[Bug tree-optimization/18768] Missed ivopts opportunity

[rakdver at gcc dot gnu dot org]

------- Additional Comments From rakdver at gcc dot gnu dot org  2004-12-01 21:44 -------
There is no additional cost counted for "(double *) coefPtr".  The reason why
the result with the cast is different is because PRE creates a (dead) phi node
when moving the cast out of the loop.  This phi node changes estimate of register
pressure used in ivopts, which leads to divergence -- we do not perform strength
reduction.

Which is a problem in DFmode, since [reg + reg] addressing for DF mode object
requires two extra additions:

[reg1 + reg2] = ...   /* lower half */
reg2+=4;
[reg1 + reg2] = ...   /* upper half */
reg2-=4;

Whereas with [reg] addressing mode, this would only be

[reg] = ...
[reg + 4] = ...

Of course when we clean up the unused phi nodes created by PRE, the regression
gets hidden (Daniel is working on this).  But the unrelying problem -- the fact
that we cannot tell that the [reg + reg] addressing for DF mode is expensive --
remains.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18768