Figured it out; turns out my code to align the stack so I could make
calls without raising exceptions was messing up and corrupting the

Running the -O2 code now makes the clock run a bit too quickly - the
calibration may have a minor issue. I'll fix that up and send patches
tomorrow or Monday hopefully.

I'll be traveling Tuesday, so I'd appreciate if we can get them merged
upstream Monday itself - I'm okay to have a call and walk someone
through the patches and whatnot if need be.


On Sun, Aug 12, 2018 at 1:25 AM, Amaan Cheval <> wrote:
> Hi!
> In the process of cleaning my work up, I've run into an odd problem
> which only shows up when I set the optimization level to -O2. At -O0,
> it's perfectly fine.
> The issue is that somehow execution ends up at address 0x0.
> This likely happens due to a _CPU_Context_switch, where the rsp is set
> to a corrupted value, leading to a corrupt (i.e. 0) return address at
> the end of the context switch.
> What's curious is that this corruption _seems_ to occur in
> _ISR_Handler's call to _Thread_Dispatch, by somehow messing the value
> of rsp up - I honestly don't know this for sure because gdb says one
> thing (i.e. that rsp = 0), but setting up some code (cmpq $0, rsp) to
> check this seems to say rsp is non-zero, at least.
> This is an odd heisenbug I'd like to investigate for sure - I just
> thought I'd shoot this email out because:
> - If I can't figure it out tomorrow, soon, I'll just drop it so I can
> create more logical commits to send as patches upstream (thereby
> leaving -O0 upstream, at least temporarily)
> - If anyone's seen an odd stack corruption like this, or has any
> advice on debugging it, could you let me know? I suspect something
> like interrupting tasks which ought not to be interrupted (perhaps I
> forgot to implement some kind of "CPU_ISR_Disable") - is there
> anything you can think of of that sort?
> Also, here's a Github PR like last time with all the work (just for
> the overall changes, not the specific commits!). I'd appreciate a
> quick review if anyone could - sorry about sending this out over the
> weekend! I've had a surprising share of Heisenbugs with QEMU in the
> past week.
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