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	revision 673, Wed Jun 21 08:57:07 2000 UTC	revision 675, Fri Jun 23 09:18:18 2000 UTC
#	Line 12	Line 12
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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