Summary of David Ditzel talk
on binary translation.
David Ditzel: worked at Transmeta, now at Intel
A 25 year perspective on binary translation: what worked, what didn’t work.
Examples of binary translation:
- pentium pro and later: translates x86 into internal UOPS via hardware
- intel ia32el runs user level x86 programs on Itanium with ~60% perf of native machine
- Java JITs, .NET (MSIL), VmWare
- Apple’s Rosetta runs PowerPC user programs on x86
- Transmeta Crusoe and Efficeon processors - transparent, full system level translation of x86
SYMBOL computer - implemented os, editor etc. in logic gates. Lessons: don’t do that. Use the right combination of software, hardware and micro-ops.
- 1987, first CMOS superscalar chip (superscalar: multiple instructions per clock)
- Reduced Instruction Set Processor targeted at C
- hw translated instructions from external, compact version into 180 bit wide UOP cache
- optimization tricks:
- branch folding - make branches disappear from pipeline
- “Stack Cache” as registers - to reduce memory references
Lessons from AT&T Crisp:
- translation from external instruction set to internal instruction set works well
MIPS had tools pixie and pixstats (~1987) to statically modify binaries to count instructions.
Sun followed (~1988) with spix, spixstats etc. Also were able to run MIPS on Sparc (at 1/3rd speed).
Sun tried to extend Sparc with instructions to help x86 emulation but decided that hw mismatch was too big - Sparc was not the right architecture for this.
Lessons realized in 1995:
- dynamic translation was reaching 1⁄3 speed of native, static 1⁄2 of native
- processor designed from scratch for binary translation might improve efficiency of dynamic translation by 2-3x
- full system level binary translation might soon become practical and even exceed perf of standard microprocessor
That led to Transmeta in 1995. Transmeta:
- spent $600M over 12 years in R&D
- 5 generations of processors although only 3 announced
Key challenges for hybrid processors:
- must be 100% compatible. When doing binary translation between commodity processors (e.g. x86<->PowerPC), there are corner cases where emulating things exactly is tricky due to hardware mismatch, which causes inefficiencies in translation => must design for translation
- precise control over user visible state, including precise exception and interrupt semantics
- delivered performance, including overhead of translation
Hardware support for hybrid processors:
- private, non-volatile storage (FLASH ROM), for storing translation software
- private memory for storing translated code (stole 5% of DRAM during boot)
- software controlled state commit/rollback/abort
- more registers than x86
- alias detection under software control
- fine grain detection of self-modifying code
- auto-typing of pure memory vs I/O (because I/O can be memory-mapped which prohibits some optimizations)
- fast traps supported by underlying runtime system
- instruction primitives for fast interpretation
Software controlled atomic execution - execute in temporary space and ability to rollback to previous commit point. Used to perform not-always-safe optimization which are considered ok as long as we hit next commit point without problems. If not, rollback and re-execute without optimizations. Needed to be able to undo stores to memory.
Transmeta’s code morphing:
- first level - interpreter
- second compiler
- translation - it can cost 10000 instructions to translate 1
Efficeon improvements used 4 levels (gears):
- first level - interpreter 15 instruction per 1, gathers
- after executing basic block 50 uses quick translation (cost 500 instructions per 1 native), also gathers more information
- after executing more than few hundred times - more optimized translation, more costly, classic optimization like common sub-expression elimination, memory re-ordering, critical path scheduling
- for hot loops, optimize multiple code blocks and use more aggressive optimization
Lessons: optimization pay off. The bigger the blocks, the bigger optimization payoff.
Binary translation myths:
- myth: translation is slow. It’s only ~20% overhead
- myth: saving translation to disk is a good idea. In efficeon they improved translation so that it was faster to translate than read from disk
- myth: static translation is faster. They compiled Linux kernel to run natively but it was slower than dynamic translation because dynamic translation could use runtime information to optimize.
- myth: software isn’t reliable. Doesn’t match transmeta’s nor Transiitive’s experience: they didn’t have any x86 compatibility bugs
Why binary translation now: power usage since increasing cores requires more power so we might not have enough power to light up all processors at full speed.