276 lines
6.8 KiB
Text
276 lines
6.8 KiB
Text
.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
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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
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objectfile ( NEW A.OUT(5L)) and just before dumping the tables this
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objectfile is converted to a Berkly 4.2BSD a.out-file. We build two versions;
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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.
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It is interesting to compare this C-compiler ( called fcemcom) with "cc -c".
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fcemcom1 (the dynamic variant of fcemcom) is tuned in such a way, that
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alloc() won't be called.
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.NH 2
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Compile time
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.PP
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fac.c is a small program that produces n! ( see below). foo.c is small program
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that loops a lot.
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.TS
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center, box, tab(:);
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c | c | c | c | c | c
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c | c | n | n | n | n.
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compiler : program : real : user : sys : object size
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=
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fcemcom : sort.c : 31.0 : 17.5 : 1.8 : 23824
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fcemcom1 : : 59.0 : 21.2 : 3.3 :
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cc -c : : 50.0 : 38.0 : 3.5 : 6788
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_
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fcemcom : ed.c : 37.0 : 23.6 : 2.3 : 41744
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fcemcom1 : : 1.16.0 : 28.3 : 4.6 :
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cc -c : : 1.19.0 : 54.8 : 4.3 : 11108
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_
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fcemcom : cp.c : 4.0 : 2.4 : 0.8 : 4652
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fcemcom1 : : 9.0 : 3.0 : 1.0 :
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cc -c : : 8.0 : 5.2 : 1.6 : 1048
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_
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fcemcom : uniq.c : 5.0 : 2.5 : 0.8 : 5568
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fcemcom1 : : 9.0 : 2.9 : 0.8 :
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cc -c : : 13.0 : 5.4 : 2.0 : 3008
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_
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fcemcom : btlgrep.c : 24.0 : 7.2 : 1.4 : 12968
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fcemcom1 : : 23.0 : 8.1 : 1.2 :
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cc -c : : 1.20.0 : 15.3 : 3.8 : 2392
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_
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fcemcom : fac.c : 1.0 : 0.1 : 0.5 : 216
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fecmcom1 : : 2.0 : 0.2 : 0.5 :
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cc -c : : 3.0 : 0.7 : 1.3 : 92
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_
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fcemcom : foo.c : 4.0 : 0.2 : 0.5 : 272
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fcemcom1 : : 11.0 : 0.3 : 0.5 :
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cc -c : : 7.0 : 0.8 : 1.6 : 108
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.TE
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.NH 2
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Run time
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.LP
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Is the runtime very bad?
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.TS
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tab(:), box, center;
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c | c | c | c | c
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c | c | n | n | n.
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compiler : program : real : user : system
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=
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fcem : sort.c : 22.0 : 17.5 : 1.5
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cc : : 5.0 : 2.4 : 1.1
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_
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fcem : btlgrep.c : 1.58.0 : 27.2 : 4.2
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cc : : 12.0 : 3.6 : 1.1
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_
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fcem : foo.c : 1.0 : 0.7 : 0.1
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cc : : 1.0 : 0.4 : 0.1
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_
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fcem : uniq.c : 2.0 : 0.5 : 0.3
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cc : : 1.0 : 0.1 : 0.2
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.TE
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.NH 2
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quality object code
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.LP
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The runtime is very bad so its interesting to have look at the code which is
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produced by fcemcom and by cc -c. I took a program which computes recursively
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n!.
