llvm dev - Nov 2013 - [LLVMdev] How Fast RegisterAllocation determine if the use-reference is the last use without isKill Flag

Hello llvmers:
      I'm study    Fast register allocation in codegen.   I get confused
with  isKill flag in MachineOperand.  I thin "isKill" flag  the last
Use
Reference. But,   Fast Register doesn't require  LiveVariable Analysis which
will mark isKill Flag for global virtual Register.  So during Fast
RegisterAllocation,  Faster Register Allocation procedure can't determine if
this use-reference is the last use  through current path ;

 , How  does Fast RegisterAllocation resolve such problem ?



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On Nov 14, 2013, at 6:20 AM, sheng wang <wansheg at qq.com> wrote:
> Hello llvmers:
>      I'm study    Fast register allocation in codegen.   I get confused
> with  isKill flag in MachineOperand.  I thin "isKill" flag  the
last Use
> Reference. But,   Fast Register doesn't require  LiveVariable Analysis
which
> will mark isKill Flag for global virtual Register.  So during Fast
> RegisterAllocation,  Faster Register Allocation procedure can't
determine if
> this use-reference is the last use  through current path ;
> 
> , How  does Fast RegisterAllocation resolve such problem ?
It is using the use-def chain of the virtual register. Virtual registers are
rewritten to physical registers in a top-down manner, and MachineRegisterInfo
updates the use-def lists as operands are rewritten. When the last use of a
virtual register is rewritten, its use-def chain becomes empty.

This only works for local virtual registers, of course. If a virtual register
has uses or refs in multiple basic blocks, it is always assumed to be live out
of the block. The fast register allocator is optimized for debug builds where
the code has almost no global virtual registers.

Thanks,
/jakob