From a911877b81a82a59fbc735ad604c01461429d1ab Mon Sep 17 00:00:00 2001
From: Andreas Kloeckner <inform@tiker.net>
Date: Tue, 27 Mar 2012 00:42:31 -0400
Subject: [PATCH] Add common subexpression finder.
---
pymbolic/cse.py | 103 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 103 insertions(+)
create mode 100644 pymbolic/cse.py
diff --git a/pymbolic/cse.py b/pymbolic/cse.py
new file mode 100644
index 0000000..f4f53f7
--- /dev/null
+++ b/pymbolic/cse.py
@@ -0,0 +1,103 @@
+from __future__ import division
+import pymbolic.primitives as prim
+from pymbolic.mapper import IdentityMapper, WalkMapper
+
+COMMUTATIVE_CLASSES = (prim.Sum, prim.Product)
+
+
+
+
+def get_normalized_cse_key(node):
+ if isinstance(node, COMMUTATIVE_CLASSES):
+ return type(node), frozenset(node.children)
+ else:
+ return node
+
+
+
+
+class CSEMapper(IdentityMapper):
+ def __init__(self, to_eliminate):
+ self.to_eliminate = to_eliminate
+
+ self.canonical_subexprs = {}
+
+ def get_cse(self, expr, key=None):
+ if key is None:
+ key = get_normalized_cse_key(expr)
+
+ try:
+ return self.canonical_subexprs[key]
+ except KeyError:
+ new_expr = prim.CommonSubexpression(
+ getattr(IdentityMapper, expr.mapper_method)(self, expr))
+ self.canonical_subexprs[key] = new_expr
+ return new_expr
+
+ def map_sum(self, expr):
+ key = get_normalized_cse_key(expr)
+ if key in self.to_eliminate:
+ result = self.get_cse(expr, key)
+ return result
+ else:
+ return getattr(IdentityMapper, expr.mapper_method)(self, expr)
+
+ map_product = map_sum
+ map_power = map_sum
+ map_quotient = map_sum
+ map_remainder = map_sum
+ map_floor_div = map_sum
+ map_call = map_sum
+
+ def map_quotient(self, expr):
+ if expr in self.to_eliminate:
+ return self.get_cse(expr)
+ else:
+ return IdentityMapper.map_quotient(self, expr)
+
+ def map_substitution(self, expr):
+ return type(expr)(
+ expr.child,
+ expr.variables,
+ tuple(self.rec(v) for v in expr.values))
+
+
+
+
+class UseCountMapper(WalkMapper):
+ def __init__(self):
+ self.subexpr_counts = {}
+
+ def visit(self, expr):
+ key = get_normalized_cse_key(expr)
+
+ if key in self.subexpr_counts:
+ self.subexpr_counts[key] += 1
+
+ # do not re-traverse (and thus re-count subexpressions)
+ return False
+ else:
+ self.subexpr_counts[key] = 1
+
+ # continue traversing
+ return True
+
+
+
+
+def tag_common_subexpressions(exprs):
+ ucm = UseCountMapper()
+
+ if isinstance(exprs, prim.Expression):
+ raise TypeError("exprs should be an iterable of expressions")
+
+ for expr in exprs:
+ ucm(expr)
+
+ to_eliminate = set([subexpr_key
+ for subexpr_key, count in ucm.subexpr_counts.iteritems()
+ if count > 1])
+ cse_mapper = CSEMapper(to_eliminate)
+ result = [cse_mapper(expr) for expr in exprs]
+ return result
+
--
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