Hi there,
I come up with an idea about implementing stack unwinding for
the i686-w64-mingw32 target using native Windows Structured
Exception Handling (a.k.a SEH) for efficiency reasons.
Unlike DWARF and SEH for x64, SEH for x86 is stack-based
and works like the SJLJ exception model: The operating system
keeps a thread specific pointer to an SEH node on the stack
that must be installed/uninstalled during run time.
The SEH-head pointer is stored in `fs:[0]`.
Typecially, an SEH handler is installed like this, in Intel syntax:
# typedef EXCEPTION_DISPOSITION
# filter_function(
# EXCEPTION_RECORD *record, void *establisher_frame,
# CONTEXT *machine_context, void *dispatcher_context)
# __attribute__((__cdecl__));
# struct x86_seh_node_header {
# struct x86_seh_node_header *next;
# filter_function *filter;
# char extra_data[];
# };
sub esp, 8 # struct x86_seh_node_header this_node;
mov ecx, dword ptr fs:[0] #
mov dword ptr[esp], ecx # this_node.next = get_thread_seh_head();
mov dword ptr[esp + 4], offset my_seh_filter
# this_node.filter = &my_seh_filter
mov dword ptr fs:[0], esp # set_thread_seh_head(&this_node);
Before the function exits and its frame is destroyed, the node
must be uninstalled like this:
mov ecx, dword ptr fs:[0] #
mov dword ptr fs:[0], ecx # set_thread_seh_head(this_node.next);
Since I am looking at the SJLJ exception model and it seems using
a slim, inlined version of `setjmp()` with `__builtin_longjmp()`
that only stores 3 or 4 pointers, extending that structure should be
a simple matter. The problem is that, installation and uninstallation
of SEH nodes require target-specific ASM code generation.
Is it possible to do in 'gcc/except.c' ?
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Best regards,
lh_mouse
2016-10-17
