Agner`s CPU blog

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Proposal for instruction set - now on Github
Author: Agner Date: 2017-09-23 01:19
- wrote:
self-modifying code, jit-compilers, etc should be minimally affected by such a restriction: Between each self-modification / jit-round, you pay a round-trip to main memory and possibly a syscall (if you don't have permissions to map a page as executable).
I am not too fond of the trend of JIt compilation and byte code. If you are JIT compiling each part of a program separately, you get erratic and unpredictable response times which users hate. I would prefer to compile everything once. ForwardCom is designed to be forward compatible in the sense that a compiled program can run optimally without recompilation on future microprocessors with longer vector registers. This means less need for JIT compiling. Another planned feature is re-linking of executable files. Instead of calling a DLL or shared object, you use static linking to include the necessary library functions in the executable, with the possibility to re-link later if a library function needs replacement. This makes the code run faster than if dynamic linking is used, and it makes it possible to adapt the code to different environments by replacing e.g. a user interface library.

I'm not sure about strictly enforcing W^X. I get that it really messes with processor optimizations and all, but applications such as emulators which make heavy use of dynamic recompilation would suffer if they are forced to syscall every time they modify code
JIT compilation is also a security problem. If we allow self-modifying code or code that makes code and executes it immediately, then we have all kind of security problems. I would like to re-think the whole idea of JIT compilation.

Emulation is a problem, I agree, if we don't allow dynamic compilation. An emulator running under ForwardCom and emulating something else would need special permission to do dynamic compilation and running the code it generates. The user will be warned when installing the emulator that it needs dangerous special permissions.

 
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