py/gc: Add MICROPY_GC_CONSERVATIVE_CLEAR option to always zero memory.
There can be stray pointers in memory blocks that are not properly zero'd
after allocation. This patch adds a new config option to always zero all
allocated memory (via gc_alloc and gc_realloc) and hence help to eliminate
stray pointers.
See issue #2195.
diff --git a/py/gc.c b/py/gc.c
index 97868c0..7ed53cf 100644
--- a/py/gc.c
+++ b/py/gc.c
@@ -480,12 +480,17 @@
GC_EXIT();
+ #if MICROPY_GC_CONSERVATIVE_CLEAR
+ // be conservative and zero out all the newly allocated blocks
+ memset((byte*)ret_ptr, 0, (end_block - start_block + 1) * BYTES_PER_BLOCK);
+ #else
// zero out the additional bytes of the newly allocated blocks
// This is needed because the blocks may have previously held pointers
// to the heap and will not be set to something else if the caller
// doesn't actually use the entire block. As such they will continue
// to point to the heap and may prevent other blocks from being reclaimed.
memset((byte*)ret_ptr + n_bytes, 0, (end_block - start_block + 1) * BYTES_PER_BLOCK - n_bytes);
+ #endif
#if MICROPY_ENABLE_FINALISER
if (has_finaliser) {
@@ -713,8 +718,13 @@
GC_EXIT();
+ #if MICROPY_GC_CONSERVATIVE_CLEAR
+ // be conservative and zero out all the newly allocated blocks
+ memset((byte*)ptr_in + n_blocks * BYTES_PER_BLOCK, 0, (new_blocks - n_blocks) * BYTES_PER_BLOCK);
+ #else
// zero out the additional bytes of the newly allocated blocks (see comment above in gc_alloc)
memset((byte*)ptr_in + n_bytes, 0, new_blocks * BYTES_PER_BLOCK - n_bytes);
+ #endif
#if EXTENSIVE_HEAP_PROFILING
gc_dump_alloc_table();
diff --git a/py/malloc.c b/py/malloc.c
index b0493d9..f48cb8d 100644
--- a/py/malloc.c
+++ b/py/malloc.c
@@ -117,7 +117,10 @@
if (ptr == NULL && num_bytes != 0) {
return m_malloc_fail(num_bytes);
}
+ // If this config is set then the GC clears all memory, so we don't need to.
+ #if !MICROPY_GC_CONSERVATIVE_CLEAR
memset(ptr, 0, num_bytes);
+ #endif
return ptr;
}
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 491a537..23591e0 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -107,6 +107,15 @@
#define MICROPY_ALLOC_GC_STACK_SIZE (64)
#endif
+// Be conservative and always clear to zero newly (re)allocated memory in the GC.
+// This helps eliminate stray pointers that hold on to memory that's no longer
+// used. It decreases performance due to unnecessary memory clearing.
+// TODO Do analysis to understand why some memory is not properly cleared and
+// find a more efficient way to clear it.
+#ifndef MICROPY_GC_CONSERVATIVE_CLEAR
+#define MICROPY_GC_CONSERVATIVE_CLEAR (1)
+#endif
+
// Support automatic GC when reaching allocation threshold,
// configurable by gc.threshold().
#ifndef MICROPY_GC_ALLOC_THRESHOLD