[Bug target/42779] [C++0x] Variadic templates + lambdas = extremely poor code quality, __m128i and aliasing sucks

["rguenth at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #5 from rguenth at gcc dot gnu dot org  2010-01-17 21:05 -------
Well, indeed - it would be far more useful to have a less convoluted testcase
where unrelated functions and source make analysis hard.

Please provide a testcase with a _single_ computation kernel applying it in
a single way (I'm trying to follow op_and ...).

>From an inlining perspective it doesn't look so bad - early inlining turns 
the innermost loop into

<bb 7>:
  D.15257_17 = thisD.13894_4(D)->m_DataD.13729;
  D.15256_19 = iD.15246_18 * 16;
  D.15255_20 = D.15257_17 + D.15256_19;
  D.15254_21 = v2D.13892_5(D)->m_DataD.13729;
  D.15256_22 = iD.15246_18 * 16;
  D.15253_23 = D.15254_21 + D.15256_22;
  D.15252_24 = *D.15253_23;
  D.15251_25 = *D.15236_2;
  D.15256_26 = iD.15246_18 * 16;
  D.15250_27 = D.15251_25 + D.15256_26;
  D.15249_28 = *D.15250_27;
  D.15247_29 = __builtin_ia32_pand128D.1150 (D.15249_28, D.15252_24);
  *D.15255_20 = D.15247_29;
  iD.15246_30 = iD.15246_18 + 1;

<bb 8>:
  # iD.15246_18 = PHI <0(6), iD.15246_30(7)>
  if (max_idxD.15245_16 != iD.15246_18)
    goto <bb 7>;

already (_ZN10bit_vector6op_andERKS_S1_).

Now the main issue why the redundant loads are not hoisted is that all
data pointers are ref-all:

  chunk_typeD.13721 * {ref-all} D.15255;

thus you tell the compiler that the store

  *D.15255_20 = D.15247_29;

might possibly clobber all loads in that loop.  Of course chunk_type is
just __m128i and I always complained that this is ref-all which makes
optimization of pointers to __m128i practically useless.

This is really a target header problem.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rth at gcc dot gnu dot org,
                   |                            |rguenth at gcc dot gnu dot
                   |                            |org, hjl at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
          Component|tree-optimization           |target
     Ever Confirmed|0                           |1
           Keywords|                            |alias, missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2010-01-17 21:05:55
               date|                            |
            Summary|[C++0x] Variadic templates +|[C++0x] Variadic templates +
                   |lambdas = extremely poor    |lambdas = extremely poor
                   |code quality                |code quality, __m128i and
                   |                            |aliasing sucks


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42779