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QTAILQ is a doubly linked list, with a pointer-to-pointer to the last element from the head, and the previous element from each node. But if you squint enough, QTAILQ becomes a combination of a singly-linked forwards list, and another singly-linked list which goes backwards and is circular. This is the idea that lets QTAILQ implement reverse iteration: only, because the backwards list points inside the node, accessing the previous element needs to go two steps back and one forwards. What this patch does is implement it in these terms, without actually changing the in-memory layout at all. The coexistence of the two lists is realized by making QTAILQ_HEAD and QTAILQ_ENTRY unions of the forwards pointer and a generic QTailQLink node. Thq QTailQLink can walk the list in both directions; the union is needed so that the forwards pointer can have the correct type, as a sort of poor man's template. While there are other ways to get the same layout without a union, this one has the advantage of simpler operation in the debugger, because the fields tqh_first and tqe_next still exist as before the patch. Those fields are also used by scripts/qemugdb/mtree.py, so it's a good idea to preserve them. The advantage of the new representation is that the two-back-one-forward dance done by backwards accesses can be done all while operating on QTailQLinks. No casting to the head struct is needed anymore because, even though the QTailQLink's forward pointer is a void *, we can use typeof to recover the correct type. This patch only changes the implementation, not the interface. The next patch will remove the head struct name from the backwards visit macros. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> |
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| .. | ||
| coccinelle | ||
| kvm | ||
| modules | ||
| qapi | ||
| qemu-guest-agent | ||
| qemugdb | ||
| qmp | ||
| tracetool | ||
| travis | ||
| analyse-9p-simpletrace.py | ||
| analyse-locks-simpletrace.py | ||
| analyze-inclusions | ||
| analyze-migration.py | ||
| archive-source.sh | ||
| checkpatch.pl | ||
| clean-header-guards.pl | ||
| clean-includes | ||
| cleanup-trace-events.pl | ||
| cocci-macro-file.h | ||
| coverity-model.c | ||
| create_config | ||
| decodetree.py | ||
| device-crash-test | ||
| disas-objdump.pl | ||
| dump-guest-memory.py | ||
| extract-vsssdk-headers | ||
| feature_to_c.sh | ||
| get_maintainer.pl | ||
| git-submodule.sh | ||
| git.orderfile | ||
| hxtool | ||
| make-release | ||
| make_device_config.sh | ||
| qapi-gen.py | ||
| qemu-binfmt-conf.sh | ||
| qemu-gdb.py | ||
| qemu.py | ||
| qtest.py | ||
| refresh-pxe-roms.sh | ||
| replay-dump.py | ||
| shaderinclude.pl | ||
| show-fixed-bugs.sh | ||
| signrom.py | ||
| simpletrace.py | ||
| switch-timer-api | ||
| tap-driver.pl | ||
| tap-merge.pl | ||
| texi2pod.pl | ||
| tracetool.py | ||
| update-linux-headers.sh | ||
| vmstate-static-checker.py | ||