3.2.15 Exception Handling Control

GNAT uses two methods for handling exceptions at run-time. The setjmp/longjmp method saves the context when entering a frame with an exception handler. Then when an exception is raised, the context can be restored immediately, without the need for tracing stack frames. This method provides very fast exception propagation, but introduces significant overhead for the use of exception handlers, even if no exception is raised.

The other approach is called “zero cost” exception handling. With this method, the compiler builds static tables to describe the exception ranges. No dynamic code is required when entering a frame containing an exception handler. When an exception is raised, the tables are used to control a back trace of the subprogram invocation stack to locate the required exception handler. This method has considerably poorer performance for the propagation of exceptions, but there is no overhead for exception handlers if no exception is raised. Note that in this mode and in the context of mixed Ada and C/C++ programming, to propagate an exception through a C/C++ code, the C/C++ code must be compiled with the -funwind-tables GCC's option.

The following switches can be used to control which of the two exception handling methods is used.

--RTS=sjlj

This switch causes the setjmp/longjmp run-time to be used for exception handling. If this is the default mechanism for the target (see below), then this has no effect. If the default mechanism for the target is zero cost exceptions, then this switch can be used to modify this default, and must be used for all units in the partition. This option is rarely used. One case in which it may be advantageous is if you have an application where exception raising is common and the overall performance of the application is improved by favoring exception propagation.

--RTS=zcx

This switch causes the zero cost approach to be used for exception handling. If this is the default mechanism for the target (see below), then this has no effect. If the default mechanism for the target is setjmp/longjmp exceptions, then this switch can be used to modify this default, and must be used for all units in the partition. This option can only be used if the zero cost approach is available for the target in use (see below).

The setjmp/longjmp approach is available on all targets, while the zero cost approach is available on selected targets. To determine whether zero cost exceptions can be used for a particular target, look at the private part of the file system.ads. Either GCC_ZCX_Support or Front_End_ZCX_Support must be True to use the zero cost approach. If both of these switches are set to False, this means that zero cost exception handling is not yet available for that target. The switch ZCX_By_Default indicates the default approach. If this switch is set to True, then the zero cost approach is used by default.