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annotate docs/HistoricalNotes/2001-05-18-ExceptionHandling.txt @ 124:4fa72497ed5d

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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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1 Meeting notes: Implementation idea: Exception Handling in C++/Java
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2
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3 The 5/18/01 meeting discussed ideas for implementing exceptions in LLVM.
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4 We decided that the best solution requires a set of library calls provided by
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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5 the VM, as well as an extension to the LLVM function invocation syntax.
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6
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7 The LLVM function invocation instruction previously looks like this (ignoring
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8 types):
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9
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10 call func(arg1, arg2, arg3)
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11
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12 The extension discussed today adds an optional "with" clause that
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13 associates a label with the call site. The new syntax looks like this:
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14
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15 call func(arg1, arg2, arg3) with funcCleanup
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16
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17 This funcHandler always stays tightly associated with the call site (being
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18 encoded directly into the call opcode itself), and should be used whenever
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19 there is cleanup work that needs to be done for the current function if
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20 an exception is thrown by func (or if we are in a try block).
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21
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22 To support this, the VM/Runtime provide the following simple library
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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23 functions (all syntax in this document is very abstract):
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24
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25 typedef struct { something } %frame;
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26 The VM must export a "frame type", that is an opaque structure used to
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27 implement different types of stack walking that may be used by various
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28 language runtime libraries. We imagine that it would be typical to
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29 represent a frame with a PC and frame pointer pair, although that is not
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30 required.
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31
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32 %frame getStackCurrentFrame();
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33 Get a frame object for the current function. Note that if the current
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34 function was inlined into its caller, the "current" frame will belong to
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35 the "caller".
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36
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37 bool isFirstFrame(%frame f);
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38 Returns true if the specified frame is the top level (first activated) frame
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39 for this thread. For the main thread, this corresponds to the main()
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40 function, for a spawned thread, it corresponds to the thread function.
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41
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42 %frame getNextFrame(%frame f);
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43 Return the previous frame on the stack. This function is undefined if f
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44 satisfies the predicate isFirstFrame(f).
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45
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46 Label *getFrameLabel(%frame f);
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47 If a label was associated with f (as discussed below), this function returns
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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48 it. Otherwise, it returns a null pointer.
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49
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50 doNonLocalBranch(Label *L);
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51 At this point, it is not clear whether this should be a function or
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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52 intrinsic. It should probably be an intrinsic in LLVM, but we'll deal with
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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53 this issue later.
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54
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55
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56 Here is a motivating example that illustrates how these facilities could be
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57 used to implement the C++ exception model:
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58
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59 void TestFunction(...) {
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60 A a; B b;
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61 foo(); // Any function call may throw
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62 bar();
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63 C c;
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64
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65 try {
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66 D d;
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67 baz();
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68 } catch (int) {
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69 ...int Stuff...
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70 // execution continues after the try block: the exception is consumed
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71 } catch (double) {
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72 ...double stuff...
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73 throw; // Exception is propogated
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74 }
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75 }
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76
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77 This function would compile to approximately the following code (heavy
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78 pseudo code follows):
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79
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80 Func:
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81 %a = alloca A
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82 A::A(%a) // These ctors & dtors could throw, but we ignore this
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83 %b = alloca B // minor detail for this example
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84 B::B(%b)
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85
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86 call foo() with fooCleanup // An exception in foo is propogated to fooCleanup
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87 call bar() with barCleanup // An exception in bar is propogated to barCleanup
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88
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89 %c = alloca C
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90 C::C(c)
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91 %d = alloca D
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92 D::D(d)
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93 call baz() with bazCleanup // An exception in baz is propogated to bazCleanup
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94 d->~D();
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95 EndTry: // This label corresponds to the end of the try block
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96 c->~C() // These could also throw, these are also ignored
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97 b->~B()
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98 a->~A()
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99 return
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100
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101 Note that this is a very straight forward and literal translation: exactly
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102 what we want for zero cost (when unused) exception handling. Especially on
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103 platforms with many registers (ie, the IA64) setjmp/longjmp style exception
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104 handling is *very* impractical. Also, the "with" clauses describe the
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105 control flow paths explicitly so that analysis is not adversly effected.
