[LLVMdev] [llvm-announce] LLVM 2.0 Release

[Chris Lattner]

<cc'ing llvmdev, please don't email me directly>

On Sun, 27 May 2007, Christophe Avoinne wrote:

> On 23/05/2007 22:05:48, Chris Lattner (sabre at nondot.org) wrote:
> >  LLVM 2.0 is done! Download it here: http://llvm.org/releases/ or view
> >  the release notes: http://llvm.org/releases/2.0/docs/ReleaseNotes.html
> > 
>
> Hello Chris.
>
> Your project sounds quite exciting and I may be interested to use it instead 
> of gcc.
>
> Well, let's me explain why I'm interested in this project :
>
> I have a PlayStation Portable and like to develop several projects using 
> all of its potential.
>
> PSP has two Allegrex cpus : SC (system control) and ME (Media Engine).
> Both are designed with a MIPS 2 core plus a dozen instructions borrowed from 
> MIPS4.
> Both also have a standard  FPU coprocessor.
> But only SC processor has a second coprocessor called VFPU which allows 
> matrix and vector computations.
>
> Actually, gcc doesn't handle VFPU at all (i'm speaking about the compiler, 
> not the assembler).
>
> I tried to add the necessary things to allow gcc to handle them but I have 
> only a half success because of the way gcc handles registers and a few other 
> messing things.
>
> So i'm looking for an alternative : llvm-gcc.

Ok.

> First, what is VFPU :
>
> it is a great coprocessor which allows us to handle up to 128 32-bit float 
> registers as a scalar or as a vector of two, three or four elements or as a 
> square matrix of 4, 9 or 16 elements.
>
> - a scalar : S000 = [$0],  S001, [$1], S002 = [$2], S003 = [$3], S100 = [$4], 
> ..., S010 = [$32], ..., S020 = [$64], ...,  S733 =  [$127]
> - a column : C000.p = [S000, S010], C020.p = [S020, S030], ..., C000.t = 
> [S000, S010, S020], C010.t = [S010, S020, S030], ..., C000.q = [S000, S010, 
> S020, S030]
> - a row : R000.p = [S000, S001], R002.p = [S002, S003], R000.t = [S000, S001, 
> S002], R001.t = [S001, S002, S003], R000.q = [S000, S001, S002, S003]
> - a matrix : M000.p = [R000.p, R010.p], M020.p = [R020.p, R030.p], ..., 
> M000.t = [R000.t, R010.t, R020.t], ..., M000.q = [R000.q, R010.q, R020.q, 
> R030q]
> - a transposed matrix : E000.p = [C000.p, C001.p], M020.p = [R020.p, R030.p], 
> ...,  M000.t = [R000.t, R010.t, R020.t], ..., M000.q = [R000.q, R010.q, 
> R020.q, R030q]
>
> (NOTE :  <t><#m><#r><#c>.<s> : t = S (scalar)/C (column)/R (row)/M (matrix)/E 
> (transposed matrix), #m = matrix, #r = row, #c = column, s = p (2d vector), t 
> (3d vector), q (4d vector))
>
> now, if you want just a 3d dot product, just issue this instruction for 
> instance : vdot.t S000.s, C100.t, C110.t
> or a matrix product : vmmul.q M000.q, M100.q, E200.q
>
> As you can see, that VFPU has a great potential but, alas, it is not very 
> well exploited by the open source psp-gcc.

Ok.

> Now, lets's speak about llvm-gcc.
>
> 1) Does a (uncompleted) MIPS target exist for  llvm-gcc ? or must I create 
> one ?

Bruno just submitted one, he's the right person to talk to.  I have not 
had a chance to review the code, so it has not been committed yet, but it 
should be soon.

> 2) Do you think that llvm-gcc can handle those combinaisons of registers :
> 2.1) Can llvm-gcc handle vector float as a super-set of scalar registers ?
> 2.2) Can llvm-gcc handle matrix float as a super-set of vector float ?

LLVM has good support for vectors, if you treat matrices as large vectors, 
you should have no problem.

> 3) Can llvm-gcc vectorize float operations ?

llvm-gcc supports explicitly vectorized code, but we have no 
autovectorization pass yet.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/