[Bug middle-end/48973] [4.3/4.4/4.5/4.6/4.7 Regression] Inliner bug with one-bit (1-bit) bitfield

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48973

--- Comment #9 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-12 11:36:21 UTC ---
(In reply to comment #8)
> (In reply to comment #6)
> > "Fixed" with bitfield lowering where we expand from
> > 
> >   v.0_1 = v;
> >   BF.1_3 = MEM[(struct S *)&s];
> >   D.2700_4 = BF.1_3 & -2;
> >   D.2702_6 = v.0_1 < 0;
> >   BF.1_7 = D.2702_6 | D.2700_4;
> >   MEM[(struct S *)&s] = BF.1_7;
> >   D.2693_9 = (<unnamed-signed:1>) BF.1_7;
> >   D.2694_10 = (unsigned int) D.2693_9;
> >   if (D.2694_10 != 4294967295)
> > 
> > similar to what Jakub proposed to do manually.
> 
> Well, if bitfield lowering does this and nothing cleans it up, there is room
> for improvement.  It would be sad if it couldn't be optimized already at the
> tree level back to:
>    v.0_1 = v;
>    BF.1_3 = MEM[(struct S *)&s];
>    D.2700_4 = BF.1_3 & -2;
>    D.2702_6 = v.0_1 < 0;
>    BF.1_7 = D.2702_6 | D.2700_4;
>    MEM[(struct S *)&s] = BF.1_7;
>    D.2693_9 = (<unnamed-signed:1>) D.2702_6;  // change here.  Assuming D.2702
>                                               // is either > 1 precision, or
>                                               // unsigned
>    D.2694_10 = (unsigned int) D.2693_9;
>    if (D.2694_10 != 4294967295)

Yeah, we can combine that with the comparison - but I'm not sure we
want to have compares with 1-bit precision quantities, do we?  Or
we want to optimize this to

  tem = D.2702_6 & CST;
  if (tem != 0)

instead?