Generate a nop instruction after some ais annotations (#137)

* Generate a nop instruction after ais annotations.

In order to prevent the merging of ais annotations with following
Labels a nop instruction is inserted, but only if the annotation
is followed immediately by a label.

The insertion of nop instructions is performed during the
expansion of builtin and pseudo assembler instructions and is
processor independent, by inserting a __builtin_nop built-in.

* Add Pnop instruction to ARM, RISC-V, and x86

ARM as well as RISC-V don't have nop instructions that can
be easily encoded by for example add with zero instructions.
For x86 we used to use `mov X0, X0` for nop but this may
not be as efficient as the true nop instruction.

* Implement __builtin_nop on all supported target architectures.

This builtin is not yet made available on the C side for all architectures.

Bug 24067

4 files changed, 10 insertions, 6 deletions

diff --git a/arm/Asm.v b/arm/Asm.v
index 07dea756..e6d1b4fc 100644
--- a/arm/Asm.v
+++ b/arm/Asm.v
@@ -231,6 +231,7 @@ Inductive instruction : Type :=
   | Prev16: ireg -> ireg -> instruction               (**r reverse bytes and reverse bits. *)
   | Prsc: ireg -> ireg -> shift_op -> instruction     (**r reverse subtract without carry. *)
   | Psbc: ireg -> ireg -> shift_op -> instruction     (**r add with carry *)
+  | Pnop : instruction                                (**r nop instruction *)
   (* Add, sub, rsb versions with s suffix *)
   | Padds: ireg -> ireg -> shift_op -> instruction    (**r integer addition with update of condition flags *)
   | Psubs: ireg -> ireg -> shift_op -> instruction    (**r integer subtraction with update of condition flags *)
@@ -805,6 +806,7 @@ Definition exec_instr (f: function) (i: instruction) (rs: regset) (m: mem) : out
   | Pfsqrt _ _
   | Prsc _ _ _
   | Psbc _ _ _
+  | Pnop
   | Padds _ _ _
   | Psubs _ _ _
   | Prsbs _ _ _
diff --git a/arm/AsmToJSON.ml b/arm/AsmToJSON.ml
index 276ceecc..3874e141 100644
--- a/arm/AsmToJSON.ml
+++ b/arm/AsmToJSON.ml
@@ -30,7 +30,7 @@ let mnemonic_names = [ "Padc"; "Padd"; "Padds"; "Pand";"Pannot"; "Pasr"; "Pb"; "
                        "Pftouizs"; "Pfuitod"; "Pfuitos"; "Pinlineasm"; "Pisb"; "Plabel"; "Pldr";
                        "Ploadsymbol_lbl"; "Pldr_p"; "Pldrb"; "Pldrb_p"; "Pldrh"; "Pldrh_p"; "Pldrsb";
                        "Pldrsh"; "Plsl"; "Plsr"; "Pmla"; "Pmov"; "Pmovite";
-                       "Pmovt"; "Pmovw"; "Pmul"; "Pmvn";  "Ploadsymbol_imm"; "Porr";
+                       "Pmovt"; "Pmovw"; "Pmul"; "Pmvn";  "Ploadsymbol_imm"; "Pnop"; "Porr";
                        "Ppush"; "Prev"; "Prev16"; "Prsb"; "Prsbs"; "Prsc"; "Psbc"; "Psbfx"; "Psdiv"; "Psmull";
                        "Pstr"; "Pstr_p"; "Pstrb"; "Pstrb_p"; "Pstrh"; "Pstrh_p"; "Psub"; "Psubs"; "Pudiv";
                        "Pumull" ]
@@ -263,6 +263,7 @@ let pp_instructions pp ic =
     | Pmovw(r1, n) -> instruction pp "Pmovw" [Ireg r1; Long n]
     | Pmul(r1, r2, r3) -> instruction pp "Pmul" [Ireg r1; Ireg r2; Ireg r3]
     | Pmvn(r1, so) -> instruction pp "Pmvn" [Ireg r1; Shift so]
+    | Pnop -> instruction pp "Pnop" []
     | Porr(r1, r2, so) -> instruction pp "Porr" [Ireg r1; Ireg r2; Shift so]
     | Ppush(rl) -> instruction pp "Ppush" (List.map (fun r -> Ireg r) rl)
     | Prev(r1, r2) -> instruction pp "Prev" [Ireg r1; Ireg r2]
diff --git a/arm/Asmexpand.ml b/arm/Asmexpand.ml
index 7c18be6b..d9424d11 100644
--- a/arm/Asmexpand.ml
+++ b/arm/Asmexpand.ml
@@ -404,6 +404,8 @@ let expand_builtin_inline name args res =
   (* Vararg stuff *)
   | "__builtin_va_start", [BA(IR a)], _ ->
      expand_builtin_va_start a
+  | "__builtin_nop", [], _ ->
+     emit Pnop
   (* Catch-all *)
   | _ ->
       raise (Error ("unrecognized builtin " ^ name))
@@ -665,10 +667,7 @@ let expand_function id fn =
   try
     set_current_function fn;
     fixup_arguments Incoming fn.fn_sig;
-    if !Clflags.option_g then
-      expand_debug id 13 preg_to_dwarf expand_instruction fn.fn_code
-    else
-      List.iter expand_instruction fn.fn_code;
+    expand id 13 preg_to_dwarf expand_instruction fn.fn_code;
     let fn = get_current_function () in
     let fn = Constantexpand.expand_constants fn in
     Errors.OK fn
diff --git a/arm/TargetPrinter.ml b/arm/TargetPrinter.ml
index 52d2ada6..bf37b0e4 100644
--- a/arm/TargetPrinter.ml
+++ b/arm/TargetPrinter.ml
@@ -306,6 +306,8 @@ struct
       fprintf oc "	vsqrt.f64 %a, %a\n" freg f1 freg f2
     | Psbc (r1,r2,sa) ->
       fprintf oc "	sbc	%a, %a, %a\n" ireg r1 ireg r2 shift_op sa
+    | Pnop ->
+      fprintf oc "	nop\n"
     | Pstr(r1, r2, sa) | Pstr_a(r1, r2, sa) ->
       fprintf oc "	str	%a, [%a, %a]\n" ireg r1 ireg r2 shift_op sa
     | Pstrb(r1, r2, sa) ->
@@ -463,7 +465,7 @@ struct
               | 1 -> let annot = annot_text preg_annot "sp" (camlstring_of_coqstring txt) args in
                 fprintf oc "%s annotation: %S\n" comment annot
               | 2 -> let lbl = new_label () in
-                fprintf oc "%a: " label lbl;
+                fprintf oc "%a:\n" label lbl;
                 AisAnnot.add_ais_annot lbl preg_annot "r13" (camlstring_of_coqstring txt) args
               | _ -> assert false
             end