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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/104529] [missed optimization] inefficient codegen around
 new/delete
Date: Mon, 14 Feb 2022 18:25:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104529

--- Comment #2 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
I don't quite see how this is a CSE problem,

There's only one of each constant and none of them are needed before the ca=
ll.
unlike in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D86892

You don't need the values of your array until you allocate memory for said
array.

x86 has the following sequence in GIMPLE

  _32 =3D operator new (6);
  MEM <unsigned int> [(char * {ref-all})_32] =3D 255;
  MEM <unsigned short> [(char * {ref-all})_32 + 4B] =3D 0;
  operator delete (_32, 6);

which is optimal, you create the object, store the values, and remove it.

AArch64 however has this

  MEM <unsigned int> [(unsigned char *)&D.24688] =3D 255;
  MEM <unsigned short> [(unsigned char *)&D.24688 + 4B] =3D 0;
  _34 =3D operator new (6);
  MEM <unsigned char[6]> [(char * {ref-all})_34] =3D MEM <unsigned char[6]>
[(char * {ref-all})&D.24688];
  D.24688 =3D{v} {CLOBBER(eol)};
  operator delete (_34, 6);

which is where the issue comes from. So this has nothing to do with CSE as =
far
as I can tell.  The GIMPLE is just suboptimal.=