Hi Ronie,

Le 15/09/2014 23:37, Ronie Salgado a ��crit��:

Hello,

I am segmenting this mail in several sections.

---------------------------------------------------------------

- On Lowcode and Cog

I have been working in the last week with the Cog VM, implementing the Lowcode instructions in Cog.

Lowcode is currently a spec of new bytecode instructions. These instructions can be used for:

- Implementing a C like language compiler.

- Making FFI calls

I am implementing these instructions using a feature of the new bytecode set for SistaV1, which is called "inline primitives". Because of this, these new instructions can be mixed freely with the standard VM bytecode set. This also allows the Sista adaptive optimizer to inline FFI calls.

These instructions provides features for:

- Int32 and Int64 integer arithmetic without type checking.

Will you provide SSE-type vector arithmetic or will you rely on the compiler to generate those?

- Pointers, with arithmetics.

- Memory access and memory manipulation.

- Single and double precision floating point arithmetics.

- Conversion between primitive types.

- Boxing and unboxing of primitive types.

- Unchecked comparisons.

- Native function call. Direct and indirect calls.

- The atomic operation compare and swap.

- Object pin/unpin (requires Spur).

- VM releasing and grabbing for threaded ffi.

Lot of very significant stuff :)

Current I have implemented the following backends:

- A C interpreter plugin.

- A LLVM based backend.

Do you mean a LLVM-IR to Lowcode backend compiler ?

Currently I am working in getting this working using the Cog code generator. So far I am already generating code for int32/pointer/float32/float64. I am starting to generate C functions calls and object boxing/unboxing.

During this work I learned a lot about Cog. Specially that Cog is missing a better Slang generator, that allows to force better inlining and more code reviews. There is a lot of code duplication in Cog, that can be attributed to limitations of Slang. In my opinion, if we could use Slang not only for building the VM we should end with a better code generator. In addition we, need more people working in Cog. We need people that performs code reviews and documentation of Cog.

So there is a path there about improving Slang to C generation.

After these weeks, I learned that working in Cogit it is not that hard. Our biggest problem is lack of documentation. Our second problem could be the lack of documentation about Slang

---------------------------------------------------------------
- Smalltalk -> LLVM ?

As for having a Smalltalk -> LLVM code generator. The truth is that we will not gain anything. LLVM is a C compiler, which is designed to optimize things such as loops with lot of arithmetics. It is designed to optimize large sections of code. In Smalltalk, most of our code is composed mostly of message sends. LLVM cannot optimize a message send.

To optimize a message send, you have to determine which is the method that is going to respond to the message. Then you have to inline the method. And then you can start performing the actual optimizations, such as constant folding, common subexpressions, dead branch elimination, loop unrolling, and a long etc.

Because we don't have information in the actual language (e.g. static types a la C/C++/Java/C#) that tells us what is going to be the actual method invoked by a message send, we have the following alternatives to determine it:
- Don't optimize anything.

- Perform a costly static global analysis of the whole program.

- Measure in runtime and hope for the best.

- Extend the language.

Do gradual typing of performance sensitive parts of the code.

In other words, our best bet is in the work of Cl��ment in Sista. The only problem with this bet are real time applications.

Real time applications requires an upper bound guarantee in their response time. In some cases, the lack of this guarantee can be just an annoyance, as happens in video games. In some mission critical applications the results can not be good, if this time constraint is not met. An example of a mission critical system could the flight controls of an airplane, or the cooling system of a nuclear reactor.

Don't worry/don't bother with thoses: you will never use Smalltalk or a VM :) It will never be certified by authorities, and the industry will never accept it.

Anyway, they are not even talking of coding anymore these days in there. They do MDE.

Consider that the target can be named performance critical applications, and those are significant but have different approaches to performance:
������ use highly tuned libraries (Intel mkl);
������ perform source to source analysis to extract performance;
������ vectorize code (via gcc intrinsics, SSE inline, OpenCL, ICC);
������ use inline asm;
������ use totally non-obvious algorithms (bitonic sort, bit slicing);
������ use GPUs;
������ use massively parallel computing resources;
������ load pre-optimised code generators in your code (compilette: a minimized Jit which generate asm at a cost of 3 inst for one generated)

Adaptative optimisation is a very impressive technique, but it goes only that far. When you consider that some compilation phases may require a 64 CPU cores machine to shorten the compilation time, then you don't do it on the fly.

For these application, it is not possible to rely in an adaptive optimizer that can be triggered sometimes. In these application you have to either:

- Extend the language to hand optimize some performance critical sections of code.

- Use another language to optimize these critical section.

- Use another language for the whole project.

And of course, you have to perform lot of profiling.

Allways!

Thierry

Greetings,

Ronie

2014-09-15 16:38 GMT-03:00 Craig Latta <craig@netjam.org>:

�� �� ��Hear hear!


-C

[1] http://tinyurl.com/m66fx8y (original message)

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