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

1 files changed, 2 insertions, 0 deletions

diff --git a/x86/Asm.v b/x86/Asm.v
index 7b4a29f4..dfa2a88a 100644
--- a/x86/Asm.v
+++ b/x86/Asm.v
@@ -284,6 +284,7 @@ Inductive instruction: Type :=
   | Pmovsb
   | Pmovsw
   | Pmovw_rm (rd: ireg) (ad: addrmode)
+  | Pnop
   | Prep_movsl
   | Psbbl_rr (rd: ireg) (r2: ireg)
   | Psqrtsd (rd: freg) (r1: freg)
@@ -1001,6 +1002,7 @@ Definition exec_instr (f: function) (i: instruction) (rs: regset) (m: mem) : out
   | Pmovsb
   | Pmovsw
   | Pmovw_rm _ _
+  | Pnop
   | Prep_movsl
   | Psbbl_rr _ _
   | Psqrtsd _ _