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tree-optimization/110204 - second level redundancy and simplification

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When PRE discovers a full redundancy during insertion it cannot unite
the two value sets.  Instead it inserts a copy old-val = new-val where
new-val can also be a constant.  The following looks through such
copies during elimination, providing one extra level of constant and
copy propagation.  For the PR this helps avoiding a bogus diagnostic
that's emitted on unreachable code during loop optimization.

	PR tree-optimization/110204
	* tree-ssa-sccvn.cc (eliminate_dom_walker::eliminate_avail):
	Look through copies generated by PRE.
This commit is contained in:
Richard Biener 2023-07-17 12:15:29 +02:00
parent c5bd0e5870
commit d0de3bf917
1 changed files with 16 additions and 1 deletions
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17
gcc/tree-ssa-sccvn.cc
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@ -6594,7 +6594,22 @@ eliminate_dom_walker::eliminate_avail (basic_block, tree op)
if (SSA_NAME_IS_DEFAULT_DEF (valnum))
return valnum;
if (avail.length () > SSA_NAME_VERSION (valnum))
return avail[SSA_NAME_VERSION (valnum)];
{
tree av = avail[SSA_NAME_VERSION (valnum)];
/* When PRE discovers a new redundancy there's no way to unite
the value classes so it instead inserts a copy old-val = new-val.
Look through such copies here, providing one more level of
simplification at elimination time. */
gassign *ass;
if (av && (ass = dyn_cast <gassign *> (SSA_NAME_DEF_STMT (av))))
if (gimple_assign_rhs_class (ass) == GIMPLE_SINGLE_RHS)
{
tree rhs1 = gimple_assign_rhs1 (ass);
if (CONSTANT_CLASS_P (rhs1) || TREE_CODE (rhs1) == SSA_NAME)
av = rhs1;
}
return av;
}
}
else if (is_gimple_min_invariant (valnum))
return valnum;