llvm dev - Jul 2006 - [LLVMdev] SSA virtuals -> Original variables

Dear LLVM'ers,

    in my register allocator, sometimes I have to spill virtuals that
represent the same variable before SSA conversion. I would like to
assign the same stack slot to these virtuals. Is there a way to
recover this information from one of LLVM data structures? In other
words, is it possible to discover which virtuals represent the same
variable before SSA conversion by reading one of LLVM data structures?

Thanks a lot,

Fernando

On Sun, 2006-07-30 at 14:57 -0700, Fernando Magno Quintao Pereira
wrote:
> Dear LLVM'ers,
> 
>     in my register allocator, sometimes I have to spill virtuals that
> represent the same variable before SSA conversion. I would like to
> assign the same stack slot to these virtuals. Is there a way to
> recover this information from one of LLVM data structures? In other
> words, is it possible to discover which virtuals represent the same
> variable before SSA conversion by reading one of LLVM data structures?
That information is lost.  What I think you really want is an analysis
of the liveness of a stack slot so that you can find spills that don't
overlap and compact those to a single stack slot.  At least, this is
something I would like to see to reduce stack usage.  Trying to do this
based on "original variable" is not going to work as optimizations may
have done a lot of rearranging, including causing two values generated
from the original variable to be live at once.

Andrew