[Bug rtl-optimization/85315] New: missed optimisation opportunity for derefences

[vegard.nossum at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85315

            Bug ID: 85315
           Summary: missed optimisation opportunity for derefences
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vegard.nossum at oracle dot com
  Target Milestone: ---

Input:

extern int x;
extern int a;
extern int b;

int f()
{
    int y = x;
    return *(&y + (a + b));
}

With -O3, trunk outputs:

f():
  movl x(%rip), %eax
  movl %eax, -4(%rsp)
  movl b(%rip), %eax
  addl a(%rip), %eax
  cltq
  movl -4(%rsp,%rax,4), %eax
  ret

Clang, on the other hand, infers that (a + b) == 0:

f(): # @f()
  movl x(%rip), %eax
  retq

From richi on IRC:

"""
we don't do this kind of optimization at the moment
a related one would be to place a if (a+b != 0) link_error (); after the memory
access
similarly for array accesses an if (i not-in-range) link_error ()
thus, we do not derive ranges for address-computation parts [at dereference
sites]
"""