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.DS
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long fac();
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main()
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{
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int n;
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scanf( "%D", &n);
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printf( "fac is %D\\\\n", fac( n));
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}
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long fac( n)
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int n;
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{
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if ( n == 0)
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return( 1);
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else
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return( n * fac( n-1));
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}
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.DE
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.br
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.br
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.br
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.br
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.LP
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"cc -c fac.c" produces :
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.DS
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fac: tstl 4(ap)
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bnequ 7f
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movl $1, r0
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ret
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7f: subl3 $1, 4(ap), r0
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pushl r0
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call $1, fac
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movl r0, -4(fp)
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mull3 -4(fp), 4(ap), r0
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ret
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.DE
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.br
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.br
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.LP
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"fcem fac.c fac.o" produces :
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.DS
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_fac: 0
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42: jmp be
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48: pushl 4(ap)
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4e: pushl $0
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54: subl2 (sp)+,(sp)
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57: tstl (sp)+
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59: bnequ 61
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5b: jmp 67
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61: jmp 79
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67: pushl $1
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6d: jmp ba
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73: jmp b9
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79: pushl 4(ap)
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7f: pushl $1
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85: subl2 (sp)+,(sp)
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88: calls $0,_fac
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8f: addl2 $4,sp
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96: pushl r0
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98: pushl 4(ap)
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9e: pushl $4
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a4: pushl $4
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aa: jsb .cii
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b0: mull2 (sp)+,(sp)
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b3: jmp ba
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b9: ret
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ba: movl (sp)+,r0
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bd: ret
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be: jmp 48
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.DE
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.NH 1
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Conclusions
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.PP
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comparing "cc -c" with "fcemcom"
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.LP
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.TS
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center, box, tab(:);
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c | c s | c | c s
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^ | c s | ^ | c s
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^ | c | c | ^ | c | c
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l | n | n | n | n | n.
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program : compile time : object size : runtime
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:_::_
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: user : sys :: user : sys
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=
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sort.c : 0.47 : 0.5 : 3.5 : 7.3 : 1.4
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_
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ed.c : 0.46 : 0.5 : 3.8 : : :
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_
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cp.c : 0.46 : 0.5 : 4.4 : : :
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_
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uniq.c : 0.46 : 0.4 : 1.8 : : :
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_
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btlgrep.c : 0.47 : 0.3 : 5.4 : 7.5 : 3.8
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_
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fac.c : 0.14 : 0.4 : 2.3 : 1.8 : 1.0
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_
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foo.c : 0.25 : 0.3 : 2.5 : 5.0 : 1.5
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.TE
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.PP
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The results for fcemcom1 are almost identical; The only thing that changes
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is that fcemcom1 is 1.2 slower than fcemcom. ( compile time) This is due to
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to an another datastructure . In the static version we use huge array's for
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the text- and
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data-segment, the relocation information, the symboltable and stringarea.
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In the dynamic version we use linked lists, wich makes it expensive to get
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and to put a byte on a abritrary memory location. So it is probably better
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to use realloc(), because in the most cases there will be enough memory.
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.PP
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The quality of the objectcode is very bad. The reason is that the frontend
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generates bad code and expects the peephole-optimizer to improve the code.
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This is also one of the main reasons that the runtime is very bad.
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(e.g. the expensive "cii" with arguments 4 and 4 could be deleted.)
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So its seems a good
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idea to put a new peephole-optimizer between the frontend and the ce.
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.PP
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Using the peephole optimizer the ce would produce :
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.DS
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_fac: 0
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pushl 4(ap)
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tstl (sp)+
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beqlu 1f
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jmp 3f
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1 : pushl $1
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jmp 2f
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3 : pushl 4(ap)
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decl (sp)
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calls $0,_fac
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addl2 $4,sp
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pushl r0
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pushl 4(ap)
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mull2 (sp)+,(sp)
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movl (sp)+,r0
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2 : ret
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.DE
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.PP
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Bruce McKenzy already implemented it and made some improvements in the
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source code of the ce. The compile-time is two to two and a half times better
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and the
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size of the objectcode is two to three times bigger.(comparing with "cc -c")
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Still we could do better.
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.PP
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Using peephole- and push/pop-optimization ce could produce :
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.DS
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_fac: 0
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tstl 4(ap)
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beqlu 1f
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jmp 2f
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1 : pushl $1
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jmp 3f
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2 : decl 4(ap)
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calls $0,_fac
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addl2 $4,sp
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mull3 4(ap), r0, -(sp)
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movl (sp)+, r0
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3 : ret
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.DE
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.PP
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prof doesn't cooperate, so no profile information.
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.PP
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