faasm: force all heap allocations to use __sbrk

Author: csegarragonz

dlmalloc/src/dlmalloc.c

@@ -6,22 +6,23 @@
 
 // Define configuration macros for dlmalloc.
 
-#ifdef __faasm
-// Faasm supports mmap/ munmap
-#define HAVE_MMAP 1
+// WebAssembly doesn't have mmap-style memory allocation.
+#define HAVE_MMAP 0
 
-// Faasm supports normal wasm memory growth, but we need to be able to reclaim
-// memory, so force everything through mmap/ munmap
+#ifdef __faasm
+// Faasm supports memory growth and shrinkage with sbrk-style positive and
+// negative allocation. We need to be explicit about sbrk usage (HAVE_MORECORE
+// set to 1) and not mmap/munmap usage (HAVE_MMAP set to 0, default in WASM).
+// Note that this does not mean that we don't support application code using
+// mmap/munmap, it means that heap is only managed with sbrk
 #define HAVE_MORECORE 1
 #define MORECORE_CONTIGUOUS 1
 
-// Faasm supports shrinking linear memory using offsets
+// We don't shrink the linear memory, but claim the pages for the runtime, and
+// move the offset down, so that subsequent allocations can re-use the free-ed
+// memory
 #define MORECORE_CANNOT_TRIM 0
-
 #else
-// WebAssembly doesn't have mmap-style memory allocation.
-#define HAVE_MMAP 0
-
 // WebAssembly doesn't support shrinking linear memory.
 #define MORECORE_CANNOT_TRIM 1
 #endif