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doc/ceg/proposal.tr
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284
doc/ceg/proposal.tr
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.TL
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Code Expander
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.br
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(proposal)
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.SH
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Introduction
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.LP
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The \fBcode expander\fR, \fBce\fR, is a program that translates EM-code to
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objectcode. The main goal is to translate very fast. \fBce\fR is an instance
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of the EM_CODE(3L)-interface. During execution of \fBce\fR, \fBce\fR will build
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in core a machine independent objectfile ( NEW A.OUT(5L)). With \fBcv\fR or
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with routines supplied by the user the machine independent objectcode will
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be converted to a machine dependent object code. \fBce\fR needs
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information about the targetmachine (e.g. the opcode's). We divide the
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information into two parts:
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.IP
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- The description in assembly instructions of EM-code instructions.
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.IP
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- The description in objectcode of assembly instructions.
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.LP
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With these two tables we can make a \fBcode expander generator\fR which
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generates a \fBce\fR. It is possible to put the information in one table
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but that will probably introduce (propable) more bugs in the table. So we
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divide and conquer. With this approach it is also possible to generate
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assembly code ( rather yhan objectcode), wich is useful for debugging.
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There is of course a link between the two tables, the link
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consist of a restriction on the assembly format. Every assembly
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instruction must have the following format:
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.sp
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INSTR ::= LABEL : MNEMONIC [ OPERAND ( "," OPERAND)* ]
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.sp
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.LP
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\fBCeg\fR uses the following algorithm:
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.IP \0\0a)
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The assembly table will be converted to a (C-)routine assemble().
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assemble() gets as argument a string, the assembler instruction,
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and can use the MNEMONIC to execute the corresponding action in the
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assembly table.
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.IP \0\0b)
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The routine assemble() can now be used to convert the EM-code table to
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a set of C-routines, wich together form an instance of the
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EM_CODE(3L).
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.SH
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The EM-instruction table
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.LP
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We use the following grammar:
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.sp
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.TS
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center box ;
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l.
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TABLE ::= (ROW)*
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ROW ::= C_instr ( SPECIAL | SIMPLE)
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SPECIAL ::= ( CONDITION SIMPLE)+ 'default' SIMPLE
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SIMPLE ::= '==>' ACTIONLIST | '::=' ACTIONLIST
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ACTIONLIST ::= [ ACTION ( ';' ACTION)* ] '.'
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ACTION ::= function-call | assembly-instruction
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.TE
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.LP
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An example for the 8086:
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.LP
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.DS
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C_lxl
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$arg1 == 0 ==> "push bp".
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$arg1 == 1 ==> "push EM_BSIZE(bp)".
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default ==> "mov cx, $arg1";
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"mov si, bp";
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"1: mov si, EM_BSIZE(si);
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"loop 1b"
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"push si".
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.DE
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.sp
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Some remarks:
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.sp
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* The C_instr is a function indentifier in the EM_CODE(3L)-interface.
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.LP
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* CONDITION is a "boolean" C-expression.
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.LP
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* The arguments of an EM-instruction can be used in CONDITION and in assembly
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instructions. They are referred by $arg\fIi\fR. \fBceg\fR modifies the
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arguments as follows:
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.IP \0\0-
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For local variables at positive offsets it increases this offset by EM_BSIZE
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.IP \0\0-
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It makes names en labels unique. The user must supply the formats (see mach.h).
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.LP
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* function-call is allowed to implement e.g. push/pop optimization.
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For example:
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.LP
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.DS
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C_adi
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$arg1 == 2 ==> combine( "pop ax");
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combine( "pop bx");
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"add ax, bx";
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save( "push ax").
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default ==> arg_error( "C_adi", $arg1).
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.DE
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.LP
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* The C-functions called in the EM-instructions table have to use the routine
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assemble()/gen?(). "assembler-instr" is in fact assemble( "assembler-instr").
