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[Bug tree-optimization/19516] missed optimization (bool)
- From: "kazu at cs dot umass dot edu" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 6 Feb 2005 17:49:50 -0000
- Subject: [Bug tree-optimization/19516] missed optimization (bool)
- References: <20050118222857.19516.rguenth@tat.physik.uni-tuebingen.de>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- 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