[LLVMdev] Type Legalizer Question.

[sanjiv gupta]

On Wed, 2008-10-01 at 23:39 -0700, Evan Cheng wrote:
> On Oct 1, 2008, at 3:01 AM, sanjiv gupta wrote:
> 
> > On Wed, 2008-10-01 at 11:12 +0200, Duncan Sands wrote:
> >> Hi,
> >>
> >>> I have another query related to type legalizer.
> >>> Can a target ignore certain nodes during legalize? Probably a hook  
> >>> for
> >>> target that can be called inside IgnoreNodeResults ()?
> >>
> >> while this could be done, I don't like the idea of doing an end-run
> >> around the whole type legalization infrastructure.  So you would have
> >> to come up with a pretty convincing argument as to why this can't be
> >> done another way!
> >>
> >>> We want to keep certain nodes illegal, especially the pointer nodes.
> >>> Our target has 16-bit pointer and a few 16-bit insns to perform
> >>> arithmetic.
> >>> Rest of the stuff is 8-bit so we want to use legalizer to expand  
> >>> them.
> >>
> >> I think x86-32 is capable of doing a small number of 64 bit  
> >> operations.
> >> I wonder how they are handled/generated given that i64 is not a  
> >> legal type?
> >>
> > They might be using DAGCombine to convert the operations back to 64- 
> > bit.
> > Also, the pointer is still a legal type (i32) in X86-32.
> >
> > In our case the pointers themselves are illegal (i16). We have 16-bit
> > registers that can only hold pointers during indirect addressing. All
> > other operations and registers are 8-bit. So we still want to expand
> > everything to 8-bit but still keep the pointer nodes as an i16 node, a
> > pair that in turn holds the expanded parts.
> 
> So you have a i16 register class which makes the type legal. You can  
> make loads and stores legal (can you?). But you will have to custom  
> lower all other i16 operations. This will work, but it requires a lot  
> of target specific code.
> 
> Evan
> 
These i16 regs are actually pairs of 8-bit regs, So the other way for us
is to have only 8-bit regclass and to use custom nodes for load/store
that hold both the hi and lo parts of the pointer.

- Sanjiv

> >
> >
> > - Sanjiv
> >
> >
> >
> >
> >
> >> Ciao,
> >>
> >> Duncan.
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