On Mon, Feb 4, 2019 at 5:07 PM Chris Johns <chr...@rtems.org> wrote: > On 2/2/19 6:53 am, Joel Sherrill wrote: > > Then we should ensure proper alignment since we NEVER want an unaligned > > exception on any architecture if it is avoidable. No point in taking the > likely > > performance hit or exception. > > Is this documented anywhere? >
Unless it is in the porting guide, I doubt it. Alignment issues for critical structures and the stack is a porting issue. I was just thinking of the m68k where it allowed unaligned accesses but there was often a hidden performance penalty. > > It is an important issue because a performance hit such as unaligned > accesses > silently happening is difficult to see and it tends to be accounted for as > RTEMS > not performing or hardware not performing. Is this something we could test > for > and check? > I don't know if it could be a static assertion but it could be a debug check. It could certainly be checked with a normal test which peeked under the hood. > > I personally have never stopped and checked a BSP for correct alignment in > all > cases or the critical cases before using and when something exposes it I > have > been surprised. > Me too. We all assume that alignment was thought through by the person who did the port. Similar to finding caching disabled on a BSP. I suppose if we identify the critical structures that a test could check them all. But we should be careful about this. What is minimum needed and what is for performance? 4 or 8 byte alignments would avoid faults/unaligned accesses. 16 or 32 byte would be performance helps on many architectures for larger structures. --joel > > Chris >
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