[Bug tree-optimization/19507] New: missed tree-optimization

[rguenth at tat dot physik dot uni-tuebingen dot de]

The following testcase:


class Flag {
public:
        Flag(bool f) : flag(f) {}
        bool test() const { return flag; }
private:
        const bool flag;
};

void bar(void);

void foo(const Flag& f)
{
        if (f.test())
                bar();
        if (f.test())
                bar();
}


Should from my point of view should generate exactly one test
and optimize the redundant one.  I miss what could be a not
ill-formed way of bar() modifying Flag::flag.

With mainline -O2 -S -fdump-tree-optimized-vops we get for
t63.optimized:

<bb 0>:
  if (f->flag != 0) goto <L1>; else goto <L2>;

<L1>:;
  #   TMT.2_17 = V_MAY_DEF <TMT.2_15>;
  bar ();

<L2>:;
  if (f->flag != 0) goto <L4>; else goto <L5>;

<L4>:;
  #   TMT.2_16 = V_MAY_DEF <TMT.2_14>;
  bar () [tail call];

<L5>:;
  return;


The RTL optimizers exploit a valid optimization, namely:

_Z3fooRK4Flag:
.LFB6:
        pushl   %ebx    #
.LCFI0:
        subl    $8, %esp        #,
.LCFI1:
        movl    16(%esp), %ebx  # f, f
        cmpb    $0, (%ebx)      # <variable>.flag
        jne     .L8     #,
.L6:
        addl    $8, %esp        #,
        popl    %ebx    #
        ret
        .p2align 4,,7
.L8:
        call    _Z3barv #
        cmpb    $0, (%ebx)      # <variable>.flag
        .p2align 4,,4
        je      .L6     #,
        addl    $8, %esp        #,
        popl    %ebx    #
        jmp     _Z3barv #


where you can see we optimized the function into the equivalent of

   if (!f.test())
     return;
   bar();
   if (!f.test())
     return;
   bar();

Who is supposed to apply the corresponding tree optimization here?

Of course, I think it is valid to omit the second test completely
as there is no valid way for bar() to change Flag::flag.

Note that this may be a frontend issue, as to the tree-optimizers
this may be no different than

void foo(const bool& f)
{
   if (f)
      bar();
   if (f)
      bar();
}

where there of course are valid ways for bar() to change f.

-- 
           Summary: missed tree-optimization
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at tat dot physik dot uni-tuebingen dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19507