[cfe-dev] Announcing "clang-ctags"

Manuel Klimek klimek at google.com
Wed Jul 25 10:25:34 PDT 2012

On Wed, Jul 25, 2012 at 7:06 PM, Douglas Gregor <dgregor at apple.com> wrote:
> On Jul 25, 2012, at 2:41 AM, Manuel Klimek <klimek at google.com> wrote:
>> On Wed, Jul 25, 2012 at 12:14 AM, Sean Silva <silvas at purdue.edu> wrote:
>>> I saw the bug report, but that gives only memory addresses and
>>> instruction offsets for what the offending code is (`0x1048b50b1 in
>>> (anonymous namespace)::ASTStatCache::getStat(char const*,
>>> stat&, int*) (in clang-tags) + 977`); given the size of `getStat` and
>>> a brief perusal of the source, it seems like at least one level of
>>> inlining is involved. It would be immensely helpful if you could tie
>>> that address back to the statement/expression which causes the
>>> reference.
>> Also, as noted on the bug, the Tooling should never pull in
>> ASTStatCache. Any pointers to how that might be instantiated would be
>> of great help.
> I suspect this is what is happening:
>   - Clang is loading a precompiled header, which wires a stat cache into the FileManager. That stat cache points into the mmap'd memory for the precompiled header.
>   - That instance of the compiler completes, and everything goes away *except* the stat cache, since the FileManager is reused. We now have a stat cache in the FileManager that points at the location of previously-mmap'd memory for the precompiled header.
>   - Later instances of the compiler wire more stat caches into the FileManager, and most lookups hit those earlier caches, so the problem doesn't reproduce easily
>   - Eventually, we have a cache miss in a later instance of the compiler, and the dangling pointer into the previously-mmap'd precompiled header ends up getter used after those addresses have been reused, and BOOM!
> Solution: clear out the stat caches attached to the FileManager when re-using that file manager.

Thx for solving the mystery :) Do you have a pointer (example test or
something) on what's the best way to create a precompiled header for a
small test?


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