[Bug c++/81972] New: Improve data tracking for simple conditional code

["bugzilla at poradnik-webmastera dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81972

            Bug ID: 81972
           Summary: Improve data tracking for simple conditional code
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugzilla@poradnik-webmastera.com
  Target Milestone: ---

[code]
__attribute__((nonnull(1)))
void f1(int*);

void f2(int* n)
{
    int* ptr = nullptr;
    if (*n > 2)
        ptr = n;
    f1(ptr);
}

void f3(int* n)
{
    if (*n > 2)
        f1(n);
    else
        f1(nullptr);
}
[/code]

Functions f2 and f3 are equivalent. Above code compiled by gcc 7.2 (with -O3
-Wall -Wextra) generates following assembler code:

[code]
f2(int*):
  cmp DWORD PTR [rdi], 2
  mov eax, 0
  cmovle rdi, rax
  jmp f1(int*)
f3(int*):
  cmp DWORD PTR [rdi], 2
  jg .L7
  xor edi, edi
.L7:
  jmp f1(int*)
[/code]

There are two issues here:
- both functions contains bug, they try to pass NULL pointer to f1. However gcc
reports this for f3 only. gcc is able to track data for optimization purposes,
so it should be possible to generate warning for f2.
- f3 is compiled to code with branch, so most probably it will be slower a bit.
It could be optimized to get the same code as f2 (branchless).