[Bug tree-optimization/113583] Main loop in 519.lbm not vectorized.

[juzhe.zhong at rivai dot ai via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113583

--- Comment #13 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
Ok. I found the optimized tree:


  _5 = 3.33333333333333314829616256247390992939472198486328125e-1 - _4;
  _8 = .FMA (_5, 1.229999999999999982236431605997495353221893310546875e-1, _4);

Let CST0 = 3.33333333333333314829616256247390992939472198486328125e-1,
CST1 = 1.229999999999999982236431605997495353221893310546875e-1

The expression is equivalent to the following:

_5 = CST0 - _4;
_8 = _5 * CST1 + 4;

That is:

_8 = (CST0 - _4) * CST1 + 4;

So, We should be able to re-associate it like Clang:

_8 = CST0 * CST1 - _4 * CST1 + 4; ---> _8 = CST0 * CST1 + _4 * (1 - CST1);

Since both CST0 * CST1 and 1 - CST1 can be pre-computed during compilation
time.

Let say CST2 = CST0 * CST1, CST3 = 1 - CST1, then we can re-associate as Clang:

_8 = FMA (_4, CST3, CST2).

Any suggestions for this re-association ?  Is match.pd the right place to do it
?

Thanks.