[Bug c++/119360] New: incomplete folding of initializers leads to dynamic initialization

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119360

            Bug ID: 119360
           Summary: incomplete folding of initializers leads to dynamic
                    initialization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nathan at gcc dot gnu.org
  Target Milestone: ---

Created attachment 60812
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=60812&action=edit
reproducer

compiling as
 g++ -O2
results in static initialization and folding.  Compilng as
 g++ -O2 -DBUG
results in dynamic init for BAR, but static init for FOO:
 _GLOBAL__sub_I__Z3foov:
        movq    $16, _ZL3BAR(%rip)
        ret
_Z3foov:
        movl    16, %eax
        ret
_Z3barv:
        movq    _ZL3BAR(%rip), %rax
        movl    (%rax), %eax
        ret



The problem is that we end up n initializer_constant_valid_p (varasm.cc), which
I think expects fully folded expressions.  When analyzing BAR's initializer it
recurses into FOO's DECL_INITIAL, which is essentially (+ 16 (* 16 0)), and
because of the multiplier thinks it's not suitable for a constant init. For
some reason we don't make that mistake with FOO itself.

I suspect we need to store a more folded initializer in DECL_INITIAL?

The original code had the multiplier as 2, not zero BTW.