Re: [Dotgnu-libjit] Arithmetic operations and type-coercions for byte, short int, ...

[Jan Wedekind]

Thanks for your response.
First of all the time for code generation seems to be negligible for my
application. I am interested in libjit, because it is difficult to write a
C++ library which provides all possible combinations of operators and
element-types.
   Looks like I overlooked quite a bit though regarding adding special
support for operations on 8- and 16-bit values.
If I understand it correctly one needs to do the following things:
* add functions supporting the additional types - and combination
  of types :( - to jit-intrinsic.c
* add instruction sets to jit-rules-*.ins for each platform
* update the jit_opcode_descr tables in jit-insn.c
* optionally upgrade the backend to support register-register operation
  such as 2-byte addition [1]. If I understand it correctly the
  code is somewhere in jit-rules-x86.c.

I'm not sure whether I will have the time (and a sufficient understanding
of the code) to do this. Especially I don't know how to do this 
without breaking the other backends for Alpha and ARM.

[1] http://home.comcast.net/~fbui/intel_a.html#add

On Sat, 13 Sep 2008, Klaus Treichel wrote:

> Hi Jan,
>
> Am Sonntag, den 07.09.2008, 11:28 +0100 schrieb Jan Wedekind:
> > 5AHi,
> >    I'm developing a Ruby-extension for doing computer vision [1] with the
> > Ruby programming language and I am thinking about replacing the current
> > C++ template implementation with an implementation based on
> > libjit and ruby-libjit [2].
> >    I need to generate code to do element-wise unary operations
> > (such as +,-) and binary (scalar-array,array-scalar,array-array)
> > operations (such as +,-,*,/,**,minor,major). The element-types of the
> > two arrays can be various combinations of char, unsigned char, unsigned
> > short int, ...
> >    I noted that adding to 8-bit values with libjit results in a 32 bit
> > integer. Since I don't want to overwrite succeeding elements in the array,
> > I need to convert the result back to 8-bit. A performance comparison shows
> > that this also has an impact on the performance (see below).
> >    I had a look at the methods "common_binary" and "apply_arith" and I am
> > wondering whether one could extend the code to support coercions resulting
> > in 8- or 16-bit values as well.
>
> We'll have to add all arithmetic opcodes for 8bit and 16bit values too which
> will about double the number of opcodes for integer arithmetics (and the
> corresponding intrinsics).
>
> The next step would be supporting backend specific addressing modes for 
> arithmetic
> operations to be able to emit code like add 0x10(%ecx, %edx, 4), %eax on x86
> instead of loading 0x10(%ecx, %edx, 4) to a register first.
>
> >    Please let me know what you think and tell me if I overlooked something.
> > It would be really cool if I could get the same performance as a C/C++
> > implementation.
>
> Libjit is still a just in time compiler where the time needed for code
> generation has to be taken into account too.
>
> Any suggestions are welcome.
>
> Klaus