py/nlr: Use MP_UNREACHABLE at the end of arch-specific nlr_jump funcs.

Recent versions of gcc perform optimisations which can lead to the
following code from the MP_NLR_JUMP_HEAD macro being omitted:

    top->ret_val = val; \
    MP_NLR_RESTORE_PYSTACK(top); \
    *_top_ptr = top->prev; \

This is noticeable (at least) in the unix coverage on x86-64 built with gcc
9.1.0.  This is because the nlr_jump function is marked as no-return, so
gcc deduces that the above code has no effect.

Adding MP_UNREACHABLE tells the compiler that the asm code may branch
elsewhere, and so it cannot optimise away the code.

1 files changed, 1 insertions, 1 deletions

diff --git a/py/nlrx64.c b/py/nlrx64.c
index 9b2b22c22..569ad84fb 100644
--- a/py/nlrx64.c
+++ b/py/nlrx64.c
@@ -108,7 +108,7 @@ NORETURN void nlr_jump(void *val) {
     :                               // clobbered registers
     );
 
-    for (;;); // needed to silence compiler warning
+    MP_UNREACHABLE
 }
 
 #endif // MICROPY_NLR_X64