[Bug middle-end/71315] New: missing strlen optimization on a POINTER_PLUS expression

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71315

            Bug ID: 71315
           Summary: missing strlen optimization on a POINTER_PLUS
                    expression
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

In addition to bug 71303 and bug 71304, another optimization missing from 
tree-ssa-strlen.c is for calls to strlen with arguments that point past the
first element of a string are not optimized into constants, resulting in
(repeated) calls to strlen with the same argument.

Clang emits optimal code for this test case.

$ cat z.cpp && /build/gcc-6-branch/gcc/xgcc -B /build/gcc-6-branch/gcc -O2 -S
-Wall -Wextra -Wpedantic -fdump-tree-optimized=/dev/stdout zz.cpp

template<class T1>
class C
{
  template<class T2>
  class C2 { };
};

template<> template<class X, class Y>
class C<int>::C2 { };
template<> template<int>
class C<float>::C2 { };


;; Function void g() (_Z1gv, funcdef_no=0, decl_uid=2254, cgraph_uid=0,
symbol_order=0)

void g() ()
{
  char s[5];
  long unsigned int _3;
  unsigned int _4;
  long unsigned int _6;
  unsigned int _7;

  <bb 2>:
  s = "1234";
  _3 = __builtin_strlen (&MEM[(void *)&s + 1B]);
  _4 = (unsigned int) _3;
  f (_4);
  _6 = __builtin_strlen (&MEM[(void *)&s + 1B]);
  _7 = (unsigned int) _6;
  f (_7);
  s ={v} {CLOBBER};
  return;

}