132 lines
3.8 KiB
Plaintext
132 lines
3.8 KiB
Plaintext
.\" $Header$
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.TL
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Addition of register variables to an existing table.
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.NH 1
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Introduction
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.PP
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This is a short description of the newest feature in the
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table driven code generator for the Amsterdam Compiler Kit.
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It describes how to add register variables to an existing table.
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This assumes a distribution of October 1983 or later.
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It is not clear whether one should read this when starting with
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a table for a new machine,
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or waiting till the table is well debugged already.
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.NH 1
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Modifications to the table itself.
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.NH 2
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Register section
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.PP
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Just before the properties of the register one
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of the following can be added:
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.IP - 2
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regvar
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.IP -
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regvar ( pointer )
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.IP -
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regvar ( loop )
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.IP -
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regvar ( float )
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.LP
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All register variables of one type must be of the same size,
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and they may have no subregisters.
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.NH 2
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Codesection
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.PP
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.IP - 2
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Two pseudo functions are added to the list allowed inside expressions:
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.RS
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.IP 1) 3
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inreg ( expr ) has as a parameter the offset of a local,
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and returns 0,1 or 2:
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.RS
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.IP 2: 3
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if the variable is in a register.
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.IP 1:
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if the variable could be in a register but isn't.
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.IP 0:
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if the variable cannot be in a register.
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.RE
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.IP 2)
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regvar ( expr ) returns the register associated with the variable.
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Undefined if it is not in a register.
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So regvar ( expr ) is defined if and only if inreg (expr ) == 2.
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.RE
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.IP -
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It is now possible to remove() a register expression,
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this is of course needed for a store into a register local.
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.IP -
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The return out of a procedure may now involve register restores,
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so the special word 'return' in the table will invoke a user defined
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function.
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.NH 1
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Modifications to mach.c
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.PP
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If register variables are used in a table, the program
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.I cgg
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will define the word REGVARS during compilation of the sources.
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So the following functions described here should be bracketed
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by #ifdef REGVARS and #endif.
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.IP - 2
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regscore(off,size,typ,freq,totyp) long off;
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.br
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This function should assign a score to a register variable,
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the score should preferably be the estimated number of bytes
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gained when it is put in a register.
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Off and size are the offset and size of the variable,
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typ is the type, that is reg_any, reg_pointer, reg_loop or reg_float.
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Freq is the number of times it occurs statically, and totyp
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is the type of the register it is planned to go into.
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.br
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Keep in mind that the gain should be net, that is the cost for
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register save/restore sequences and the cost of initialisation
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in the case of parameters should already be included.
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.IP -
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i_regsave()
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.br
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This function is called at the start of a procedure, just before
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register saves are done.
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It can be used to initialise some variables if needed.
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.IP -
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f_regsave()
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.br
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This function is called at end of the register save sequence.
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It can be used to do the real saving if multiple register move
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instructions are available.
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.IP -
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regsave(regstr,off,size) char *regstr; long off;
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.br
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Should either do the real saving or set up a table to have
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it done by f_regsave.
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Note that initialisation of parameters should also be done,
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or planned here.
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.IP -
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regreturn()
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.br
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Should restore saved registers and return.
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The function result is already in the function return area by now.
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.NH 1
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Examples
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.PP
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Here are some examples out of the PDP 11 table
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.DS
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lol inreg($1)==2| | | regvar($1) | |
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lil inreg($1)==2| | | {regdef2, regvar($1)} | |
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stl inreg($1)==2| xsource2 |
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remove(regvar($1))
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move(%[1],regvar($1)) | | |
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inl inreg($1)==2| | remove(regvar($1))
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"inc %(regvar($1)%)"
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setcc(regvar($1)) | | |
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.DE
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.NH 1
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Afterthoughts.
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.PP
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At the time of this writing the tables for the PDP 11 and the M68000 and
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the VAX are converted, in all cases the two byte wordsize versions.
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No big problems have occurred, but experience has shown that it is
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necessary to check the table carefully for all patterns with locals in them.
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Code may be generated that uses the memoryslot the local is not in.
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