[all-commits] [llvm/llvm-project] 49e5a9: Add an algorithm for performing "optimal" layout o...

[John McCall via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 49e5a97ec363a844b167340a82b54a01dc4e671a
      https://github.com/llvm/llvm-project/commit/49e5a97ec363a844b167340a82b54a01dc4e671a
  Author: John McCall <rjmccall at apple.com>
  Date:   2020-03-23 (Mon, 23 Mar 2020)

  Changed paths:
    A llvm/include/llvm/Support/OptimalLayout.h
    M llvm/lib/Support/CMakeLists.txt
    A llvm/lib/Support/OptimalLayout.cpp
    M llvm/unittests/Support/CMakeLists.txt
    A llvm/unittests/Support/OptimalLayoutTest.cpp

  Log Message:
  -----------
  Add an algorithm for performing "optimal" layout of a struct.

The algorithm supports both assigning a fixed offset to a field prior to
layout and allowing fields to have sizes that aren't multiples of their
required alignments.  This means that the well-known algorithm of sorting
by decreasing alignment isn't always good enough.  Still, we start with
that, and only if that leaves padding around do we fall back on a greedy
padding-minimizing algorithm.

There is no known efficient algorithm for producing a guaranteed-minimal
layout in all cases.  In fact, allowing arbitrary fixed-offset fields means
there's a straightforward reduction from bin-packing, making this NP-hard.
But as usual with such problems, we can still efficiently produce adequate
solutions to the cases that matter most to us.

I intend to use this in coroutine frame layout, where the retcon lowerings
very badly want to minimize total space usage, and where the switch lowering
can indeed produce a header with interior padding if the promise field is
highly-aligned.  But it may be useful in a much wider variety of situations.