[Bug tree-optimization/49513] PRE inhibits if-conversion and vectorization

["vincenzo.innocente at cern dot ch" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49513

--- Comment #2 from vincenzo Innocente <vincenzo.innocente at cern dot ch> 2011-06-23 12:16:59 UTC ---
thanks for the fast analysis.

the code in question was there also to test what the vectorized code would do
for x=0 and y=0 (it is extracted from a simplified version of atan2f that gcc
can vectorize).
Being a test, I can workaround (I've to test extreme conditions explicitely
anyhow).

In any case I think that any peeling will ruin alignmement.

 hoping gcc can find ways to avoid PRE and vectorization to clash.

       vincenzo




On 23 Jun, 2011, at 1:55 PM, rguenth at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49513
> 
> Richard Guenther <rguenth at gcc dot gnu.org> changed:
> 
>           What    |Removed                     |Added
> ----------------------------------------------------------------------------
>             Status|UNCONFIRMED                 |NEW
>           Keywords|                            |missed-optimization
>   Last reconfirmed|                            |2011.06.23 11:55:09
>                 CC|                            |rguenth at gcc dot gnu.org
>     Ever Confirmed|0                           |1
>            Summary|introducing a product       |PRE inhibits if-conversion
>                   |inhibit vectorization       |and vectorization
> 
> --- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-23 11:55:09 UTC ---
> That is because PRE decided to optimize the first iteration where it knows
> that z == 0 and thus z*s[i] and z*c[i] are 0 and thus a[i] will be 0.  Which
> means we confuse if-conversion which in turn causes this missed vectorization.
> IL before if-conversion:
> 
> <bb 2>:
>  goto <bb 6>;
> 
> <bb 3>:
>  z_3 = (float) i_9;
>  D.3280_4 = c[i_9];
>  D.3281_5 = D.3280_4 * z_3;
>  D.3282_6 = s[i_9];
>  D.3283_7 = D.3282_6 * z_3;
>  yy_13 = ABS_EXPR <D.3281_5>;
>  yy_14 = ABS_EXPR <D.3283_7>;
>  if (yy_13 < yy_14)
>    goto <bb 5>;
>  else
>    goto <bb 4>;
> 
> <bb 4>:
> 
> <bb 5>:
>  # yy_28 = PHI <yy_13(4), yy_14(3)>
>  # yy_29 = PHI <yy_14(4), yy_13(3)>
> 
> <bb 6>:
>  # yy_16 = PHI <yy_28(5), 0.0(2)>
>  # yy_15 = PHI <yy_29(5), 0.0(2)>
>  # i_30 = PHI <i_9(5), 0(2)>
>  # ivtmp.44_24 = PHI <ivtmp.44_10(5), 1024(2)>
>  t_17 = yy_15 / yy_16;
>  a[i_30] = t_17;
>  i_9 = i_30 + 1;
>  ivtmp.44_10 = ivtmp.44_24 - 1;
>  if (ivtmp.44_10 != 0)
>    goto <bb 3>;
>  else
>    goto <bb 7>;
> 
> <bb 7>:
>  return;
> 
> The PHI nodes in bb 6 is what makes if-conversion fail (thus, the
> irregular loop entry which really should be peeled off).
> 
> This situation commonly occurs when PRE can compute the first iterations
> result.  Manually peeling off the iteration like the following is a
> workaround:
> 
> void foo2() {
>  a[0] = 0;
>  for (int i=1; i!=1024; ++i) {
>    float z = i;
>    a[i] = bar(z*s[i],z*c[i]);
> }
> }
> 
> Not sure if your original issue you derived this testcase from really
> matches the above problem though.
> 
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--
Il est bon de suivre sa pente, pourvu que ce soit en montant. 
A.G.
http://www.flickr.com/photos/vin60/1320965757/