Re: How expensive are exceptions?
David Abrahams wrote:
on Thu Jun 14 2007, Walter Bright <walter-AT-digitalmars-nospamm.com> wrote:
at least on Windows, having EH enabled does add some cost to
successful paths. The reason is because EH on x86 Windows is
implemented using Microsoft's "Structured Exception Handling" scheme
which requires:
1) a special stack frame
2) incrementing/decrementing a special 'scope index' variable when going
in and out of try blocks
It does not have to be implemented that way on Windows.
Right, but it's nice being compatible with the standard way the
operating system works. For example, if a hardware exception occurs, you
get an SEH which can be caught by C++. It's nice being compatible with
DLLs that throw standard exceptions. If you invent your own ABI, you're
compatible with nobody.
That's just
how Microsoft did it on 32-bit windows, and several vendors naturally
followed suit.
Digital Mars C++ certainly did it that way <g>. DMC++ uses range tabled
EH for non-Windows executables (like 32 bit DOS). Digital Mars D uses
SEH for Windows and range tables for Linux.
at the cost of much slower execution of catch blocks.
Slower unwinding, actually. The catch block itself is just regular
C++ code.
Right.
While the modern trend is to accept slow exception paths, such does
preclude using the hardware to implement write barriers in gc
implementations.
Care to say more? I don't follow that.
For a generational GC, you can avoid scanning sections of memory that
you know haven't changed since the last time it was scanned. This is
done by implementing a "write barrier", which marks the section as
"changed and needs rescanning" when it is written to. One way to do this
is to instrument every write instruction. Another way is to mark those
sections' pages as "read only". Then, the hardware throws a "write
fault" exception when an instruction tries to write to it. You set up a
handler that intercepts the write fault, logs the faulting page as
"changed and need rescanning", then turns on the write enable bit for
the page, and restarts the instruction.
This is not practical if the exception unwinding is slow.
-------------
Walter Bright
http://www.digitalmars.com C, C++, D programming language compilers
http://www.astoriaseminar.com Extraordinary C++
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