Making alloca more safe

[bearophile]

Walter Bright:

>is that biology achieves reliability by using redundancy, not by requiring individual components to be perfect. The redundancy goes down to the DNA level, even. Another way is it uses quantity, rather than quality. Many organisms produce millions of offspring in the hope that one or two survive.<

Quantity is a form of redundancy.

In biology reliability is achieved in many ways.
Your genetic code is degenerated, so many single letter mutations lead to no mutated proteins. This leads to neural mutations.

Bones and tendons use redundancy too to be resilient, but they aren't flat organizations, they are hierarchies of structures inside larger structures, at all levels, from the molecular level up; this allows for a different kind of failures, like in earthquakes (many small ones, few large ones, with a power law distribution).

A protein is a chain of small parts, and its function is partially determined by its form. This forms is mostly self-created. But once in a while few other proteins help shape up the other proteins, especially when the temperature is too much high.

Most biological systems are able to self-repair, that usually means cells that die and duplicate and sometimes they build harder structures like bones. This happens at a sub-cellular level too, cells have many systems to repair and clean themselves, they keep destroying and rebuilding their parts at all levels, and you can see it among neurons too: your memory is encoded (among other things) by the connections between neurons, but they die. So new connections among even very old neurons can be created, and they replace the missing wiring, keeping the distributed memory functional even 100 years after the events, in very old people.

Genetic information is encoded in multiple copies, and sometimes in bacteria distribuited in the population. Reliability is necessary when you copy or read the genetic information, this comes from a balance from the energy used to copy and how much reliable you want such read/copy, and how much fast you want it (actually ribosomes and DNA polymerase are about on the theoretical minimum of this 3-variable optimization, you can't do better even in theory).

Control systems, like those in the brain, seek reliability in several different ways. One of them is encoding vectors in a small population of neurons. The final direction of where your finger points is found by such vectorized average. Parkinson's disease can kill 90% of the cells in certain zones, yet I can keep being able to move the hand to grab a glass of water (a little shaky, because the average is computed on much less vectors).

There is enough stuff to write more than one science popularization article :-)


>how would you write a program that would be expected to survive having a random bit in it flipped at random intervals?<

That's a nice question. The program and all its data is stored somewhere, usually on RAM, caches, and registers. How can you use a program if bits in your RAM can flip at random with a certain (low) probability? There are error-correction RAM memories, based on redundancy codes, like Reed-Solomon one. ECC memory is today common enough. Similar error correction schemes can be added to inner parts of the CPU too (and probably someone has done it, for example in CPUs that must work in space on satellites, where the Sun radiation is not shielded by the earth atmosphere).
I am sure related schemes can be used to test if a CPU instruction has done its purpose of if during its execution something has gone wrong. You can fix such things in hardware too.

But there are other solutions beside fixing all errors. Today chips keep getting smaller, and power for each transistor keeps going down. Eventually noise and errors will start to grow. Recently some people have realized that on the screen of a mobile telephone you can tolerate few wrongly decompressed pixels from a video, if this allows the chip to use only 1/10 of the normal energy used. Sometimes you want few wrong pixels here and there if they allow you to keep seeing videos on your mobile telephone for twice long. In future CPUs will probably become less reliable, so they software (mostly operating system, I think) will need to invent ways to fix those errors. This will allow to keep programs globally reliable even with fast low powered CPUs. Molecular-scale adders will need software to fix their errors. Eventually this is going to become more and more like cellular biochemistry, with all its active redundancy :-)

There's no end to the amount of things you can say on this topic.

Bye,
bearophile