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.LP
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* \fBceg\fR takes care not only about the conversions of arguments but also
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about
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changes between segments. There are situation when one doesn't want
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conversion of arguments. This can be done by using ::= in stead of ==>.
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This is usefull when two C_instr are equivalent. For example:
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.IP
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C_slu ::= C_sli( $arg1)
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.LP
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* There are EM-CODE instructions wich are machine independent (e.g. C_open()).
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For these EM_CODE instructions \fBceg\fR will generate \fIdefault\fR-
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instructions. There is one exception: in the case of C_pro() the tablewriter
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has to supply a function prolog().
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.LP
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* Also the EM-pseudoinstructions C_bss_\fIcstp\fR(), C_hol_\fIcstp\fR(),
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C_con_\fIcstp\fR() and C_rom_\fIcstp\fR can be translated automaticly.
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\fBceg\fR only has to know how to interpretate string-constants:
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.DS
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\&..icon $arg2 == 1 ==> gen1( (char) atoi( $arg1))
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$arg2 == 2 ==> gen2( atoi( $arg1))
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$arg2 == 4 ==> gen4( atol( $arg1))
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\&..ucon $arg2 == 1 ==> gen1( (char) atoi( $arg1))
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$arg2 == 2 ==> gen2( atoi( $arg1))
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$arg2 == 4 ==> gen4( atol( $arg1))
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\&..fcon ::= not_implemented( "..fcon")
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.DE
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.LP
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* Still, life can be made easier for the tablewriter; For the routines wich
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he/she didn't implement \fBceg\fR will generate a default instruction wich
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generates an error-message. \fBceg\fR seems to generate :
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.IP
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C_xxx ::= not_implemented( "C_xxx")
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.SH
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The assembly table
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.LP
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How to map assembly on objectcode.
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.LP
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Each row in the table consists of two fields, one field for the assembly
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instruction, the other field for the corresponding objectcode. The tablewriter
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can use the following primitives to generate code for the machine
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instructions :
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.IP "\0\0gen1( b)\0\0:" 17
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generates one byte in de machine independent objectfile.
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.IP "\0\0gen2( w)\0\0:" 17
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generates one word ( = two bytes), the table writer can change the byte
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order by setting the flag BYTES_REVERSED.
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.IP "\0\0gen4( l)\0\0:" 17
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generates two words ( = four bytes), the table writer can change the word
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order by setting the flag WORDS_REVERSED.
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.IP "\0\0reloc( n, o, r)\0\0:" 17
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generates relocation information for a label ( = name + offset +
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relocationtype).
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.LP
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Besides these primitives the table writer may use his self written
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C-functions. This allows the table writer e.g. to write functions to set
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bitfields within a byte.
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.LP
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There are more or less two methods to encode the assembly instructions:
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.IP \0\0a)
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MNEMONIC and OPERAND('s) are encoded independently of each other. This can be
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done when the target machine has an orthogonal instruction set (e.g. pdp-11).
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.IP \0\0b)
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MNEMONIC and OPERAND('s) together determine the opcode. In this case the
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assembler often uses overloading: one MNEMONIC is used for several
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different machine-instructions. For example : (8086)
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.br
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mov ax, bx
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.br
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mov ax, variable
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.br
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These instructions have different opcodes.
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.LP
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As the transformation MNEMONIC-OPCODE is not one to
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one the table writer must be allowed to put restrictions on the operands.
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This can be done with type declarations. For example:
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.LP
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.DS
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mov dst:REG, src:MEM ==>
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gen1( 0x8b);
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modRM( op2.reg, op1);
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.DE
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.DS
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mov dst:REG, src:REG ==>
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gen1( 0x89);
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modRM( op2.reg, op1);
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.DE
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.LP
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modRM() is a function written by the tablewriter and is used to encode
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the operands. This frees the table writer of endless typing.
