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

dpgeorge · tannewt · commit 1cf0ce094a4a · 2019-12-12T11:01:40.000-08:00
Recent versions of gcc perform optimisations which can lead to the
following code from the MP_NLR_JUMP_HEAD macro being omitted:

    top-&gt;ret_val = val; \
    MP_NLR_RESTORE_PYSTACK(top); \
    *_top_ptr = top-&gt;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.
diff --git 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
diff --git a/py/nlrx86.c b/py/nlrx86.c
@@ -100,7 +100,7 @@ NORETURN void nlr_jump(void *val) {
     :                               // clobbered registers
     );
 
-    for (;;); // needed to silence compiler warning
+    MP_UNREACHABLE
 }
 
 #endif // MICROPY_NLR_X86
diff --git a/py/nlrxtensa.c b/py/nlrxtensa.c
@@ -77,7 +77,7 @@ NORETURN void nlr_jump(void *val) {
     :                               // clobbered registers
     );
 
-    for (;;); // needed to silence compiler warning
+    MP_UNREACHABLE
 }
 
 #endif // MICROPY_NLR_XTENSA

Original file line number	Diff line number	Diff line change
`@@ -108,7 +108,7 @@ NORETURN void nlr_jump(void *val) {`
`108`	`108`	`: // clobbered registers`
`109`	`109`	`);`
`110`	`110`
`111`		`- for (;;); // needed to silence compiler warning`
	`111`	`+ MP_UNREACHABLE`
`112`	`112`	`}`
`113`	`113`
`114`	`114`	`#endif // MICROPY_NLR_X64`
Original file line number	Diff line number	Diff line change
`@@ -100,7 +100,7 @@ NORETURN void nlr_jump(void *val) {`
`100`	`100`	`: // clobbered registers`
`101`	`101`	`);`
`102`	`102`
`103`		`- for (;;); // needed to silence compiler warning`
	`103`	`+ MP_UNREACHABLE`
`104`	`104`	`}`
`105`	`105`
`106`	`106`	`#endif // MICROPY_NLR_X86`
Original file line number	Diff line number	Diff line change
`@@ -77,7 +77,7 @@ NORETURN void nlr_jump(void *val) {`
`77`	`77`	`: // clobbered registers`
`78`	`78`	`);`
`79`	`79`
`80`		`- for (;;); // needed to silence compiler warning`
	`80`	`+ MP_UNREACHABLE`
`81`	`81`	`}`
`82`	`82`
`83`	`83`	`#endif // MICROPY_NLR_XTENSA`