[Bug tree-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong

["mikpe at it dot uu dot se" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #6 from mikpe at it dot uu dot se  2009-07-08 09:59 -------
(In reply to comment #2)
> Replacing *tbl++ by tbl[i] gives this ARM code:
> .L2:
>         mov     r3, #10
>         str     r3, [r2], #4
>         cmp     r2, #0
>         bne     .L2
>         bx      lr
> 
> See, gcc knows about the wrapping but still the *tbl++ version misses the
> end-condition which is the bug.

The difference is that in the tbl[i] version there will not be a wraparound at
runtime because &tbl[i] for i == 64 is never computed, while in the *tbl++
version the iteration with i == 63 will do tbl++ moving tbl from -4U to 0
before the loop termination test, which triggers undefined behaviour.

And the issue is not ARM, the same infinite loop occurs for e.g. target i686.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679