rt: Stop zeroing out all allocations. Unobservable perf win. Closes #2682. rs=negligible-perf-win

Author: pcwalton

src/rt/rust_upcall.cpp

@@ -158,8 +158,7 @@ upcall_s_exchange_malloc(s_exchange_malloc_args *args) {
     LOG_UPCALL_ENTRY(task);
 
     size_t total_size = get_box_size(args->size, args->td->align);
-    // FIXME--does this have to be calloc? (Issue #2682)
-    void *p = task->kernel->calloc(total_size, "exchange malloc");
+    void *p = task->kernel->malloc(total_size, "exchange malloc");
 
     rust_opaque_box *header = static_cast<rust_opaque_box*>(p);
     header->ref_count = -1; // This is not ref counted
@@ -234,8 +233,7 @@ upcall_s_malloc(s_malloc_args *args) {
     LOG_UPCALL_ENTRY(task);
     LOG(task, mem, "upcall malloc(0x%" PRIxPTR ")", args->td);
 
-    // FIXME--does this have to be calloc? (Issue #2682)
-    rust_opaque_box *box = task->boxed.calloc(args->td, args->size);
+    rust_opaque_box *box = task->boxed.malloc(args->td, args->size);
     void *body = box_body(box);
 
     debug::maybe_track_origin(task, box);