Gcc 3.1 performance regressions with respect to 2.95.3

[Jason Merrill]

>> Gcc 3.1 is slower in the areas: E exception handling, L loop
>> overhead, G io and S Stepanov.

The loop/Stepanov slowdown is an optimizer bug which also affects C.  For
the loop in

  double accumulate(double* first, double* last, double result)
  {
    for (; first != last; ++first)
      result += *first;
    return result;
  }

we used to produce

        cmpl %edx,%eax
        je .L4
.L6:
        faddl (%eax)
        addl $8,%eax
        cmpl %edx,%eax
        jne .L6
.L4:

but now we produce

.L9:
        cmpl    %edx, %eax
        je      .L8
        faddl   (%eax)
        addl    $8, %eax
        jmp     .L9
.L8:

which is one insn shorter but about 20% slower on i686.  This pessimization
seems to be performed by the flow2 pass; until that point the rtl looks
like the old form.

Jason