[all-commits] [llvm/llvm-project] bf434a: Improve the representation of <compare>'s zero-onl...

[Richard Smith via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: bf434a5f173eed4112a10e28e8a6236d48f9da07
      https://github.com/llvm/llvm-project/commit/bf434a5f173eed4112a10e28e8a6236d48f9da07
  Author: Richard Smith <richard at metafoo.co.uk>
  Date:   2020-09-29 (Tue, 29 Sep 2020)

  Changed paths:
    M libcxx/include/__config
    M libcxx/include/compare
    A libcxx/test/std/language.support/cmp/cmp.categories.pre/zero_type.verify.cpp

  Log Message:
  -----------
  Improve the representation of <compare>'s zero-only type.

* Use an empty struct instead of a member pointer to represent this
  type, so that we don't actually pass a zero member pointer at runtime.

* Mark the constructor as consteval to ensure that no code is emitted
  for it whenever possible.

* Add a honeypot constructor to reject all non-int arguments, so that
  the only argument that can arrive at the real constructor is the
  literal 0.

This results in better generated code, and rejecting invalid comparisons
against nullptr, 0L, and so on, while also rejecting invalid comparisons
against (1-1) and similar that would be allowed if we required an
integer constant expression with value 0.

Differential Revision: https://reviews.llvm.org/D85051