[LLVMdev] Shouldn't DAGCombine insert legal nodes?

[Scott Michel]

I just ran across something interesting: DAGCombine inserts a 64-bit  
constant as the result of converting a (bitconvert (fabs val)) to a  
(and (bitconvert val), i64const).

The problem: i64 constants have to be legalized for the CellSPU  
platform. DAGCombine is doing the right thing but it's not doing the  
right thing for CellSPU and it's damed difficult to work around this  
"feature". Moreover, the way all of SelectionDAGLegalize and  
DAGCombne's code is written, it's particularly difficult to "re- 
legalize" nodes unless one more legalization pass is invoked after  
DAGCombine.

It's not like it's actually easy to load an i64 constant while  
maintaining the vector register uniformity that CellSPU expects. In  
fact, there're a lot of different ways to load a 64-bit constant  
depending on the constant's value. At worse case, it's a constant  
pool load on a memory-constrained machine (256K). But it can be a lot  
better than paying the 6-cycle penalty for memory loads.

OTOH, that's why I created all of that legalization code in the first  
place so the backend could get it right.

Suggestions apart from doing custom fabs legalization in CellSPU  
(sorta defeats the purpose of DAGCombine, no)?


-scooter