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.LP
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The table writer has to do the "typechecking" by himself. But typechecking
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is almost the same as operand decoding. So it's more efficient to do this
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in one function. We now have all the tools to describe the function
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assemble().
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.IP
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assemble() first calls the function
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decode_operand() ( by the table writer written), with two arguments: a
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string ( the operand) and a
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pointer to a struct. The struct is declared by the table writer and must
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consist of at least a field called type. ( the other fields in the struct can
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be used to remember information about the decoded operand.) Now assemble()
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fires a row wich is selected by mapping the MNEMONIC and the type of the
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operands.
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.br
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In the second field of a row there may be references to other
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fields in the struct (e.g. op2.reg in the example above).
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.LP
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We ignored one problem. It's possible when the operands are encoded, that
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not everything is known. For example $arg\fIi\fR arguments in the
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EM-instruction table get their value at runtime. This problem is solved by
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introducing a function eval(). eval() has a string as argument and returns
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an arith. The string consists of constants and/or $arg\fIi\fR's and the value
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returned by eval() is the value of the string. To encode the $arg\fIi\fR's
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in as few bytes as possible the table writer can use the statements %if,
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%else and %endif. They can be used in the same manner as #if, #else and
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#endif in C and result in a runtime test. An example :
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.LP
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.DS
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-- Some rows of the assembly table
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mov dst:REG, src:DATA ==>
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%if sfit( eval( src), 8) /* does the immediate-data fit in 1 byte? */
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R53( 0x16 , op1.reg);
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gen1( eval( src));
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%else
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R53( 0x17 , op1.reg);
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gen2( eval( src));
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%endif
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.LD
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mov dst:REG, src:REG ==>
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gen1( 0x8b);
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modRM( op1.reg, op2);
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.DE
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.DS
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-- The corresponding part in the function assemble() :
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case MNEM_mov :
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decode_operand( arg1, &op1);
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decode_operand( arg2, &op2);
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if ( REG( op1.type) && DATA( op2.type)) {
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printf( "if ( sfit( %s, 8)) {\\\\n", eval( src));
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R53( 0x16 , op1.reg);
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printf( "gen1( %s)\\\\n", eval( arg2));
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printf( "}\\\\nelse {\\\\n");
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R53( 0x17 , op1.reg);
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printf( "gen2( %s)\\\\n", eval( arg2));
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printf( "}\\\\n");
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}
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else if ( REG( op1.type) && REG( op2.type)) {
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gen1( 0x8b);
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modRM( op1.reg, op2);
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}
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.DE
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.DS
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-- Some rows of the right part of the EM-instruction table are translated
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-- in the following C-functions.
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"mov ax, $arg1" ==>
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if ( sfit( w, 8)) { /* w is the actual argument of C_xxx( w) */
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gen1( 176); /* R53() */
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gen1( w);
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}
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else {
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gen1( 184);
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gen2( w);
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}
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.LD
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"mov ax, bx" ==>
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gen1( 138);
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gen1( 99); /* modRM() */
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.DE
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.SH
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Restrictions
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.LP
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.IP \0\01)
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The EM-instructions C_exc() is not implemented.
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.IP \0\03)
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All messages are ignored.
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276
doc/ceg/prototype.tr
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276
doc/ceg/prototype.tr
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@ -0,0 +1,276 @@
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.TL
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A prototype Code expander
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.NH
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Introduction
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.PP
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A program to be compiled with ACK is first fed into the preprocessor.
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The output of the preprocessor goes into the appropiate front end,
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whose job it is to produce EM. The EM code generated is
|
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fed into the peephole optimizer, wich scans it with a window of few
|
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instructions, replacing certain inefficient code sequences by better
|
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ones. Following the peephole optimizer follows a backend wich produces
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good assembly code. The assembly code goes into the assembler and the objectcode
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then goes into the loader/linker, the final component in the pipeline.
