[Bug tree-optimization/19516] missed optimization (bool)

[kazu@cs.umass.edu]

------- Additional Comments From kazu at cs dot umass dot edu  2005-02-06 17:49 -------
Hi Richard,

I didn't say that load elimination is the *only* way to take this
optimization opportunity.

DOM *can* thread incoming edges to a basic block with more than COND_EXPR
or SWITCH_EXPR in limited circumstances.

One problem here is that upon taking edge from <bb 0> to <L1>,
DOM does not record that D.1317_2 == 0.

Once we fix that, then D.1317_4 = *flag_1; is folded to D.1317_4 = 0;,
but DOM wants to see an SSA_NAME on rhs on every MODIFY_EXPR leading up to
COND_EXPR or SWITCH_EXPR.

If DOM also knew that D.1317_2 == D.1317_4, it could thread the edge in
question, but I cannot think of an easy way of teaching DOM how to
figure out equivalences of two SSA_NAMEs when we already have a known constant
value.

CCing Jeff as he is the DOM person.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19516