Hello, On Mon, Jul 29, 2019 at 11:16:45PM +0100, Maciej W. Rozycki wrote: > On Mon, 29 Jul 2019, Nick Desaulniers wrote: > > The code in question is modifying a variable declared const through > > pointer manipulation. Such code is explicitly undefined behavior, and > > is the lone issue preventing malta_defconfig from booting when built > > with Clang: > > > > If an attempt is made to modify an object defined with a const-qualified > > type through use of an lvalue with non-const-qualified type, the > > behavior is undefined. > > > > LLVM is removing such assignments. A simple fix is to not declare > > variables const that you plan on modifying. Limiting the scope would be > > a better method of preventing unwanted writes to such a variable. > > > > Further, the code in question mentions "compiler bugs" without any links > > to bug reports, so it is difficult to know if the issue is resolved in > > GCC. The patch was authored in 2006, which would have been GCC 4.0.3 or > > 4.1.1. The minimal supported version of GCC in the Linux kernel is > > currently 4.6. > > It's somewhat older than that. My investigation points to: > > commit c94e57dcd61d661749d53ee876ab265883b0a103 > Author: Ralf Baechle <r...@linux-mips.org> > Date: Sun Nov 25 09:25:53 2001 +0000 > > Cleanup of include/asm-mips/io.h. Now looks neat and harmless. > > However the purpose of the arrangement does not appear to me to be > particularly specific to a compiler version.
Agreed - I don't think the code here talks about compiler bugs at all, it talks about emitting extra unnecessary loads & says there's a codegen "issue" which I interpret in this context to simply mean that the generated code is suboptimal. See also this previous patch which aimed to remove the const too, though for other reasons; namely LTO: https://lore.kernel.org/linux-mips/20180616154745.28230-1-ha...@hauke-m.de/T/#u As I measured there this does indeed have an impact on code size, though it's not infeasibly large or anything. > > For what its worth, there was UB before the commit in question, it just > > added a barrier and got lucky IRT codegen. I don't think there's any > > actual compiler bugs related, just runtime bugs due to UB. > > Does your solution preserves the original purpose of the hack though as > documented in the comment you propose to be removed? > > Clearly it was defined enough to work for almost 18 years, so it would be > good to keep the optimisation functionally by using different means that > do not rely on UB. This variable is assigned at most once throughout the > life of the kernel and then early on, so considering it r/w with all the > consequences for all accesses does not appear to me to be a good use of > it. > > Maybe a piece of inline asm to hide the initialisation or suchlike then? That could work as a replacement hack. As I mentioned in the thread linked above a less hacky, though more extensive & invasive change might be to move our I/O area to a fixmap which ought to produce even better code since the addresses would become compile-time constant. I'd settle for either approach for now though. Thanks, Paul
