GNU Compiler Collection (GCC) Internals: Cleanups

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10.7.6 Cleanups

Destructors for local C++ objects and similar dynamic cleanups are represented in GIMPLE by a TRY_FINALLY_EXPR. TRY_FINALLY_EXPR has two operands, both of which are a sequence of statements to execute. The first sequence is executed. When it completes the second sequence is executed.

The first sequence may complete in the following ways:

1. Execute the last statement in the sequence and fall off the end.
2. Execute a goto statement (GOTO_EXPR) to an ordinary label outside the sequence.
3. Execute a return statement (RETURN_EXPR).
4. Throw an exception. This is currently not explicitly represented in GIMPLE.

The second sequence is not executed if the first sequence completes by calling setjmp or exit or any other function that does not return. The second sequence is also not executed if the first sequence completes via a non-local goto or a computed goto (in general the compiler does not know whether such a goto statement exits the first sequence or not, so we assume that it doesn’t).

After the second sequence is executed, if it completes normally by falling off the end, execution continues wherever the first sequence would have continued, by falling off the end, or doing a goto, etc.

TRY_FINALLY_EXPR complicates the flow graph, since the cleanup needs to appear on every edge out of the controlled block; this reduces the freedom to move code across these edges. Therefore, the EH lowering pass which runs before most of the optimization passes eliminates these expressions by explicitly adding the cleanup to each edge. Rethrowing the exception is represented using RESX_EXPR.

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