[Bug tree-optimization/18576] [3.4/4.0 Regression] missing jump threading because of type changes

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-12-12 17:42 -------
Another (possibly expensive) approach would be to "ignore" statements whose
results are used only in the basic block we are threading through.

Consider:

  # iftmp.0_1 = PHI <1(2), 0(3), 0(1)>;
<L4>:;
  D.1171_13 = (unsigned char) iftmp.0_1;
  D.1160_14 = (int) D.1171_13;
  D.1145_16 = (unsigned char) D.1160_14;
  if (D.1145_16 != 0) goto <L8>; else goto <L14>;

Note that the LHS of all MODIFY_EXPRs are used only in this basic block.
So if we get to COND_EXPR and find out that COND_EXPR_COND is a constant
(due to temporary propagation), then we can safely duplicate this basic block
and thread an incoming edge (without causing code bloat).
All the duplicate copies of MODIFY_EXPRs will be removed as dead code.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18576