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.PP
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For various applications this scheme is too slow. For example for testing
|
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programs; In this case the program has to be translated fast and the
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runtime of the objectcode may be slower. A solution is to build a code
|
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expander ( \fBce\fR) wich translates EM code to objectcode. Of course this
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has to
|
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be done automaticly by a code expander generator, but to get some feeling
|
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for the problem we started out to build prototypes.
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We built two types of ce's. One wich tranlated EM to assembly, one
|
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wich translated EM to objectcode.
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.NH
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EM to assembly
|
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.PP
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We made one for the 8086 and one for the vax4. These ce's are instances of the
|
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EM_CODE(3L)-interface and produce for a single EM instruction a set
|
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of assembly instruction wich are semantic equivalent.
|
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We implemented in the 8086-ce push/pop-optimalization.
|
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.NH
|
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EM to objectcode
|
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.PP
|
||||
Instead of producing assembly code we tried to produce vax4-objectcode.
|
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During execution of ce, ce builds in core a machine independent
|
||||
objectfile ( NEW A.OUT(5L)) and just before dumping the tables this
|
||||
objectfile is converted to a Berkly 4.2BSD a.out-file. We build two versions;
|
||||
One with static memory allocation and one with dynamic memory allocation.
|
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If the first one runs out of memory it will give an error message and stop,
|
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the second one will allocate more memory and proceed with producing
|
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objectcode.
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.PP
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The C-frontend calls the EM_CODE-interface. So after linking the frontend
|
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and the ce we have a pipeline in a program saving a lot of i/o.
|
||||
It is interesting to compare this C-compiler ( called fcemcom) with "cc -c".
|
||||
fcemcom1 (the dynamic variant of fcemcom) is tuned in such a way, that
|
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alloc() won't be called.
|
||||
.NH 2
|
||||
Compile time
|
||||
.PP
|
||||
fac.c is a small program that produces n! ( see below). foo.c is small program
|
||||
that loops a lot.
|
||||
.TS
|
||||
center, box, tab(:);
|
||||
c | c | c | c | c | c
|
||||
c | c | n | n | n | n.
|
||||
compiler : program : real : user : sys : object size
|
||||
=
|
||||
fcemcom : sort.c : 31.0 : 17.5 : 1.8 : 23824
|
||||
fcemcom1 : : 59.0 : 21.2 : 3.3 :
|
||||
cc -c : : 50.0 : 38.0 : 3.5 : 6788
|
||||
_
|
||||
fcemcom : ed.c : 37.0 : 23.6 : 2.3 : 41744
|
||||
fcemcom1 : : 1.16.0 : 28.3 : 4.6 :
|
||||
cc -c : : 1.19.0 : 54.8 : 4.3 : 11108
|
||||
_
|
||||
fcemcom : cp.c : 4.0 : 2.4 : 0.8 : 4652
|
||||
fcemcom1 : : 9.0 : 3.0 : 1.0 :
|
||||
cc -c : : 8.0 : 5.2 : 1.6 : 1048
|
||||
_
|
||||
fcemcom : uniq.c : 5.0 : 2.5 : 0.8 : 5568
|
||||
fcemcom1 : : 9.0 : 2.9 : 0.8 :
|
||||
cc -c : : 13.0 : 5.4 : 2.0 : 3008
|
||||
_
|
||||
fcemcom : btlgrep.c : 24.0 : 7.2 : 1.4 : 12968
|
||||
fcemcom1 : : 23.0 : 8.1 : 1.2 :
|
||||
cc -c : : 1.20.0 : 15.3 : 3.8 : 2392
|
||||
_
|
||||
fcemcom : fac.c : 1.0 : 0.1 : 0.5 : 216
|
||||
fecmcom1 : : 2.0 : 0.2 : 0.5 :
|
||||
cc -c : : 3.0 : 0.7 : 1.3 : 92
|
||||
_
|
||||
fcemcom : foo.c : 4.0 : 0.2 : 0.5 : 272
|
||||
fcemcom1 : : 11.0 : 0.3 : 0.5 :
|
||||
cc -c : : 7.0 : 0.8 : 1.6 : 108
|
||||
.TE
|
||||
.NH 2
|
||||
Run time
|
||||
.LP
|
||||
Is the runtime very bad?
