The ability to generate a C library was lost in recent versions

[julienfreche]

In the past, we worked on the ability to generate a C library with Sail rather than a C executable. That was useful to use the generated code in the context of a full emulator or a VMM.

However, it looks like refactorings in the C backend eliminated that option. Was that on purpose and/or is there still a desire from the maintainers to keep this as a working option in the future?

For example, those changes were lost: https://github.com/rems-project/sail/pull/81 https://github.com/rems-project/sail/pull/83 https://github.com/rems-project/sail/pull/164

[arichardson]

I have used sail generated code as a library somewhat recently. For exxample: https://github.com/CTSRD-CHERI/cheri-compressed-cap/blob/master/test/contrib/sail_compression_128.c contains sail generated code that I used to compare against a C implementation of the same logic. To generate it I used https://github.com/CTSRD-CHERI/cheri-compressed-cap/blob/master/test/regen_sail_c_code.sh: The flags -c -c_no_main -c_prefix sailgen_ -c_specialize seemed sufficient to get it working. That said those files are linked into a test binary rather than built as a standalone .so so it's possible there are some things that are broken.

[julienfreche]

Thank you for the quick reply and for giving this example. That's a start, but, it does not seem to cover a few features that are very useful to a VMM use case like:

• Ability to override the read and write to the CPU registers as seen by Sail (useful to remap them to the actual registers and/or a shared state stored somewhere)
• Ability to manually provide some function outside of sail (read or write to RAM for example)

or am I mistaken?

[Alasdair]

Maintaining two separate copies of the Sail C backend was too much work with the code duplication involved. I naively thought at the time that I could transition uses of the older C backend to the new one and fix the code duplication by removing the older backend, but that just wasn't realistic. When I switched the build system to dune (about 2 years ago roughly) and moved each Sail backend into a separate plugin, I wasn't sure if anyone was actively using the c2 output and it didn't fit neatly into the more cleanly separated architecture, so I decided to drop it - thinking that if anyone was they would probably notice and raise an issue. I see that finally happened!

My plan after that was to add these features back in by parameterising the original backend, preserving backwards-compatibility, but I just never had either the time or a pressing use-case to actually implement that.

[julienfreche]

There is definitely an interest from our side as we still use sail to perform emulation in certain cases. I just only recently looked into upgrading our ARM model to a newer version hence the belated feedback. Thanks for the explanation.

[trdthg]

We also have people here who are interested in Sail generating C libraries(with header file used for ffi calls) maybe related to #215

[Alasdair]

@julienfreche By the way, how do you feel about C++? I think I can do something quite a bit nicer (and easier) if I don't restrict myself to just plain C, i.e. rather than changing the compilation to thread all the state through manually I could make a C++ class that encapsulates all the state - that might be a nicer interface for using the model in general.

[julienfreche]

We use C++ on our side so that would make my life even easier :) thank you for asking!

[arichardson]

C++ would be much nicer! Last year I started trying to use the Boost.Multiprecision for the larger arithmetic, but never got around to fully implementing that. When profiling I saw that most of the time is being spent in malloc/free for GMP, and using a header-only C++ library would allow inlining all those functions to avoid heap allocations for smaller integers.

For example, https://godbolt.org/z/j3hjWhr4K shows that the compiler is able to remove all the logic to perform uint65 arithmetic whereas constant_fold_gmp still involves gmp API calls even for constants.

[julienfreche]

My experiments also correlate with your observations.

• A lot of malloc/free that could fit on the stack
• GMP is used for numbers that fit naturally the native CPU capabilities.

Tackling those two issues to some degree would lead to much better perf.