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106
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107 The foo/barCleanup labels are implemented as:
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108
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109 TryCleanup: // Executed if an exception escapes the try block
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110 c->~C()
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111 barCleanup: // Executed if an exception escapes from bar()
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112 // fall through
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113 fooCleanup: // Executed if an exception escapes from foo()
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114 b->~B()
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115 a->~A()
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116 Exception *E = getThreadLocalException()
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117 call throw(E) // Implemented by the C++ runtime, described below
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118
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119 Which does the work one would expect. getThreadLocalException is a function
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120 implemented by the C++ support library. It returns the current exception
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121 object for the current thread. Note that we do not attempt to recycle the
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122 shutdown code from before, because performance of the mainline code is
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123 critically important. Also, obviously fooCleanup and barCleanup may be
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124 merged and one of them eliminated. This just shows how the code generator
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125 would most likely emit code.
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126
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127 The bazCleanup label is more interesting. Because the exception may be caught
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128 by the try block, we must dispatch to its handler... but it does not exist
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129 on the call stack (it does not have a VM Call->Label mapping installed), so
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130 we must dispatch statically with a goto. The bazHandler thus appears as:
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
131
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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132 bazHandler:
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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133 d->~D(); // destruct D as it goes out of scope when entering catch clauses
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
134 goto TryHandler
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
135
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
136 In general, TryHandler is not the same as bazHandler, because multiple
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
137 function calls could be made from the try block. In this case, trivial
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
138 optimization could merge the two basic blocks. TryHandler is the code
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
139 that actually determines the type of exception, based on the Exception object
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
140 itself. For this discussion, assume that the exception object contains *at
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
141 least*:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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142
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
143 1. A pointer to the RTTI info for the contained object
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
144 2. A pointer to the dtor for the contained object
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
145 3. The contained object itself
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
146
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
147 Note that it is necessary to maintain #1 & #2 in the exception object itself
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
148 because objects without virtual function tables may be thrown (as in this
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
149 example). Assuming this, TryHandler would look something like this:
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
150
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
151 TryHandler:
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
152 Exception *E = getThreadLocalException();
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
153 switch (E->RTTIType) {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
154 case IntRTTIInfo:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
155 ...int Stuff... // The action to perform from the catch block
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
156 break;
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
157 case DoubleRTTIInfo:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
158 ...double Stuff... // The action to perform from the catch block
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
159 goto TryCleanup // This catch block rethrows the exception
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
160 break; // Redundant, eliminated by the optimizer
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
161 default:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
162 goto TryCleanup // Exception not caught, rethrow
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
163 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
164
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
165 // Exception was consumed
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
166 if (E->dtor)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
167 E->dtor(E->object) // Invoke the dtor on the object if it exists
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
168 goto EndTry // Continue mainline code...
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
169
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
170 And that is all there is to it.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
171
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
172 The throw(E) function would then be implemented like this (which may be
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
173 inlined into the caller through standard optimization):
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
174
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
175 function throw(Exception *E) {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
176 // Get the start of the stack trace...
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
177 %frame %f = call getStackCurrentFrame()
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
178
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
179 // Get the label information that corresponds to it
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
180 label * %L = call getFrameLabel(%f)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
181 while (%L == 0 && !isFirstFrame(%f)) {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
182 // Loop until a cleanup handler is found
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
183 %f = call getNextFrame(%f)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
184 %L = call getFrameLabel(%f)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
185 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
186
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
187 if (%L != 0) {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
188 call setThreadLocalException(E) // Allow handlers access to this...
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
189 call doNonLocalBranch(%L)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
190 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
191 // No handler found!
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
192 call BlowUp() // Ends up calling the terminate() method in use
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
193 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
194
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
195 That's a brief rundown of how C++ exception handling could be implemented in
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
196 llvm. Java would be very similar, except it only uses destructors to unlock
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
197 synchronized blocks, not to destroy data. Also, it uses two stack walks: a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
198 nondestructive walk that builds a stack trace, then a destructive walk that
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
199 unwinds the stack as shown here.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
200
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
201 It would be trivial to get exception interoperability between C++ and Java.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
202