78 lines
2.9 KiB
Text
78 lines
2.9 KiB
Text
.NH 2
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Implementation
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.PP
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UD first builds a number of tables:
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.IP locals: 9
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contains information about the local variables of the
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current procedure (offset,size,whether a register message was found
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for it and, if so, the score field of that message)
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.IP defs:
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a table of all explicit definitions appearing in the
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current procedure.
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.IP copies:
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a table of all copies appearing in the
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current procedure.
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.LP
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Every variable (local as well as global), definition and copy
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is identified by a unique number, which is the index
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in the table.
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All tables are constructed by traversing the EM code.
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A fourth table, "vardefs" is used, indexed by a 'variable number',
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which contains for every variable the set of explicit definitions of it.
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Also, for each basic block b, the set CHGVARS containing all variables
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changed by it is computed.
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.PP
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The GEN sets are obtained in one scan over the EM text,
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by analyzing every EM instruction.
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The KILL set of a basic block b is computed by looking at the
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set of variables
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changed by b (i.e. CHGVARS[b]).
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For every such variable v, all explicit definitions to v
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(i.e. vardefs[v]) that are not in GEN[b] are added to KILL[b].
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Also, the implicit defininition of v is added to KILL[b].
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Next, the data flow equations for use-definition information
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are solved,
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using a straight forward, iterative algorithm.
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All sets are represented as bitvectors, so the operations
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on sets (union, difference) can be implemented efficiently.
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.PP
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The C_GEN and C_KILL sets are computed simultaneously in one scan
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over the EM text.
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For every copy A := B appearing in basic block b we do
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the following:
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.IP 1.
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for every basic block n /= b that changes B, see if the definition A := B
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reaches the beginning of n (i.e. check if the index number of A := B in
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the "defs" table is an element of IN[n]);
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if so, add the copy to C_KILL[n]
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.IP 2.
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if B is redefined later on in b, add the copy to C_KILL[b], else
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add it to C_GEN[b]
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.LP
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C_IN and C_OUT are computed from C_GEN and C_KILL via the second set of
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data flow equations.
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.PP
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Finally, in one last scan all opportunities for optimization are
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detected.
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For every use u of a variable A, we check if
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there is a unique explicit definition d reaching u.
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.sp
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If the definition is a copy A := B and B has the same value at d as
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at u, then the use of A at u may be changed into B.
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The latter condition can be verified as follows:
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.IP -
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if u and d are in the same basic block, see if there is
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any assignment to B in between d and u
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.IP -
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if u and d are in different basic blocks, the condition is
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satisfied if there is no assignment to B in the block of u prior to u
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and d is in C_IN[b].
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.LP
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Before the transformation is actually done, UD first makes sure the
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alteration is really desirable, as described before.
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The information needed for this purpose (access costs of local and
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global variables) is read from a machine descriptor file.
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.sp
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If the only definition reaching u has the form "A := constant", the use
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of A at u is replaced by the constant.
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