[LLVMdev] Simpler types in TableGen isel patterns

Chris Lattner clattner at apple.com
Thu Mar 21 13:39:57 PDT 2013

On Mar 21, 2013, at 11:26 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:
> I think in most cases it would be much simpler and safer to specify pattern types directly:
>  def : Pat<(and (not i32:$src1), i32:$src2),
>            (ANDN32rr i32:$src1,  i32:$src2)>;
>  def : Pat<(and (not i64:$src1), i64:$src2),
>            (ANDN64rr i64:$src1,  i64:$src2)>;
> This doesn't lose any type checking because the register classes of the instructions in the output pattern are still checked. It avoids the problem where type inference makes it impractical to add types to a register class to model instruction set extensions, and it makes it clear that patterns operate on types, not register classes.

This makes a huge amount of sense to me.  Besides your motivating problem, writing an isel pattern that matches things in terms of register classes doesn't make *any* sense: the input graph has type labels that are MVTs.

If you want to get *really* crazy, the ideal type system for dag nodes would be to allow either an EVT or a register class: after isel (and after some custom dag combines) the types of nodes should be register classes, since they are more specific than an MVT (or EVT).


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