[Bug c++/68678] New: Initialization of pointer by constant expression using static ctor.

[sorganov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68678

            Bug ID: 68678
           Summary: Initialization of pointer by constant expression using
                    static ctor.
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sorganov at gmail dot com
  Target Milestone: ---

g++ 5 produces initialization for the following case using static constructor,
while earlier versions of g++ as well as gcc 5 in C compilation mode do it
using simple entry in .data section.

This sounds like missed optimization.

$ cat dynamic-init.cc
typedef struct S { int x; } S;

void const* p = (void*)(0x1234 + sizeof(S));

$ ~/try/gcc-5.2.0/bin/gcc -O2 -c -save-temps dynamic-init.cc
$ cat dynamic-init.s
        .file   "dynamic-init.cc"
        .section        .text.unlikely,"ax",@progbits
.LCOLDB0:
        .section        .text.startup,"ax",@progbits
.LHOTB0:
        .p2align 4,,15
        .type   _GLOBAL__sub_I_p, @function
_GLOBAL__sub_I_p:
.LFB1:
        .cfi_startproc
        movq    $4664, p(%rip)
        ret
        .cfi_endproc
.LFE1:
        .size   _GLOBAL__sub_I_p, .-_GLOBAL__sub_I_p
        .section        .text.unlikely
.LCOLDE0:
        .section        .text.startup
.LHOTE0:
        .section        .init_array,"aw"
        .align 8
        .quad   _GLOBAL__sub_I_p
        .globl  p
        .bss
        .align 8
        .type   p, @object
        .size   p, 8
p:
        .zero   8
        .ident  "GCC: (GNU) 5.2.1 20151116"
        .section        .note.GNU-stack,"",@progbits

Here is what gcc 4.4.5 did:

$ gcc -O2 -c -save-temps dynamic-init.cc 
$ cat dynamic-init.s
        .file   "dynamic-init.cc"
.globl p
        .data
        .align 8
        .type   p, @object
        .size   p, 8
p:
        .quad   4664
        .ident  "GCC: (GNU) 4.4.5 20101112 (Red Hat 4.4.5-2)"
        .section        .note.GNU-stack,"",@progbits
$