[Bug c++/86605] New: Suboptimal code for pointer arithmetic with 'this'

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86605

            Bug ID: 86605
           Summary: Suboptimal code for pointer arithmetic with 'this'
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

In GCC 5.5 an optimization of following code was broken:

struct Test {
    Test(Test* p) : i{this-p ? 10 : 20} {}
    int i;
};

int f() {
    Test test{(Test*)4};
    return test.i;
}



GCC 5.4 generates optimal code:
f():
  mov eax, 10
  ret

GCC 5.5 generates:
f():
  lea rax, [rsp-5]
  cmp rax, 7
  sbb eax, eax
  and eax, 10
  add eax, 10
  ret

GCC (trunk) generates also suboptimal code:
f():
  cmp rsp, 8
  mov edx, 20
  mov eax, 10
  cmove eax, edx
  ret



Note, that in GCC 5.5 the optimizer was fixed to generate optimal code for the
following:
struct Test {
    Test(Test* p) : i{this-p ? 10 : 20} {}
    int i;
};

int f() {
    Test test{(Test*)3}; // 3 instead of 4
    return test.i;
}

So that now GCC (trunk) generates optimal code for that case:
f():
  mov eax, 10
  ret
but fail if we change 3 back to 4