[Bug middle-end/70274] New: optimization goes astray and adds completely redundant code

["l_belev at yahoo dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70274

            Bug ID: 70274
           Summary: optimization goes astray and adds completely redundant
                    code
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: l_belev at yahoo dot com
  Target Milestone: ---

here is c code that demonstrates the problem:

double x;
unsigned n;
void foo() {
        while (n > 8) {
                n -= 8;
                x *= 3.0;
        }
}

compiled with options: -O2 -fomit-frame-pointer -quiet -ffast-math -march=core2

then observe the result assembly (i will paste part of it here for
convenience):
....
_foo:
        movl    _n, %edx
        cmpl    $8, %edx
        jbe     L1
        movsd   _x, %xmm0
        movl    %edx, %eax
        movsd   LC0, %xmm1
        .p2align 4,,10
L3:
        mulsd   %xmm1, %xmm0
        subl    $8, %eax
        cmpl    $8, %eax
        ja      L3
        leal    -9(%edx), %eax  // redundant code
        subl    $8, %edx        // redundant code
        movsd   %xmm0, _x
        andl    $-8, %eax       // redundant code
        subl    %eax, %edx      // redundant code
        movl    %edx, _n
L1:
        ret
...

Apparently GCC notices opportunity for strength-reduction (or something else,
i'm not sure what) about the variable n and produces code that directly
calculates n without the need of the loop (the instructions i marked with
"redundant code").

But GCC fails to consider that the loop is run anyway (because it has other
effects besides calculating n, which can't be strength-reduced).
So after the end of the loop, the final value of n is already available, but
GCC decides to discard it and re-calculates it again by means of the witty
"strength-reduced" code. That is completely redundant.