vm: allow signals during vm execution.

I have experimented with several approaches, and found that
this one has an immeasurably small effect on performance,
below the noise floor.

* vm.c (vm_jmp): Call sig_check_fast whenever there is a
backwards branch.
(vm_execute): Check for signals once, on entry.

1 files changed, 6 insertions, 1 deletions

diff --git a/vm.c b/vm.c
index 3fc03d68..da06d156 100644
--- a/vm.c
+++ b/vm.c
@@ -670,7 +670,10 @@ NOINLINE static void vm_movrr(struct vm *vm, vm_word_t insn)
 
 static void vm_jmp(struct vm *vm, vm_word_t insn)
 {
-  vm->ip = vm_insn_bigop(insn);
+  unsigned ip = vm_insn_bigop(insn);
+  if (ip < vm->ip)
+    sig_check_fast();
+  vm->ip = ip;
 }
 
 NOINLINE static void vm_if(struct vm *vm, vm_word_t insn)
@@ -944,6 +947,8 @@ NOINLINE static void vm_close(struct vm *vm, vm_word_t insn)
 
 NOINLINE static val vm_execute(struct vm *vm)
 {
+  sig_check_fast();
+
   for (;;) {
     vm_word_t insn = vm->code[vm->ip++];
     vm_op_t opcode = vm_insn_opcode(insn);