Re: [tree-ssa] Removing useless/redundant "const" calls

[law at redhat dot com]

In message <Pine.LNX.4.44.0306120921320.4105-100000@wotan.suse.de>, Michael Mat
z writes:
 >Hi,
 >
 >On Thu, 12 Jun 2003 law@redhat.com wrote:
 >> If the answer is no, then we'll need to distinguish between pure and
 >> const functions later -- simply clearing TREE_SIDE_EFFECTS would result
 >> in incorrect code.
 >
 >I see.  But I guess this can be done?
Certainly.  It wouldn't be terribly hard.  First we clear TREE_SIDE_EFFECTS
for "pure" functions.  Then we change this code in tree-ssa.c:

  /* Step 2.  Check for redundant computations.  Do this optimization only
     for assignments that make no calls and have no aliased stores
     nor volatile references and no side effects (i.e., no VDEFs).  */
  may_optimize_p = !ann->makes_aliased_stores
                   && !ann->has_volatile_ops
                   && vdefs == NULL
                   && def_p
                   && ! TREE_SIDE_EFFECTS (TREE_OPERAND (stmt, 1));
                                                                              
to reject 'pure' functions.  


Then you have DCE not consider pure or const functions are inherently
necessary.  

I'm not currently planning to do this (I've been down in this rathole too
long), but I'm more than happy to help walk you through it.

jeff