Fix Heartbleed demo legacy lock-up due to 32-bit mcycle read #75
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The legacy demo unintentionally aliased a 32-bit `rdcycle` function to `rdcycle64`, and used its 32-bit output as a 64-bit value. This caused an overflow in `mcycle` after a few minutes of running, which sometimes left the demo stuck and unable to progress in its legacy component. Replace this function with an actual 64-bit mcycle read using the low/high registers appropriately. Signed-off-by: Alex Jones <alex.jones@lowrisc.org>
engdoreis
approved these changes
May 22, 2025
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Note: Ideally merge after #74 to pull in the fixes necessary to get the Heartbleed demo to compile first.
The legacy demo unintentionally aliased a 32-bit
$$\frac{2^{32} - 1}{40 * 1000 * 1000} \approx 1$$ min 47 s.
rdcyclefunction tordcycle64, and used its 32-bit output as a 64-bit value. This caused an overflow in the value ofmcyclethat was read after a short while of running, which left the demo stuck and unable to progress in its legacy component. Specifically,So after around 2 minutes the legacy component of the demo will stop working.
This PR replaces this function with an actual 64-bit mcycle read using the low/high registers appropriately. It will now take >10000 years to overflow, so I've opted not to add additional logic to catch overflows here.