[Bug tree-optimization/22184] tree vectorizer depends on context

["rakdver at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From rakdver at gcc dot gnu dot org  2005-07-07 10:52 -------
More precisely, the code when it comes to loop optimizer looks basically as

if (len > 3)
  something;
else if (len > 0)
  something_else;
else
  return;

for (i = 0; i < len; i++)
  whatever;

So indeed, len > 0 on each path that reaches the loop, but it is not trivial to 
deduce.

One solution is to use the results of VRP here. However, VRP uses scev 
analysis, and adding yet another cycle into the current completely 
incomprehensible nest of dependences between # of iterations analysis and SCEV 
seems quite scary to me (and there are other technical difficulties with this 
solution -- running VRP twice is bad from efficiency reasons, but keeping 
results of VRP valid is not entirely trivial and may prevent some 
optimizations).

The other possibility is to extend the current oracle-like approach (# of 
iterations analysis basically asks whether a given condition -- len > 0 in this 
case -- is true, and answer is computed on-demand by traversing SSA and 
dominance tree) to handle also this case.  This would however need to have the 
ASSERT_EXPRs (so we either would need to insert them before loop optimizer, or 
keep them valid since VRP, neither of which seems to be good for performance 
and memory consumption).

-- 
           What    |Removed                     |Added
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                 CC|                            |dnovillo at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22184