[Bug libstdc++/80331] New: unused const std::string not optimized away

["hiraditya at msn dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80331

            Bug ID: 80331
           Summary: unused const std::string not optimized away
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hiraditya at msn dot com
  Target Milestone: ---

$ cat t.cpp
#include<string>
int sain() {
  const std::string s("a");
  return 0;
}

# gcc version 7.0.0 20170118 (experimental) (GCC)
$ g++ -S -o t.s t.cpp -O2 -fno-exceptions -std=c++11

$ cat t.s
        .type   _Z4sainv, @function
_Z4sainv:
.LFB940:
        .cfi_startproc
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        movl    $.LC0+1, %esi
        subq    $32, %rsp
        .cfi_def_cfa_offset 48
        leaq    16(%rsp), %rbx
        movq    %rsp, %rdi
        movq    %rbx, (%rsp)
        call   
_ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEE12_M_constructIPKcEEvT_S8_St20forward_iterator_tag.isra.16.constprop.20
        movq    (%rsp), %rdi
        cmpq    %rbx, %rdi
        je      .L13
        call    _ZdlPv
.L13:
        addq    $32, %rsp
        .cfi_def_cfa_offset 16
        xorl    %eax, %eax
        popq    %rbx
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc
.LFE940:
        .size   _Z4sainv, .-_Z4sainv


clang++, on the other hand, completely optimizes the const string.

        .type   _Z4sainv,@function
_Z4sainv:                               # @_Z4sainv
        .cfi_startproc
# BB#0:
        xorl    %eax, %eax
        retq
.Lfunc_end0:
        .size   _Z4sainv, .Lfunc_end0-_Z4sainv