Messages in this thread | ![/](/images/icornerl.gif) | | From | Nick Desaulniers <> | Date | Fri, 21 Aug 2020 16:16:56 -0700 | Subject | Re: [PATCH] x86: work around clang IAS bug referencing __force_order |
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On Fri, Aug 21, 2020 at 4:04 PM Arvind Sankar <nivedita@alum.mit.edu> wrote: > > On Fri, Aug 21, 2020 at 02:37:48AM +0200, Thomas Gleixner wrote: > > On Thu, Aug 20 2020 at 09:06, Arvind Sankar wrote: > > > I don't think that's an issue, or at least, not one where force_order > > > helps. > > > > > > If the source for foo() is not visible to the compiler, the only reason > > > force_order prevents the reordering is because foo() might have > > > references to it, but equally foo() might have volatile asm, so the > > > reordering isn't possible anyway. > > > > > > If the source is visible, force_order won't prevent any reordering > > > except across references to force_order, but the only references are > > > from the volatile asm's which already prevent reordering. > > > > > > I think force_order can only help with buggy compilers, and for those it > > > should really have been an input-output operand -- it wouldn't currently > > > do anything to prevent cr writes from being reordered.
I agree 100%. From the link to GCC docs, the code in question doesn't even follow the pattern from the doc from informing the compiler of any dependency, it just looks like !@#$.
> > > > Fair enough. Care to provide a patch which has the collected wisdom of > > this thread in the changelog? > > > > Thanks, > > > > tglx > > The gcc bug I linked to earlier is only fixed in gcc-6 onwards. Is that
(based on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602#c14)
> good enough to remove force_order? I can test gcc-4.9 and gcc-5 to check > if it would currently have any impact.
I think checking the disassemblies with a pre-gcc-6 would be good enough then; that bug isn't specific to this particular case.
> CBL guys, can you confirm that clang also will not reorder volatile asm?
Full disassemblies of vmlinux pre vs post __force_order removal are the same. That's pretty good actually; I was worried for a code base of this size whether two compiles would produce the exact same disassemblies; I know the version strings are timestamped, for instance, but I didn't compare data, just .text. I should triple check i386, and some of the ko's that use modified functions. I'd be happy to help provide a tested by tag for numerous configurations with Clang.
Attaching the diff I was testing, feel free to add a commit message. -- Thanks, ~Nick Desaulniers [unhandled content-type:application/octet-stream] | ![\](/images/icornerr.gif) |