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Meeting notes: Implementation idea: Exception Handling in C++/Java
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The 5/18/01 meeting discussed ideas for implementing exceptions in LLVM.
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We decided that the best solution requires a set of library calls provided by
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the VM, as well as an extension to the LLVM function invocation syntax.
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The LLVM function invocation instruction previously looks like this (ignoring
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call func(arg1, arg2, arg3)
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The extension discussed today adds an optional "with" clause that
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associates a label with the call site. The new syntax looks like this:
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call func(arg1, arg2, arg3) with funcCleanup
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This funcHandler always stays tightly associated with the call site (being
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encoded directly into the call opcode itself), and should be used whenever
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there is cleanup work that needs to be done for the current function if
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an exception is thrown by func (or if we are in a try block).
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To support this, the VM/Runtime provide the following simple library
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functions (all syntax in this document is very abstract):
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typedef struct { something } %frame;
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The VM must export a "frame type", that is an opaque structure used to
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implement different types of stack walking that may be used by various
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language runtime libraries. We imagine that it would be typical to
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represent a frame with a PC and frame pointer pair, although that is not
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%frame getStackCurrentFrame();
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Get a frame object for the current function. Note that if the current
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function was inlined into its caller, the "current" frame will belong to
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bool isFirstFrame(%frame f);
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Returns true if the specified frame is the top level (first activated) frame
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for this thread. For the main thread, this corresponds to the main()
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function, for a spawned thread, it corresponds to the thread function.
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%frame getNextFrame(%frame f);
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Return the previous frame on the stack. This function is undefined if f
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satisfies the predicate isFirstFrame(f).
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Label *getFrameLabel(%frame f);
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If a label was associated with f (as discussed below), this function returns
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it. Otherwise, it returns a null pointer.
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doNonLocalBranch(Label *L);
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At this point, it is not clear whether this should be a function or
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intrinsic. It should probably be an intrinsic in LLVM, but we'll deal with
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Here is a motivating example that illustrates how these facilities could be
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used to implement the C++ exception model:
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void TestFunction(...) {
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foo(); // Any function call may throw
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// execution continues after the try block: the exception is consumed
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throw; // Exception is propogated
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This function would compile to approximately the following code (heavy
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A::A(%a) // These ctors & dtors could throw, but we ignore this
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%b = alloca B // minor detail for this example
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call foo() with fooCleanup // An exception in foo is propogated to fooCleanup
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call bar() with barCleanup // An exception in bar is propogated to barCleanup
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call baz() with bazCleanup // An exception in baz is propogated to bazCleanup
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EndTry: // This label corresponds to the end of the try block
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c->~C() // These could also throw, these are also ignored
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Note that this is a very straight forward and literal translation: exactly
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what we want for zero cost (when unused) exception handling. Especially on
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platforms with many registers (ie, the IA64) setjmp/longjmp style exception
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handling is *very* impractical. Also, the "with" clauses describe the
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control flow paths explicitly so that analysis is not adversly effected.
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The foo/barCleanup labels are implemented as:
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TryCleanup: // Executed if an exception escapes the try block
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barCleanup: // Executed if an exception escapes from bar()
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fooCleanup: // Executed if an exception escapes from foo()
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Exception *E = getThreadLocalException()
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call throw(E) // Implemented by the C++ runtime, described below
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Which does the work one would expect. getThreadLocalException is a function
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implemented by the C++ support library. It returns the current exception
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object for the current thread. Note that we do not attempt to recycle the
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shutdown code from before, because performance of the mainline code is
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critically important. Also, obviously fooCleanup and barCleanup may be
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merged and one of them eliminated. This just shows how the code generator
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would most likely emit code.
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The bazCleanup label is more interesting. Because the exception may be caught
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by the try block, we must dispatch to its handler... but it does not exist
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on the call stack (it does not have a VM Call->Label mapping installed), so
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we must dispatch statically with a goto. The bazHandler thus appears as:
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d->~D(); // destruct D as it goes out of scope when entering catch clauses
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In general, TryHandler is not the same as bazHandler, because multiple
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function calls could be made from the try block. In this case, trivial
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optimization could merge the two basic blocks. TryHandler is the code
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that actually determines the type of exception, based on the Exception object
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itself. For this discussion, assume that the exception object contains *at
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1. A pointer to the RTTI info for the contained object
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2. A pointer to the dtor for the contained object
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3. The contained object itself
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Note that it is necessary to maintain #1 & #2 in the exception object itself
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because objects without virtual function tables may be thrown (as in this
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example). Assuming this, TryHandler would look something like this:
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Exception *E = getThreadLocalException();
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switch (E->RTTIType) {
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...int Stuff... // The action to perform from the catch block
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...double Stuff... // The action to perform from the catch block
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goto TryCleanup // This catch block rethrows the exception
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break; // Redundant, eliminated by the optimizer
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goto TryCleanup // Exception not caught, rethrow
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// Exception was consumed
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E->dtor(E->object) // Invoke the dtor on the object if it exists
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goto EndTry // Continue mainline code...
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And that is all there is to it.
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The throw(E) function would then be implemented like this (which may be
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inlined into the caller through standard optimization):
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function throw(Exception *E) {
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// Get the start of the stack trace...
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%frame %f = call getStackCurrentFrame()
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// Get the label information that corresponds to it
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label * %L = call getFrameLabel(%f)
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while (%L == 0 && !isFirstFrame(%f)) {
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// Loop until a cleanup handler is found
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%f = call getNextFrame(%f)
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%L = call getFrameLabel(%f)
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call setThreadLocalException(E) // Allow handlers access to this...
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call doNonLocalBranch(%L)
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call BlowUp() // Ends up calling the terminate() method in use
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That's a brief rundown of how C++ exception handling could be implemented in
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llvm. Java would be very similar, except it only uses destructors to unlock
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synchronized blocks, not to destroy data. Also, it uses two stack walks: a
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nondestructive walk that builds a stack trace, then a destructive walk that
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unwinds the stack as shown here.
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It would be trivial to get exception interoperability between C++ and Java.