[Issue 7243] New: Compiler should call separate function when allocating a struct on the heap

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=7243

           Summary: Compiler should call separate function when allocating
                    a struct on the heap
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: schveiguy at yahoo.com


--- Comment #0 from Steven Schveighoffer <schveiguy at yahoo.com> 2012-01-07 12:00:56 PST ---
Note, this requires druntime fixes too, but I can't select both DMD and
druntime.


Currently, when creating a struct on the heap, _d_newarrayT is called with a
length of 1.

However, the runtime must assume this is an array of length 1.  Therefore, it
must set the appendable bit, and reserve space for the 'used' length.

This can waste space.

For instance, the following could fit in a 16-byte block, but will go into a
32-byte block due to the used field:

import core.memory;

struct S
{
  int[4] buf;
}

void main()
{
  auto s = new S;
  assert(GC.sizeof(s) == 32);
}

I think instead, a _d_newstruct function should be called (and implemented in
druntime) to initialize a single struct on the heap in the most efficient
manner possible.

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