Dear Albert ARIBAUD, > Hi Marek, > > On Sat, 1 Sep 2012 00:16:43 +0200, Marek Vasut <ma...@denx.de> wrote: > > Dear Albert ARIBAUD, > > > > > I think you are talking about lumping small-sized accesses together > > > into a bigger access possibly aligned. > > > > This is exactly what I mean. > > > > > If I am correct, then I don't > > > think this is related to misaligned accesses. > > > > Why won't it be? Such access can in the end turn out to be aligned, > > therefore leveraging all the penalty. > > I have not expressed myself clearly. Yes, access lumping is related to > access alignment. What I meant is: disallowing misaligned native > accesses will not prevent access lumping. Misalignment restrictions do > indeed restrict how such lumpings will happen, but it does not prevent > lumping per se. > > One place where lumping and misalignement prevention did clash was > raised in the previous discussion: a 7+1 bytes function-local char array > was allocated on a non-aligned address (which is possible and normal > because it is a char) and was initialized with some content. The > compiler lumped the initialization as two misaligned 32-byte native > accesses, despite misaligned native accesses being forbidden by > compiler command line options. This was a compiler bug.
But that'd mean that instead of fixing a compiler, we'd be doing a workaround in our code? > > > If I am not correct, can > > > you please detail what you meant? > > > > > > > Besides, right now, the code is much more readable. So I really > > > > don't like adding some strange macros to force crazy aligned > > > > access if the compiler can do it for us and can do it better. > > > > > > I personally would let the compiler do it too, but I prefer it to be > > > clearly indicated to the reader of the code when an access is > > > known to be misaligned. > > > > I'd enable enable the Alignment trapping in the CPU and die on an > > unaligned access at runtime -- to indicate the user that he should > > fix his bloody compiler. > > ... or fix his bloody structure, or fix his bloody f...ixing pointer > arithmetic, or... but I do agree with the trapping, and that's my plan. > > However other architectures may need, or choose, another stance on > alignments, and it is best if they don't have to discover intended > misaligned accesses the hard way. Yet still, in such case, valid compiler has to generate valid workaround code. > Thus my opinion that any misaligned > access which cannot be fixed should not be sliently left for the > compiler to handle, but should (also) be clearly marked as such, if only > for humans to notice. I can't say I agree here ... since it's a really ad-hoc solution. I can't say I see any real benefit other than that it's hiding possible compiler bugs :-( > > Best regards, > > Marek Vasut > > Amicalement, Best regards, Marek Vasut _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
