[Bug c++/90372] New: [x64][missed optimization] pushes unused r12 onto stack on unique_ptr use

[marc at kdab dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90372

            Bug ID: 90372
           Summary: [x64][missed optimization] pushes unused r12 onto
                    stack on unique_ptr use
           Product: gcc
           Version: 9.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marc at kdab dot com
  Target Milestone: ---

In the following example: https://godbolt.org/z/jHSYue
one(), using std::unique_ptr, pushes r12 to the stack just to pop it back at
the end. r12 does not appear to be used in-between.

The similar two() function does not do this, even though the rest of the
function (except the stack unwind trampoline) is instruction-by-instruction
identical.

Expected: r12 is not pushed/popped.