forked from OSchip/llvm-project
dbe00d51b4
(i == 5334 || i == 5335)
to:
((i & -2) == 5334)
This transformation has some incorrect side conditions. Specifically, the
transformation is only applied when the right-hand side constant (5334 in
the example) is a power of two not equal and not equal to the negated mask.
These side conditions were added in r258904 to fix PR26323. The correct side
condition is that: ((Constant & Mask) == Constant)[(5334 & -2) == 5334].
It's a little bit hard to see why these transformations are correct and what
the side conditions ought to be. Here is a CVC3 program to verify them for
64-bit values:
ONE : BITVECTOR(64) = BVZEROEXTEND(0bin1, 63);
x : BITVECTOR(64);
y : BITVECTOR(64);
z : BITVECTOR(64);
mask : BITVECTOR(64) = BVSHL(ONE, z);
QUERY( (y & ~mask = y) =>
((x & ~mask = y) <=> (x = y OR x = (y | mask)))
);
Please note that each pattern must be a dual implication (<--> or iff). One
directional implication can create spurious matches. If the implication is
only one-way, an unsatisfiable condition on the left side can imply a
satisfiable condition on the right side. Dual implication ensures that
satisfiable conditions are transformed to other satisfiable conditions and
unsatisfiable conditions are transformed to other unsatisfiable conditions.
Here is a concrete example of a unsatisfiable condition on the left
implying a satisfiable condition on the right:
mask = (1 << z)
(x & ~mask) == y --> (x == y || x == (y | mask))
Substituting y = 3, z = 0 yields:
(x & -2) == 3 --> (x == 3 || x == 2)
The version of this code before r258904 had no side-conditions and
incorrectly justified itself in comments through one-directional
implication.
Thanks to Chandler for the suggestion!
Author: Thomas Jablin (tjablin)
Reviewers: chandlerc majnemer hfinkel cycheng
http://reviews.llvm.org/D21417
llvm-svn: 272873
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