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Diff of /sml/trunk/NOTES/HISTORY

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revision 674, Thu Jun 22 06:45:00 2000 UTC revision 675, Fri Jun 23 09:18:18 2000 UTC
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12  Tag: <post-commit CVS tag>  Tag: <post-commit CVS tag>
13  Description:  Description:
14  ----------------------------------------------------------------------  ----------------------------------------------------------------------
15    Name: Matthias Blume
16    Date: 2000/06/23 18:20:00 JST
17    Tag: blume-20000623-btrace
18    Description:
19    
20    This updates adds a backtrace facility to aid programmers in debugging
21    their programs.  This involves the following changes:
22    
23    1. Module system/smlnj/init/core.sml (structure _Core) now has hooks for
24       keeping track of the current call stack.  When programs are compiled
25       in a special mode, the compiler will insert calls to these hooks
26       into the user program.
27       "Hook" means that it is possible for different implementations of
28       back-tracing to register themselves (at different times).
29    
30    2. compiler/MiscUtil/profile/btrace.sml implements the annotation phase
31       as an Absyn.dec->Absyn.dec rewrite.  Normally this phase is turned off.
32       It can be turned on using this call:
33         SMLofNJ.Internals.BTrace.mode (SOME true);
34       Turning it off again:
35         SMLofNJ.Internals.BTrace.mode (SOME false);
36       Querying the current status:
37         SMLofNJ.Internals.BTrace.mode NONE;
38       Annotated programs are about twice as big as normal ones, and they
39       run a factor of 2 to 4 slower with a dummy back-trace plugin (one
40       where all hooks do nothing).  The slowdown with a plugin that is
41       actually useful (such as the one supplied by default) is even greater,
42       but in the case of the default plugin it is still only an constant
43       factor (amortized).
44    
45    3. system/Basis/Implementation/NJ/internals.{sig,sml} have been augmented
46       with a sub-structure BTrace for controlling back-tracing.  In particular,
47       the above-mentioned function "mode" controls whether the annotation
48       phase is invoked by the compiler.  Another important function is
49       "trigger": when called it aborts the current execution and causes
50       the top-level loop to print a full back-trace.
51    
52    4. compiler/MiscUtil/profile/btimp.sml is the current default plugin
53       for back-tracing.  It keeps track of the dynamic call stack and in
54       addition to that it keeps a partial history at each "level" of that
55       stack.  For example, if a tail-calls b, b tail-calls c, and c tail-calls
56       d and b (at separate times, dynamically), then the report will show:
57    
58       GOTO   d
59             /c
60       GOTO  \b
61       CALL   a
62    
63       This shows that there was an initial non-tail call of a, then a
64       tail-call to b or c, looping behavior in a cluster of functions that
65       consist of b and c, and then a goto from that cluster (i.e., either from
66       b or from c) to d.
67    
68       Note that (depending on the user program) the amount of information
69       that the back-trace module has to keep track of at each level is bounded
70       by a constant.  Thus, the whole implementation has the same asymptotical
71       complexity as the original program (both in space and in time).
72    
73    5. compiler/TopLevel/interact/evalloop.sml has been modified to
74       handle the special exception SMLofNJ.Internals.BTrace.BTrace
75       which is raised by the "trigger" function mentioned above.
76    
77    Notes on usage:
78    
79    - Annotated code works well together with unannotated code:
80    Unannotated calls simply do not show up at all in the backtrace.
81    
82    - It is not a good idea to let modules that were annotated during
83    different sessions run at the same time.  This is because the compiler
84    chooses small integers to identify individual functions, and there
85    will be clashes if different modules were compiled in separate sessions.
86    (Nothing will crash, and you will even be told about the clashes, but
87    back-trace information will in general not be useful.)
88    
89    - Back-tracing can be confused by callcc and capture.
90    
91    - The only way of getting a back-trace right now is to explicitly
92    invoke the "trigger" function from your user program.  Eventually, we
93    should make every exception carry back-trace information (if
94    available).  But since this creates more overhead at "raise"-time
95    (similar to the current exnHistory overhead), I have not yet
96    implemented this.  (The implementation will be rather easy.)  With
97    exceptions carrying back-trace information, this facility will be even
98    more useful because users don't need to modify their programs...
99    
100    - While it is possible to compile the compiler with back-trace
101    annotations turned on (I did it to get some confidence in
102    correctness), you must make absolutely sure that core.sml and
103    btimp.sml are compiled WITHOUT annotation!  (core.sml cannot actually
104    be compiled with annotation because there is no core access yet, but
105    if you compile btimp.sml with annotation, then the system will go into
106    an infinite recursion and crash.)
107    Since CM currently does not know about BTrace, the only way to turn
108    annotations on and off for different modules of the compiler is to
109    interrupt CMB.make, change the settings, and re-invoke it.  Of course,
110    this is awkward and clumsy.
