[LLVMbugs] [Bug 768] NEW: Register scavenging

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=768

           Summary: Register scavenging
           Product: libraries
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Register Allocator
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: natebegeman at mac.com


I'm not sure this is technically a register allocator to-do, but we need a way to scavenge registers during 
frame index elimination and spilling for targets that need additional registers for creating large offsets, or 
cannot use immediate offsets in addresses during spilling.

Examples of this in the PPC backend include using r0 as a scratch register during frame index elimination 
for creating constant offsets larger than 32k, and using r0 as an index register for saving and restoring 
vector registers, which can only use reg+reg addressing.

Once this is done, all use of r0 should be removed from the PPC backend.



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