[cfe-dev] Clang incorrectly optimizing out for conditional in Linux

[Keane, Erich via cfe-dev]

I filed an issue with the Kernel (here https://bugzilla.kernel.org/show_bug.cgi?id=195235) to capture this issue.  Hopefully this is something that they will fix!  I'll note that my suggested answer assumes they want the behavior to remain the same, though I'm not sure that is what they MEANT.

-Erich

On 4/3/17 9:33 AM, Derrick McKee via cfe-dev wrote:
> Hi,
>
> I am trying to get the Linux kernel to compile and boot using clang.
> The code compiles fine, but the kernel does not boot.  It looks like 
> the following macro in mm/vmalloc.c (line 650) is optimizing out the 
> exit condition on the for loop:
>
> #define llist_for_each_entry(pos, node, member)             \
>     for ((pos) = llist_entry((node), typeof(*(pos)), member);   \
>          &(pos)->member != NULL;

This comparison is tautologically true because it'd be UB to construct the address if pos were null, and true otherwise.


Jon

         \
>          (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
>

snip

> --
> Derrick McKee
> Ph.D. Student at Purdue University
>
>
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>

-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded


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