[llvm-dev] Where and how to report an optimisation issue that doesn't cause a crash

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Thu Oct 24 14:44:03 PDT 2019

Here's a simpler version which I believe still captures the essence of the
issue: https://godbolt.org/z/BUoLq1

Yep, looks like GCC manages this optimization and Clang does not - the
runtime bound on the array is stopping clang from going further here
(perhaps that's intentional - maybe to allow the array indexed store to
possibly exceed the bounds of the array and assign to 'size' that comes
after it - UB, but I don't know just how far non-strict aliasing goes)

Yeah, looks like it might be something like that ^ if you change the index
to be unsigned, and reorder the members so 'size' comes before the array
(so indexing into the array can't alias 'size'), you do get:
https://godbolt.org/z/Ax62Fv which avoids reloading size after the
assignment to the array.

‪On Thu, Oct 24, 2019 at 12:53 PM ‫אלכס לופ'‬‎ <alex_lop at walla.com> wrote:‬

> Hi David,
> Thanks for your reply. The fact that
> "queue_ptr->queue[queue_ptr->wr_idx++]" could be somewhere in the object
> pointed by "queue_ptr" came to my mind too.
> I also added this in the stackoverflow question at the bottom.
> In such case, I assumed, that if I change the struct "queue_t" to have an
> array of "event_t" instead of just pointer to "event_t", like this:
> typedef struct{
>     event_t                     queue[256]; // changed from pointer to array with max size
>     size_t                      size;
>     uint16_t                    num_of_items;
>     uint8_t                     rd_idx;
>     uint8_t                     wr_idx;} queue_t;
> there would be no valid way (without breaking the C standard definitions) that "queue_ptr->queue[queue_ptr->wr_idx++]" ends up somewhere inside "*queue_ptr", right?
> However the generated assembly code still contained the same re-reads...
> Any idea why would this happen even with the new "queue_t" struct definition?
> P.S.
> I can easily reproduce it and provide any additional data which can be gathered during the compilation process, if it helps.
> Thanks,
> Alex.
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