Make dlmalloc and emmalloc align to max_align_t

Required by C and C++ standards

Author: Akaricchi

system/lib/dlmalloc.c

@@ -26,6 +26,12 @@
 #define USE_SPIN_LOCKS 0 // Ensure we use pthread_mutex_t.
 #endif
 
+/* `mallloc`ed pointers must be aligned at least as strictly as max_align_t. */
+#ifndef MALLOC_ALIGNMENT
+#include <stddef.h>
+#define MALLOC_ALIGNMENT (__alignof__(max_align_t))
+#endif
+
 #endif
 
 

system/lib/emmalloc.cpp

@@ -96,7 +96,8 @@ static size_t lowerBoundPowerOf2(size_t x) {
 // Constants
 
 // All allocations are aligned to this value.
-static const size_t ALIGNMENT = 8;
+// FIXME: compile as C++11 and change this to alignof(max_align_t)
+static const size_t ALIGNMENT = __alignof__(long double);
 
 // Even allocating 1 byte incurs this much actual payload
 // allocation. This is our minimum bin size.
@@ -131,8 +132,13 @@ struct Region;
 // the payload is a multiple of MIN_ALLOC).
 struct FreeInfo {
   // free lists are doubly-linked lists
-  FreeInfo* _prev;
-  FreeInfo* _next;
+  union {
+    struct {
+        FreeInfo* _prev;
+        FreeInfo* _next;
+    };
+    char _padding[ALLOC_UNIT];
+  };
 
   FreeInfo*& prev() { return _prev; }
   FreeInfo*& next() { return _next; }
@@ -173,7 +179,7 @@ struct Region {
   // the payload, or freelist info.
   union {
     FreeInfo _freeInfo;
-    char _payload[];
+    __attribute__((aligned(ALIGNMENT))) char _payload[];
   };
 
   size_t getTotalSize() { return _totalSize; }

tests/malloc_alignment.c

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2019 The Emscripten Authors.  All rights reserved.
+ * Emscripten is available under two separate licenses, the MIT license and the
+ * University of Illinois/NCSA Open Source License.  Both these licenses can be
+ * found in the LICENSE file.
+ */
+
+#include <assert.h>
+#include <stdalign.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+int main(int argc, char **argv) {
+    for(int i = 0; i < 16; ++i) {
+        void *p = malloc(i);
+        assert(((uintptr_t)p & (alignof(max_align_t) - 1)) == 0);
+    }
+
+    return 0;
+}

tests/test_core.py

@@ -8493,6 +8493,16 @@ def test():
     self.emcc_args += ['-DPOOL']
     test()
 
+  @parameterized({
+    'dlmalloc': ['dlmalloc'],
+    'emmalloc': ['emmalloc'],
+  })
+  def test_malloc_alignment(self, malloc):
+    self.set_setting('MALLOC', malloc)
+    self.emcc_args += ['-std=gnu11']
+    src = open(path_from_root('tests', 'malloc_alignment.c')).read()
+    self.do_run(src, '', basename='src.c')
+
 
 # Generate tests for everything
 def make_run(name, emcc_args, settings=None, env=None):