166 lines
5 KiB
Text
166 lines
5 KiB
Text
.NH 2
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The Intermediate Code construction phase
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.PP
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The first phase of the global optimizer,
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called
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.UL IC,
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constructs a major part of the intermediate code.
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To be specific, it produces:
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.IP -
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the EM text
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.IP -
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the object table
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.IP -
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part of the procedure table
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.LP
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The calling, change and use attributes of a procedure
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and all its flags except the external and bodyseen flags
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are computed by the next phase (Control Flow phase).
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.PP
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As explained before,
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the intermediate code does not contain
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any names of variables or procedures.
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The normal identifiers are replaced by identifying
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numbers.
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Yet, the output of the global optimizer must
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contain normal identifiers, as this
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output is in Compact Assembly Language format.
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We certainly want all externally visible names
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to be the same in the input as in the output,
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because the optimized EM module may be a library unit,
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used by other modules.
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IC dumps the names of all procedures and data labels
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on two files:
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.IP -
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the procedure dump file, containing tuples (P_ID, procedure name)
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.IP -
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the data dump file, containing tuples (D_ID, data label name)
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.LP
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The names of instruction labels are not dumped,
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as they are not visible outside the procedure
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in which they are defined.
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.PP
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The input to IC consists of one or more files.
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Each file is either an EM module in Compact Assembly Language
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format, or a Unix archive file (library) containing such modules.
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IC only extracts those modules from a library that are
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needed somehow, just as a linker does.
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It is advisable to present as much code
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of the EM program as possible to the optimizer,
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although it is not required to present the whole program.
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If a procedure is called somewhere in the EM text,
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but its body (text) is not included in the input,
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its bodyseen flag in the procedure table will still
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be off.
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Whenever such a procedure is called,
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we assume the worst case for everything;
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it will change and use all variables it has access to,
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it will call every procedure etc.
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.sp
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Similarly, if a data label is used
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but not defined, the PSEUDO attribute in its data block
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will be set to UNKNOWN.
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.NH 3
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Implementation
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.PP
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Part of the code for the EM Peephole Optimizer
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.[
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staveren peephole toplass
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.]
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has been used for IC.
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Especially the routines that read and unravel
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Compact Assembly Language and the identifier
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lookup mechanism have been used.
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New code was added to recognize objects,
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build the object and procedure tables and to
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output the intermediate code.
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.PP
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IC uses singly linked linear lists for both the
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procedure and object table.
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Hence there are no limits on the size of such
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a table (except for the trivial fact that it must fit
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in main memory).
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Both tables are outputted after all EM code has
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been processed.
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IC reads the EM text of one entire procedure
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at a time,
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processes it and appends the modified code to
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the EM text file.
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EM code is represented internally as a doubly linked linear
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list of EM instructions.
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.PP
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Objects are recognized by looking at the operands
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of instructions that reference global data.
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If we come across the instructions:
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.DS
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.TS
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l l.
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LDE X+6 -- Load Double External
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LAE X+20 -- Load Address External
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.TE
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.DE
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we conclude that the data block
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preceded by the data label X contains an object
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at offset 6 of size twice the word size,
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and an object at offset 20 of unknown size.
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.sp
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A data block entry of the object table is allocated
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at the first reference to a data label.
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If this reference is a defining occurrence
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or a INA pseudo instruction,
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the label is not externally visible
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.[~[
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keizer architecture
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.], section 11.1.4.3]
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In this case, the external flag of the data block
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is turned off.
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If the first reference is an applied occurrence
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or a EXA pseudo instruction, the flag is set.
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We record this information, because the
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optimizer may change the order of defining and
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applied occurrences.
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The INA and EXA pseudos are removed from the EM text.
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They may be regenerated by the last phase
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of the optimizer.
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.sp
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Similar rules hold for the procedure table
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and the INP and EXP pseudos.
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.NH 3
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Source files of IC
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.PP
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The source files of IC consist
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of the files ic.c, ic.h and several packages.
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.UL ic.h
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contains type definitions, macros and
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variable declarations that may be used by
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ic.c and by every package.
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.UL ic.c
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contains the definitions of these variables,
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the procedure
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.UL main
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and some high level I/O routines used by main.
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.sp
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Every package xxx consists of two files.
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ic_xxx.h contains type definitions,
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macros, variable declarations and
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procedure declarations that may be used by
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every .c file that includes this .h file.
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The file ic_xxx.c provides the
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definitions of these variables and
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the implementation of the declared procedures.
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IC uses the following packages:
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.IP lookup: 18
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procedures that loop up procedure, data label
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and instruction label names; procedures to dump
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the procedure and data label names.
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.IP lib:
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one procedure that gets the next useful input module;
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while scanning archives, it skips unnecessary modules.
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.IP aux:
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several auxiliary routines.
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.IP io:
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low-level I/O routines that unravel the Compact
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Assembly Language.
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.IP put:
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routines that output the intermediate code
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.LP
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