|
||||
.TS
|
||||
tab(:), box, center;
|
||||
c | c | c | c | c
|
||||
c | c | n | n | n.
|
||||
compiler : program : real : user : system
|
||||
=
|
||||
fcem : sort.c : 22.0 : 17.5 : 1.5
|
||||
cc : : 5.0 : 2.4 : 1.1
|
||||
_
|
||||
fcem : btlgrep.c : 1.58.0 : 27.2 : 4.2
|
||||
cc : : 12.0 : 3.6 : 1.1
|
||||
_
|
||||
fcem : foo.c : 1.0 : 0.7 : 0.1
|
||||
cc : : 1.0 : 0.4 : 0.1
|
||||
_
|
||||
fcem : uniq.c : 2.0 : 0.5 : 0.3
|
||||
cc : : 1.0 : 0.1 : 0.2
|
||||
.TE
|
||||
.NH 2
|
||||
quality object code
|
||||
.LP
|
||||
The runtime is very bad so its interesting to have look at the code which is
|
||||
produced by fcemcom and by cc -c. I took a program which computes recursively
|
||||
n!.
|
||||
.DS
|
||||
long fac();
|
||||
|
||||
main()
|
||||
{
|
||||
int n;
|
||||
|
||||
scanf( "%D", &n);
|
||||
printf( "fac is %D\\\\n", fac( n));
|
||||
}
|
||||
|
||||
long fac( n)
|
||||
int n;
|
||||
{
|
||||
if ( n == 0)
|
||||
return( 1);
|
||||
else
|
||||
return( n * fac( n-1));
|
||||
}
|
||||
.DE
|
||||
.br
|
||||
.br
|
||||
.br
|
||||
.br
|
||||
.LP
|
||||
"cc -c fac.c" produces :
|
||||
.DS
|
||||
fac: tstl 4(ap)
|
||||
bnequ 7f
|
||||
movl $1, r0
|
||||
ret
|
||||
7f: subl3 $1, 4(ap), r0
|
||||
pushl r0
|
||||
call $1, fac
|
||||
movl r0, -4(fp)
|
||||
mull3 -4(fp), 4(ap), r0
|
||||
ret
|
||||
.DE
|
||||
.br
|
||||
.br
|
||||
.LP
|
||||
"fcem fac.c fac.o" produces :
|
||||
.DS
|
||||
_fac: 0
|
||||
42: jmp be
|
||||
48: pushl 4(ap)
|
||||
4e: pushl $0
|
||||
54: subl2 (sp)+,(sp)
|
||||
57: tstl (sp)+
|
||||
59: bnequ 61
|
||||
5b: jmp 67
|
||||
61: jmp 79
|
||||
67: pushl $1
|
||||
6d: jmp ba
|
||||
73: jmp b9
|
||||
79: pushl 4(ap)
|
||||
7f: pushl $1
|
||||
85: subl2 (sp)+,(sp)
|
||||
88: calls $0,_fac
|
||||
8f: addl2 $4,sp
|
||||
96: pushl r0
|
||||
98: pushl 4(ap)
|
||||
9e: pushl $4
|
||||
a4: pushl $4
|
||||
aa: jsb .cii
|
||||
b0: mull2 (sp)+,(sp)
|
||||
b3: jmp ba
|
||||
b9: ret
|
||||
ba: movl (sp)+,r0
|
||||
bd: ret
|
||||
be: jmp 48
|
||||
.DE
|
||||
.NH 1
|
||||
Conclusions
|
||||
.PP
|
||||
comparing "cc -c" with "fcemcom"
|
||||
.LP
|
||||
.TS
|
||||
center, box, tab(:);
|
||||
c | c s | c | c s
|
||||
^ | c s | ^ | c s
|
||||
^ | c | c | ^ | c | c
|
||||
l | n | n | n | n | n.
