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by fleventynine 223 days ago
Rigid ABIs aren't necessary for statically linked programs. Ideally, the compiler would look at the usage of the function in context and figure out an ABI specifically for that function that minimizes unnecessary copies and register churn.

IMHO this is the next logical step in LTO; today we leave a lot of code size and performance on the floor in order to meet some arbitrary ABI.
2 comments
ozgrakkurt 223 days ago
Can’t offload everything into the compiler. It is already too slow.

Soon people will demand it just figures out what you are implementing and rewrites your whole codebase

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fleventynine 222 days ago
> Can’t offload everything into the compiler. It is already too slow.

Speak for yourself. On embedded platforms I'd happily make my compiles twice as slow for 10% code size improvements.

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acedTrex 223 days ago
> Soon people will demand it just figures out what you are implementing and rewrites your whole codebase

We have this now, it is indeed very slow lol. Gemini is pretty fast however.

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thfuran 223 days ago
Isn't that what people are using Claude for now?
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layer8 223 days ago
I would argue that most of today’s performance problems in software are unrelated to ABI.
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stmw 223 days ago
I would argue that is largely true because we got the ABIs and the hardware to support them to be highly optimized. Thing slow down very quickly if one gets off that hard-won autobahn of ABI efficiency.
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pornel 222 days ago
Compilers inline everything they can.

Partly it's due to lack of better ideas for effective inter-procedural analysis and specialization, but it could also be a symptom of working around the cost of ABIs.

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layer8 222 days ago
The point of interfaces is to decouple caller implementation details from callee implementation details, which almost by definition prevents optimization opportunities that rely on the respective details. There is no free lunch, so to speak. Whole-program optimization affords more optimizations, but also reduces tractability of the generated code and its relation to the source code, including the modularization present in the source code.

In the current software landscape, I don’t see these additional optimizations as a priority.

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fleventynine 222 days ago
When looking at the rv32imc emitted by the Rust compiler, it's clear that there would be a lot less code if the compiler could choose different registers than those defined in the ABI for the arguments of leaf functions.

Not to mention issues like the op mentions making it impossible to properly take advantage of RVO with stuff like Result<T> and the default ABI.

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