Top-down ISA evolution
The design process of an Instruction Set Architecture (ISA) has always struck me as being backwards. I imagine that they are mostly designed by hardware engineers whose decisions are largely influenced by the medium in which they work - hardware description languages. Every decision is influenced to some degree by implementation difficulty or limitations of the hardware.
Only the lucky compiler developers have a chance to review an ISA before it is fixed in stone. The few times I've seen this happen it has always been to everyone's benefit. Perhaps there are instructions
that the compiler could never use, or perhaps the compiler would generate better code if only it had some overlooked addressing mode.
I've often wondered what would happen if we reversed this process. What would an ISA look like that was solely influenced by the needs of the compiler writer, without regard to the hardware side of the house? Instead of designing from the hardware up, start from the compiler and design top-down.
As an experiment, I've developed the start of a GNU tools port to a new architecture that was wholly undefined at the start of the project. There was no instruction set architecture definition, no register file definition, no defined ABI or OS interfaces. Instead, I defined these things on the fly, letting things evolve naturally based on the needs of the compiler. The C compiler, assembler, linker, binutils and simulator were developed concurrently. So, for instance, when the compiler required an "add" instruction, each tool would be taught about "add" and so on.
I'm going to blog a patch a day to show my progress. Each patch will be small and buildable. I hope to show that you can get from nothing to running real programs using this top-down ISA design method in a surprisingly short amount of time.
A couple of caveats first... I am not an experienced compiler writer. I've been involved in many new GNU ports over the years at Cygnus and Red Hat, but mostly in a bizdev capacity. I have a good idea how all the pieces go together, but this will be my first attempt at it and I'm certain to botch some things. Second, I'm by no means an expert in ISA design. I'm taking the lazy man's route to ISA design:
1. Try to build something with gcc
2. See what the compiler is complaining about
3. Implement it
4. Go to 1.
Every design decision is driven by whatever is easiest to implement. What I expect I'll end up with is a simple although wildly inefficient architecture. Then perhaps we can optimize this toy ISA based on real-world code generation. At the end of the day, however, my goal is for this to be a fun and interesting experiment.
I'll post my first patch later today.
