-current build failure

[Konstantin Belousov]

On Tue, Jul 24, 2012 at 10:08:13PM +0100, David Chisnall wrote:
> On 23 Jul 2012, at 20:53, David Chisnall wrote:
> 
> > On 23 Jul 2012, at 20:18, Konstantin Belousov wrote:
> > 
> >> Longer description is that pc_curthread is offset 0 if %gs-based.
> >> The dereferenced pointer point to the struct thread, which contains
> >> td_proc pointer at offset 8. Instead, clang seems to dereference
> >> td_proc from offset 8 based on %gs, or something similar.
> > 
> > This appears to be a bug in the LLVM X86 back end.  It is performing an invalid fold of the two loads.  I have filed this bug:
> > 
> > http://llvm.org/bugs/show_bug.cgi?id=13438
> 
> And fixed it in LLVM r160687.  Since it's a single-line change, we can probably pull it into our version.
> 
> dim: http://llvm.org/viewvc/llvm-project?view=rev&revision=160687

As kan rightfully notes, the assumption that &%fs:0 == *%fs:0 holds for
userspace on amd64, and the same is true for %gs userspace on i386.
The change you committed to clang/llvm/whatever it called just breaks
useful optimization for FreeBSD.

Sigh.
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