[Bug tree-optimization/82732] malloc+zeroing other than memset not optimized to calloc, so asm output is malloc+memset

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82732

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
We do recognize the memset early enough. What we fail to recognize is that the
size argument to malloc is the same as the length of the memset:

  _1 = (long unsigned int) size_8(D);
  _2 = _1 * 4;
  p_11 = malloc (_2);
  if (size_8(D) != 0)
    goto <bb 3>; [85.00%] [count: INV]
  else
    goto <bb 4>; [15.00%] [count: INV]

  <bb 3> [12.75%] [count: INV]:
  _18 = size_8(D) + 4294967295;
  _21 = (sizetype) _18;
  _7 = _21 + 1;
  _6 = _7 * 4;
  __builtin_memset (p_11, 0, _6);

VRP could be taught to simplify (unsigned long)(u-1)+1 to (unsigned long)u for
unsigned int u non-zero (though there is no VRP between ldist and strlen), or
we could try to generate some simpler code in ldist...