[LLVMdev] Vector select/compare support in LLVM

[Rotem, Nadav]

David, 

The problem with the sparse representation is that it is word-width dependent. For 32-bit data-types, the mask is the 32nd bit, while fore 64bit types the mask is the 64th bit. 

How would you legalize the mask for the following code ?

%mask = cmp nge <4 x float> %A, %B                   ; <4 x i1>
%val  = select <4 x i1>% mask,  <4 x double> %X, %Y ; <4 x double>

Moreover, in some cases the generator of the mask and the consumer of the mask are in different basic blocks. The legalizer works on one basic block at a time. This makes it impossible for the legalizer to find the 'native' representation. 

I wrote down some of the comments which were made in this email thread:

http://wiki.llvm.org/Vector_select


Cheers,
Nadav


-----Original Message-----
From: David A. Greene [mailto:greened at obbligato.org] 
Sent: Thursday, March 10, 2011 18:57
To: Rotem, Nadav
Cc: David A. Greene; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Vector select/compare support in LLVM

"Rotem, Nadav" <nadav.rotem at intel.com> writes:

> One of the arguments for packing masks is that it reduces
> vector-registers pressure.  Auto-vectorizing compilers maintain
> multiple masks for different execution paths (for each loop nesting,
> etc).  Saving masks in xmm registers may result in vector-register
> pressure which will cause spilling of these registers.  I agree with
> you that GP registers are also a precious resource.

GPRs are more precious than vector registers in my experience.  Spilling
a vector register isn't that painful.  Spilling a GPR holding an address
is disastrous.

> In my private branch, I added the [v4i1 .. v64i1] types. I also
> implemented a new type of target lowering: "PACK". This lowering packs

Is PACK in the X86 namespace?  It seems a pretty target-specific thing.

> I also plan to experiment with promoting <4 x i1> to <4 x i32>.  At
> this point I can't really say what needs to be done.  Implementing
> this kind of promotion also requires adding legalization support for
> strange vector types such as <4 x i65>.

How often do we see something like that?  Baby steps, baby steps...  :)

                                -Dave
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