|
||||
program : compile time : object size : runtime
|
||||
:_::_
|
||||
: user : sys :: user : sys
|
||||
=
|
||||
sort.c : 0.47 : 0.5 : 3.5 : 7.3 : 1.4
|
||||
_
|
||||
ed.c : 0.46 : 0.5 : 3.8 : : :
|
||||
_
|
||||
cp.c : 0.46 : 0.5 : 4.4 : : :
|
||||
_
|
||||
uniq.c : 0.46 : 0.4 : 1.8 : : :
|
||||
_
|
||||
btlgrep.c : 0.47 : 0.3 : 5.4 : 7.5 : 3.8
|
||||
_
|
||||
fac.c : 0.14 : 0.4 : 2.3 : 1.8 : 1.0
|
||||
_
|
||||
foo.c : 0.25 : 0.3 : 2.5 : 5.0 : 1.5
|
||||
.TE
|
||||
.PP
|
||||
The results for fcemcom1 are almost identical; The only thing that changes
|
||||
is that fcemcom1 is 1.2 slower than fcemcom. ( compile time) This is due to
|
||||
to an another datastructure . In the static version we use huge array's for
|
||||
the text- and
|
||||
data-segment, the relocation information, the symboltable and stringarea.
|
||||
In the dynamic version we use linked lists, wich makes it expensive to get
|
||||
and to put a byte on a abritrary memory location. So it is probably better
|
||||
to use realloc(), because in the most cases there will be enough memory.
|
||||
.PP
|
||||
The quality of the objectcode is very bad. The reason is that the frontend
|
||||
generates bad code and expects the peephole-optimizer to improve the code.
|
||||
This is also one of the main reasons that the runtime is very bad.
|
||||
(e.g. the expensive "cii" with arguments 4 and 4 could be deleted.)
|
||||
So its seems a good
|
||||
idea to put a new peephole-optimizer between the frontend and the ce.
|
||||
.PP
|
||||
Using the peephole optimizer the ce would produce :
|
||||
.DS
|
||||
_fac: 0
|
||||
pushl 4(ap)
|
||||
tstl (sp)+
|
||||
beqlu 1f
|
||||
jmp 3f
|
||||
1 : pushl $1
|
||||
jmp 2f
|
||||
3 : pushl 4(ap)
|
||||
decl (sp)
|
||||
calls $0,_fac
|
||||
addl2 $4,sp
|
||||
pushl r0
|
||||
pushl 4(ap)
|
||||
mull2 (sp)+,(sp)
|
||||
movl (sp)+,r0
|
||||
2 : ret
|
||||
.DE
|
||||
.PP
|
||||
Bruce McKenzy already implemented it and made some improvements in the
|
||||
source code of the ce. The compile-time is two to two and a half times better
|
||||
and the
|
||||
size of the objectcode is two to three times bigger.(comparing with "cc -c")
|
||||
Still we could do better.
|
||||
.PP
|
||||
Using peephole- and push/pop-optimization ce could produce :
|
||||
.DS
|
||||
_fac: 0
|
||||
tstl 4(ap)
|
||||
beqlu 1f
|
||||
jmp 2f
|
||||
1 : pushl $1
|
||||
jmp 3f
|
||||
2 : decl 4(ap)
|
||||
calls $0,_fac
|
||||
addl2 $4,sp
|
||||
mull3 4(ap), r0, -(sp)
|
||||
movl (sp)+, r0
|
||||
3 : ret
|
||||
.DE
|
||||
.PP
|
||||
prof doesn't cooperate, so no profile information.
|
||||
.PP
|
||||
Loading…
Reference in a new issue