This assumes jemalloc3 is enabled.

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.mozconfig

ac_add_options --enable-replace-malloc

================================================
memory/replace/moz.build

+ DIRS += ['randchunks']

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memory/replace/randchunks/moz.build

SharedLibrary('replace_randchunks')

SOURCES += [
    'malloc.cpp',
    ]

DEFINES['abort'] = 'moz_abort'

DISABLE_STL_WRAPPING = True

================================================
memory/replace/randchunks/malloc.cpp

#include "replace_malloc.h"
#include <unistd.h>
#include <execinfo.h>
#include <stdlib.h>
#include <stdio.h>

//Requires https://bug1052573.bugzilla.mozilla.org/attachment.cgi?id=8573433

#define NUM_RAND_ARENAS 10

static const malloc_table_t *funcs = NULL;
static unsigned int mallocs = 0, frees = 0, reallocs = 0, callocs = 0;
static unsigned int arenas[NUM_RAND_ARENAS];
static const uintptr_t randseed = 5;

extern "C" {

void replace_init(const malloc_table_t *table)
{
  funcs = table;
  for(int i=0; i<NUM_RAND_ARENAS; i++) {
    arenas[i] = funcs->malloc_create_partition();
  }
  printf("In init!\n");
}

void replace_jemalloc_stats(jemalloc_stats_t *stats)
{
  void* fps[4];
  backtrace(fps, 4);
  

  printf("%d mallocs, %d frees, %d reallocs, %d callocs\n4-point stacktrace:\n", mallocs, frees, reallocs, callocs);
  backtrace_symbols_fd(fps, 4, STDERR_FILENO);
}

void* replace_malloc(size_t size)
{
  int arena_indx;
  uintptr_t fps[4];
  //backtrace((void**)fps, 4);
  
  arena_indx = (randseed * fps[1] * fps[2] * fps[3]) % NUM_RAND_ARENAS;

  mallocs++;
  return funcs->malloc_from_partition(arenas[arena_indx], size);
}

void replace_free(void* ptr)
{
  frees++;
  funcs->free(ptr);
}

void* replace_realloc(void* ptr, size_t size)
{
  int arena_indx;
  uintptr_t fps[4];
  //backtrace((void**)fps, 4);
  
  arena_indx = (randseed * fps[1] * fps[2] * fps[3]) % NUM_RAND_ARENAS;

  reallocs++;
  return funcs->realloc_from_partition(arenas[arena_indx], ptr, size);
}

void* replace_calloc(size_t nmemb, size_t size)
{
  int arena_indx;
  uintptr_t fps[4];
  //backtrace((void**)fps, 4);
  
  arena_indx = (randseed * fps[1] * fps[2] * fps[3]) % NUM_RAND_ARENAS;

  callocs++;
  return funcs->calloc_from_partition(arenas[arena_indx], nmemb, size);
}

} //extern C