On Feb 19, 2012, Richard Sandiford <rdsandif...@googlemail.com> wrote:
> I have to admit I still don't like these changes > I'd much rather we kept to the original dag. I'm not sure I mentioned before, but it remains a DAG unless cselib_add_permanent_equiv is called. Only var-tracking calls it, and even then, only when VTA is enabled, so if anyone ever runs into a problem, it's easy enough to disable VTA, var-tracking or even -g altogether to work around the problem. Now, I confess I didn't expect problems in the first place, for I'd missed this use of alias.c by var-tracking. The other use, in find_base_term, had been properly adjusted already. There aren't any other uses of CSELIB_VAL_PTR, so I'm now pretty confident we won't run into any other problems like this. (famous last words ;-) > and it still isn't obvious to me when canonical_cselib_val is supposed > to be used. For comparison of VALUEs, it avoids the need for recursive or combinatorial compares, for all equivalent VALUEs map directly to the single canonical value. For recursive searches and other the like, it's just an optimization to avoid an additional recursion (avoiding recursing into *any* VALUEs is recommended along with it). Other algorithms that iterate over loc lists and recurse should take care to avoid infinite recursion, by marking already-visited nodes (cselib and var-tracking do some of this), temporarily zeroing out their loc lists (like find_base_term), or using other mechanisms to break recursion cycles (like get_addr). Algorithms that didn't expect naked VALUEs in loc lists (like get_addr) may need adjusting to iterate over the loc lists of the canonical value (for non-canonical values have a single loc, the canonical value), and to disregard values in canonical value's lists (unless e.g. we happen to be looking for VALUEs that turn out to be noncanonical). In other cases, the use of canonical values instead of noncanonical ones when filling in data structures (say, building expressions to record in cselib) may save memory by avoiding duplication, but since it causes cselib to compute different hashes, we'd better use whatever is most likely to be searched for by hashing. (We could tweak lookups to use canonical values and to recompute hashes when adding equivalences between values already used in expressions, but this hasn't been done yet). I hope this makes some sense ;-) -- Alexandre Oliva, freedom fighter http://FSFLA.org/~lxoliva/ You must be the change you wish to see in the world. -- Gandhi Be Free! -- http://FSFLA.org/ FSF Latin America board member Free Software Evangelist Red Hat Brazil Compiler Engineer
