callee_save_area b ocs) 4); - set (olink := ora + 4); - set (ostkdata := align (olink + 4) 8). + set (ostkdata := align (size_callee_save_area b ocs) 8). generalize b.(bound_local_pos) b.(bound_outgoing_pos) b.(bound_stack_data_pos); intros. - assert (4 * b.(bound_outgoing) <= ol) by (apply align_le; omega). + assert (0 <= olink) by (unfold olink; omega). + assert (olink <= ora) by (unfold ora; omega). + assert (ora + 4 <= ol) by (apply align_le; omega). assert (ol + 4 * b.(bound_local) <= ocs) by (unfold ocs; omega). assert (ocs <= size_callee_save_area b ocs) by apply size_callee_save_area_incr. - assert (size_callee_save_area b ocs <= ora) by (apply align_le; omega). - assert (ora <= olink) by (unfold olink; omega). - assert (olink + 4 <= ostkdata) by (apply align_le; omega). + assert (size_callee_save_area b ocs <= ostkdata) by (apply align_le; omega). (* Reorder as: outgoing - local - callee-save + back link retaddr - back link *) + local + callee-save *) rewrite sep_swap12. - rewrite sep_swap45. + rewrite sep_swap23. rewrite sep_swap34. - rewrite sep_swap45. (* Apply range_split and range_split2 repeatedly *) unfold fe_ofs_arg. - apply range_split_2. fold ol; omega. omega. apply range_split. omega. - apply range_split_2. fold ora; omega. omega. + apply range_split. omega. + apply range_split_2. fold ol; omega. omega. apply range_split. omega. apply range_drop_right with ostkdata. omega. eapply sep_drop2. eexact H. @@ -105,18 +103,18 @@ Lemma frame_env_range: 0 <= fe_stack_data fe /\ fe_stack_data fe + bound_stack_data b <= fe_size fe. Proof. intros; simpl. - set (ol := align (4 * b.(bound_outgoing)) 8); + set (olink := 4 * b.(bound_outgoing)); + set (ora := olink + 4); + set (ol := align (ora + 4) 8); set (ocs := ol + 4 * b.(bound_local)); - set (ora := align (size_callee_save_area b ocs) 4); - set (olink := ora + 4); - set (ostkdata := align (olink + 4) 8). + set (ostkdata := align (size_callee_save_area b ocs) 8). generalize b.(bound_local_pos) b.(bound_outgoing_pos) b.(bound_stack_data_pos); intros. - assert (4 * b.(bound_outgoing) <= ol) by (apply align_le; omega). + assert (0 <= olink) by (unfold olink; omega). + assert (olink <= ora) by (unfold ora; omega). + assert (ora + 4 <= ol) by (apply align_le; omega). assert (ol + 4 * b.(bound_local) <= ocs) by (unfold ocs; omega). assert (ocs <= size_callee_save_area b ocs) by apply size_callee_save_area_incr. - assert (size_callee_save_area b ocs <= ora) by (apply align_le; omega). - assert (ora <= olink) by (unfold olink; omega). - assert (olink + 4 <= ostkdata) by (apply align_le; omega). + assert (size_callee_save_area b ocs <= ostkdata) by (apply align_le; omega). split. omega. apply align_le; omega. Qed. @@ -130,14 +128,13 @@ Lemma frame_env_aligned: /\ (4 | fe_ofs_retaddr fe). Proof. intros; simpl. - set (ol := align (4 * b.(bound_outgoing)) 8); + set (olink := 4 * b.(bound_outgoing)); + set (ora := olink + 4); + set (ol := align (ora + 4) 8); set (ocs := ol + 4 * b.