-Author: Dietmar Maurer (dietmar@ximian.com)
-(C) 2001 Ximian, Inc.
+ Exception Implementation in the Mono Runtime
+ Dietmar Maurer (dietmar@ximian.com)
+ (C) 2001 Ximian, Inc.
Exception implementation (jit):
===============================
instruction is inside a finally block (but not inside another handler)
we call the finally handler before we branch to the target.
-finally/endfinally: is translated into subroutine ending with a "return"
-statement. The subroutine does not save EBP/ESP, because we need access to the
-local variables of the enclosing method. We have to use a "call"
-instruction to execute such finally handlers. This makes it possible to
-execute them inside the stack unwinding code.
+finally/endfinally, filter/endfilter: is translated into subroutine ending with
+a "return" statement. The subroutine does not save EBP, because we need access
+to the local variables of the enclosing method. Its is possible that
+instructions inside those handlers modify the stack pointer, thus we save the
+stack pointer at the start of the handler, and restore it at the end. We have
+to use a "call" instruction to execute such finally handlers. This makes it
+also possible to execute them inside the stack unwinding code. The exception
+object for filters is passed in a local variable (cfg->exvar).
-throw: we first save all regs into a sigcontext struct (we pass the
-exception object in register ECX), and then call the stack unwinding
-code.
+throw: we first save all regs into a sigcontext struct and then call the stack
+unwinding code.
-catch handler: receives the exception object in ECX. They store that
-object into a local variable, so that rethrow can access the object.
-
-
-gcc support for Exceptions
-==========================
-
-gcc supports exceptions in files compiled with the -fexception option. gcc
-generates DWARF exceptions tables in that case, so it is possible to unwind the
-stack. The method to read those exception tables is contained in libgcc.a, and
-in newer versions of glibc (glibc 2.2.5 for example), and it is called
-__frame_state_for(). Another usable glibc function is backtrace_symbols() which
-returns the function name corresponding to a code address.
-
-We dynamically check if those features are available using g_module_symbol(),
-and we use them only when available. If not available we use the LMF as
-fallback.
-
-Using gcc exception information prevents us from saving the LMF at each native
-call, so this is a way to speed up native calls. This is especially valuable
-for internal calls, because we can make sure that all internal calls are
-compiled with -fexceptions (we compile the whole mono runtime with that
-option).
-
-All native function are able to call function without exception tables, and so
-we are unable to restore all caller saved registers if an exception is raised
-in such function. Well, its possible if the previous function already saves all
-registers. So we only omit the the LMF if a function has an exception table
-able to restore all caller saved registers.
-
-One problem is that gcc almost never saves all caller saved registers, because
-it is just unnecessary in normal situations. But there is a trick forcing gcc
-to save all register, we just need to call __builtin_unwind_init() at the
-beginning of a function. That way gcc generates code to save all caller saved
-register on the stack.
-
-
-
-
-
\ No newline at end of file
+catch handler: catch hanlders are always called from the stack unwinding
+code. The exception object is passed in a local variable (cfg->exvar).
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