Assert on proper alignment in dlmalloc (#14634)

Fixes #10072

Author: kripken

system/lib/dlmalloc.c

@@ -24,8 +24,19 @@
 #define USE_SPIN_LOCKS 0 // Ensure we use pthread_mutex_t.
 #endif
 
+#ifndef MALLOC_ALIGNMENT
+#include <stddef.h>
+/* `malloc`ed pointers must be aligned at least as strictly as max_align_t. */
+#define MALLOC_ALIGNMENT (__alignof__(max_align_t))
+/*
+  Emscripten aligns even float128 to 64-bits, to save size and increase speed.
+  See https://github.com/emscripten-core/emscripten/issues/10072
+*/
+_Static_assert(MALLOC_ALIGNMENT == 8, "max_align_t must be 8");
 #endif
 
+#endif // __EMSCRIPTEN__
+
 
 #define __THROW
 #define __attribute_malloc__

system/lib/emmalloc.cpp

@@ -39,6 +39,7 @@
  *    malloc.
  */
 
+#include <stddef.h>
 #include <stdint.h>
 #include <unistd.h>
 #include <memory.h>
@@ -60,7 +61,8 @@ extern "C"
 
 // Configuration: specifies the minimum alignment that malloc()ed memory outputs. Allocation requests with smaller alignment
 // than this will yield an allocation with this much alignment.
-#define MALLOC_ALIGNMENT 8
+#define MALLOC_ALIGNMENT alignof(max_align_t)
+_Static_assert(alignof(max_align_t) == 8, "max_align_t must be correct");
 
 #define EMMALLOC_EXPORT __attribute__((weak, __visibility__("default")))