[hwasan] Do not memset allocation if it comes from the secondary allocator

[leonardchan]

Summary

The secondary allocator calls mmap which should return zero-inited pages, so we don't need to explicitly memset it with zeros. This is similar to what asan's calloc does.

[mcgrathr]

lgtm

[vitalybuka]

lgtm

compiler-rt/lib/hwasan/hwasan_allocator.cpp

static void *HwasanAllocate(StackTrace *stack, uptr orig_size, uptr alignment,
   }
   if (UNLIKELY(!allocated)) {
     SetAllocatorOutOfMemory();
     if (AllocatorMayReturnNull())
       return nullptr;
     ReportOutOfMemory(size, stack);
   }
   if (zeroise) {
+    // The secondary allocator mmaps memory, which should be zero-inited so we
+    // don't need to explicitly clear it.
+    if (allocator.FromPrimary(allocated))
       internal_memset(allocated, 0, size);
   } else if (flags()->max_malloc_fill_size > 0) {
     uptr fill_size = Min(size, (uptr)flags()->max_malloc_fill_size);
     internal_memset(allocated, flags()->malloc_fill_byte, fill_size);
   }
   if (size != orig_size) {
     u8 *tail = reinterpret_cast<u8 *>(allocated) + orig_size;
     uptr tail_length = size - orig_size;
     internal_memcpy(tail, tail_magic, tail_length - 1);