https://bugs.kde.org/show_bug.cgi?id=392373
Bug ID: 392373
Summary: Valgrind could have an option to search for pointers
at unaligned addresses
Product: valgrind
Version: unspecified
Platform: Other
OS: Linux
Status: UNCONFIRMED
Severity: normal
Priority: NOR
Component: memcheck
Assignee: jsew...@acm.org
Reporter: anti...@gmail.com
Target Milestone: ---
Memcheck determines if the block is reachable from pointers within the
root-set, checking for pointed-sized memory locations that are stored into an
*aligned* address. This may leads to false positives in case some code, for the
sake of memory efficiency or representation locality, packs pointers with other
data, effectively storing pointers at unaligned addresses. Note that using
memcpy() instead of deferencing the pointer, such a memory layout is perfectly
safe even in architectures that do not have support for accessing unaligned
memory locations: anyway when this is possible, compilers will translate memcpy
with a fixed pointer-size "count" to a load/store operation instead of actually
calling the function, so there is not lack of performances most of the times.
An example of code using this trick is an hash table where, instead of storing
at each dictionary bucket a single value, we store a blob of prefixed-len keys
followed by the respective pointer in a single allocated memory blob:
"[3]foo<ptr-for-foo-value>[5]apple<ptr-for-apple-value>". This is similar to
chaining but without paying the pointers and new bucket memory cost. In this
case, Valgrind will not be able to look for the pointers when they are stored
at unaligned offsets, reporting the leak. I imagine that scanning for aligned
addresses is faster, because the search space is reduced to 1/8 of what would
be otherwise. However this feature could just be opt-in, so that normally
Valgrind users will not incur in any penalty. I have no expertise on the
internals of Valgrind, but I can imagine that this feature could also be quite
simple to add, if there is to change just the step of the search. Thanks for
checking this issue. Regards, Salvatore.
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