Blogging from ISCA: AMAS-BT: Pardo, SMC

Dave Keppel, Pardo, Google: Self Modifying Code

Everyone knows SMC is dead, but SMC is alive in the very tools that complain about it: dynamic optimizers, dynamic linkers, etc.

Pardo talks fast.

Detecting SMC via page protection. Slow if data and code in same page.

BitBLT - recomile every 10K instructions.

Debugger watchpoints. Change immediates in code.

Present in real commercial workloads.

Coherency events:

x86 - non.

Hardware instructions: ISCP "something changed". iflush addr. coherency(base,length).
Poor match between application and simulator/emulator. Need to detect what really changed.

Adaptive: default-write protect. Change strategy if too many faults. Fall back to default after a while.

Self checking strategy: check current ibytes against saved copy of original ibytes.

Pardo noted that invalidated code often reappears - thing like debugger watchpoints maychange back to original. "Revalidation". Another use for invaid cache entries.

Shade. SPARC. Iflush addr. But, there were some applications that did not use iflush, but which worked on real hardware.

Transmeta Crusoe. Subpage write protection.

"Fetch imediates" - translate code, but fetch immediates that might have been patched.

Crusoe: lots of retranslations when falling through the gears.

Deoptimized translators: fetch immediates. Translation calls interpreter.

Bad: BT leads to more implementations, more chances of bugs, reduced test coverage.

Performance stability, lack of. Consistent sometimes better than fast.

SMC/ISC. Q: what does ISC stand for?

Hardware support:

Crusoe: 2 write protect bits per page. Subpage WP cache.

Shade: 100 instructions to translate an instruction.

Gill51 - universa simulator.