111    
112    Sample sessions:
113    
114    Standard ML of New Jersey v110.28.1 [FLINT v1.5], June 5, 2000
115    - SMLofNJ.Internals.BTrace.mode (SOME true);
116    [autoloading]
117    [autoloading done]
118    val it = false : bool
119    - structure X = struct
120    -     fun main n = let
121    -         fun a (x, 0) = d x
122    -           | a (x, n) = b (x, n - 1)
123    -         and b (x, n) = c (x, n)
124    -         and c (x, n) = a (x, n)
125    -         and d x = e (x, 3)
126    -         and e (x, 0) = f x
127    -           | e (x, n) = e (x, n - 1)
128    -         and f 0 = SMLofNJ.Internals.BTrace.trigger ()
129    -           | f n = n * g (n - 1)
130    -         and g n = a (n, 3)
131    -     in
132    -         f n
133    -     end
134    - end;
135    structure X : sig val main : int -> int end
136    - X.main 3;
137    *** BACK-TRACE ***
138    GOTO   stdIn:4.2-13.20: X.main[2].f
139    GOTO-( stdIn:4.2-13.20: X.main[2].e
140    GOTO   stdIn:4.2-13.20: X.main[2].d
141         / stdIn:4.2-13.20: X.main[2].a
142         | stdIn:4.2-13.20: X.main[2].b
143    GOTO-\ stdIn:4.2-13.20: X.main[2].c
144    CALL   stdIn:4.2-13.20: X.main[2].g
145    GOTO   stdIn:4.2-13.20: X.main[2].f
146    GOTO-( stdIn:4.2-13.20: X.main[2].e
147    GOTO   stdIn:4.2-13.20: X.main[2].d
148         / stdIn:4.2-13.20: X.main[2].a
149         | stdIn:4.2-13.20: X.main[2].b
150    GOTO-\ stdIn:4.2-13.20: X.main[2].c
151    CALL   stdIn:4.2-13.20: X.main[2].g
152    GOTO   stdIn:4.2-13.20: X.main[2].f
153    GOTO-( stdIn:4.2-13.20: X.main[2].e
154    GOTO   stdIn:4.2-13.20: X.main[2].d
155         / stdIn:4.2-13.20: X.main[2].a
156         | stdIn:4.2-13.20: X.main[2].b
157    GOTO-\ stdIn:4.2-13.20: X.main[2].c
158    CALL   stdIn:4.2-13.20: X.main[2].g
159    GOTO   stdIn:4.2-13.20: X.main[2].f
160    CALL   stdIn:2.15-17.4: X.main[2]
161    -
162    
163    (Note that because of a FLINt bug the above code currently does not
164    compile without BTrace turned on.)
165    
166    Here is another example, using my modified Tiger compiler:
167    
168    Standard ML of New Jersey v110.28.1 [FLINT v1.5], June 5, 2000
169    - SMLofNJ.Internals.BTrace.mode (SOME true);
170    [autoloading]
171    [autoloading done]
172    val it = false : bool
173    - CM.make "sources.cm";
174    [autoloading]
175    ...
176    [autoloading done]
177    [scanning sources.cm]
178    [parsing (sources.cm):parse.sml]
179    [creating directory CM/SKEL ...]
180    [parsing (sources.cm):tiger.lex.sml]
181    ...
182    [wrote CM/sparc-unix/semant.sml]
183    [compiling (sources.cm):main.sml]
184    [wrote CM/sparc-unix/main.sml]
185    [New bindings added.]
186    val it = true : bool
187    - Main.compile ("../testcases/merge.tig", "foo.out");
188    *** BACK-TRACE ***
189    CALL   lib/semant.sml:99.2-396.21: SemantFun[2].transExp.trvar
190    CALL   lib/semant.sml:99.2-396.21: SemantFun[2].transExp.trexp
191    CALL   lib/semant.sml:289.3-295.22: SemantFun[2].transExp.trexp.check[2]
192    GOTO   lib/semant.sml:289.3-295.22: SemantFun[2].transExp.trexp.check[2]
193    CALL   lib/semant.sml:99.2-396.21: SemantFun[2].transExp.trexp
194    CALL   lib/semant.sml:99.2-396.21: SemantFun[2].transExp.trexp
195    CALL   lib/semant.sml:488.3-505.6: SemantFun[2].transDec.trdec[2].transBody[2]
196         / lib/semant.sml:411.65-543.8: SemantFun[2].transDec
197    CALL-\ lib/semant.sml:413.2-540.9: SemantFun[2].transDec.trdec[2]
198    CALL   lib/semant.sml:99.2-396.21: SemantFun[2].transExp.trexp
199    CALL   lib/semant.sml:8.52-558.4: SemantFun[2].transProg[2]
200    CALL   main.sml:1.18-118.4: Main.compile[2]
201    -
202    
203    ----------------------------------------------------------------------
204  Name: Matthias Blumen  Name: Matthias Blumen
205  Date: 2000/06/21 18:00:00 JST  Date: 2000/06/21 18:00:00 JST
206  Tag: blume-20000621-manual  Tag: blume-20000621-manual

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