clang/llvm 3.9.0 mysteriously zeroing variables?

Starting with r309124 (when clang/llvm 3.9.0 was imported) I'm seeing EC2
instances panic on boot with a division-by-zero error; the code in question
is in blkfront.c, printing out the size of disks:

> 		device_printf(dev, "%juMB <%s> at %s",
> 		    (uintmax_t) sectors / (1048576 / sector_size),
> 		    device_get_desc(dev),
> 		    xenbus_get_node(dev));

My first thought was that 'sector_size' must be either zero or very large...
but no, when I add printf("sector_size = %ju\n", (uintmax_t)sector_size), it's
entirely normal.  What's more, adding that printf makes the division-by-zero
panic go away.

I'd think I was just hallucinating, but earlier today I heard that a similarly
"impossible" panic had been observed in the NFS client code when compiled with
clang/llvm 3.9.0.

So... is anyone else seeing unexpected panics or other odd behaviour starting
after clang/llvm 3.9.0 was imported?

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid