py/gc: Avoid valgrind false positives.

When you want to use the valgrind memory analysis tool on MicroPython, you
can arrange to define MICROPY_DEBUG_VALGRIND to enable use of special
valgrind macros.  For now, this only fixes `gc_get_ptr` so that it never
emits the diagnostic "Conditional jump or move depends on uninitialised
value(s)".

Signed-off-by: Jeff Epler <jepler@gmail.com>

py/gc.c

@@ -32,6 +32,10 @@
 #include "py/gc.h"
 #include "py/runtime.h"
 
+#if MICROPY_DEBUG_VALGRIND
+#include <valgrind/memcheck.h>
+#endif
+
 #if MICROPY_ENABLE_GC
 
 #if MICROPY_DEBUG_VERBOSE // print debugging info
@@ -449,6 +453,11 @@ void gc_collect_start(void) {
 __attribute__((no_sanitize_address))
 #endif
 static void *gc_get_ptr(void **ptrs, int i) {
+    #if MICROPY_DEBUG_VALGRIND
+    if (!VALGRIND_CHECK_MEM_IS_ADDRESSABLE(&ptrs[i], sizeof(*ptrs))) {
+        return NULL;
+    }
+    #endif
     return ptrs[i];
 }
 

py/mpconfig.h

@@ -510,6 +510,11 @@
 #define MICROPY_DEBUG_VM_STACK_OVERFLOW (0)
 #endif
 
+// Whether to enable extra instrumentation for valgrind
+#ifndef MICROPY_DEBUG_VALGRIND
+#define MICROPY_DEBUG_VALGRIND (0)
+#endif
+
 /*****************************************************************************/
 /* Optimisations                                                             */