(bound_local)); - set (ora := align (size_callee_save_area b ocs) 4); - set (olink := ora + 4); - set (ostkdata := align (olink + 4) 8). + set (ostkdata := align (size_callee_save_area b ocs) 8). split. apply Zdivide_0. split. apply align_divides; omega. split. apply align_divides; omega. - split. apply Z.divide_add_r. apply align_divides; omega. apply Z.divide_refl. - apply align_divides; omega. + unfold ora, olink; auto using Z.divide_mul_l, Z.divide_add_r, Z.divide_refl. Qed. diff --git a/arm/TargetPrinter.ml b/arm/TargetPrinter.ml index 60f52efd25..1dfe8af6d7 100644 --- a/arm/TargetPrinter.ml +++ b/arm/TargetPrinter.ml @@ -539,23 +539,13 @@ struct | Psbc (r1,r2,sa) -> fprintf oc " sbc %a, %a, %a\n" ireg r1 ireg r2 shift_op sa; 1 | Pstr(r1, r2, sa) | Pstr_a(r1, r2, sa) -> - fprintf oc " str %a, [%a, %a]\n" ireg r1 ireg r2 shift_op sa; - begin match r1, r2, sa with - | IR14, IR13, SOimm n -> cfi_rel_offset oc "lr" (camlint_of_coqint n) - | _ -> () - end; - 1 + fprintf oc " str %a, [%a, %a]\n" ireg r1 ireg r2 shift_op sa; 1 | Pstrb(r1, r2, sa) -> fprintf oc " strb %a, [%a, %a]\n" ireg r1 ireg r2 shift_op sa; 1 | Pstrh(r1, r2, sa) -> fprintf oc " strh %a, [%a, %a]\n" ireg r1 ireg r2 shift_op sa; 1 | Pstr_p(r1, r2, sa) -> - fprintf oc " str %a, [%a], %a\n" ireg r1 ireg r2 shift_op sa; - begin match r1, r2, sa with - | IR14, IR13, SOimm n -> cfi_rel_offset oc "lr" (camlint_of_coqint n) - | _ -> () - end; - 1 + fprintf oc " str %a, [%a], %a\n" ireg r1 ireg r2 shift_op sa; 1 | Pstrb_p(r1, r2, sa) -> fprintf oc " strb %a, [%a], %a\n" ireg r1 ireg r2 shift_op sa; 1 | Pstrh_p(r1, r2, sa) -> @@ -751,6 +741,7 @@ struct assert false end | Pcfi_adjust sz -> cfi_adjust oc (camlint_of_coqint sz); 0 + | Pcfi_rel_offset ofs -> cfi_rel_offset oc "lr" (camlint_of_coqint ofs); 0 let no_fallthrough = function | Pb _ -> true diff --git a/backend/Asmgenproof0.v b/backend/Asmgenproof0.v index 53ecf73ade..0250628b34 100644 --- a/backend/Asmgenproof0.v +++ b/backend/Asmgenproof0.v @@ -285,6 +285,23 @@ Proof. exploit preg_of_injective; eauto. congruence. Qed. +Lemma agree_undef_regs2: + forall ms sp rl rs rs', + agree (Mach.undef_regs rl ms) sp rs -> + (forall r', data_preg r' = true -> preg_notin r' rl -> rs'#r' = rs#r') -> + agree (Mach.undef_regs rl ms) sp rs'. +Proof. + intros. destruct H. split; auto. + rewrite <- agree_sp0. apply H0; auto. + rewrite preg_notin_charact. intros. apply not_eq_sym. apply preg_of_not_SP. + intros. destruct (In_dec mreg_eq r rl). + rewrite Mach.undef_regs_same; auto. + rewrite H0; auto. + apply preg_of_data. + rewrite preg_notin_charact. intros; red; intros. elim n. + exploit preg_of_injective; eauto. congruence. +Qed. + Lemma agree_set_undef_mreg: forall ms sp rs r v rl rs', agree ms sp rs -> @@ -738,6 +755,18 @@ Ltac TailNoLabel := | _ => idtac end. +Remark tail_nolabel_find_label: + forall lbl k c, tail_nolabel k c -> find_label lbl c = find_label lbl k. +Proof. + intros. destruct H. auto. +Qed. + +Remark tail_nolabel_is_tail: + forall k c, tail_nolabel k c -> is_tail k c. +Proof. + intros. destruct H. auto. +Qed. + (** * Execution of straight-line code *) Section STRAIGHTLINE.