[llvm-dev] Re-computing Live-in/Live-out Physical Registers for Basic Blocks Using LivePhysRegs

[章明 via llvm-dev]

Thank you for your reply!

If liveness of APSR is not tracked, how does the code generator determine whether instructions like cmp are dead code or not?
Is it true that, for every use of APSR (such as a conditional branch), the used def (such as cmp, add, etc.) is always in the same basic block?
Is it true that APSR is never assumed to live across basic block boundaries?

Thank you!


Ming Zhang



>On Aug 2, 2017, at 9:07 AM, Matthias Braun via llvm-dev llvm-dev at lists.llvm.org llvm-dev at lists.llvm.org>> wrote: 

>Yes LivePhysRegs can do that for you. It has addLiveIns() and addLiveOuts() functions to get the live-in or live-out set for a basic block (make the set is empty when you call these functions).

>Yes APSR is marked as a reserved register by the ARM target, this means we do not track liveness for it.

>- Matthias

>>On Aug 2, 2017, at 2:45 AM, 章明 via llvm-dev llvm-dev at lists.llvm.org llvm-dev at lists.llvm.org>> wrote: 

>>
>>Hi, LLVM developers! 
>>
>>
>>I am working on software control flow checking. I have to work at the native assembly level, and rely on the code generator to provide various information. I'm considering ARM Cortex-M3 as my target CPU. 
>> 
>>
>>I would like to determine live-in and live-out PHYSICAL registers for every machine basic block after all code generation passes, such as the ARM Constant Island pass, and before code emission. Unfortunately, at this point, information about live-in/live-out registers has been invalidated by the ARM Constant Island pass. So I am considering re-computing it.
>>
>>
>>Class LivePhysRegs seems to be able to compute live-in/live-out registers for a given machine basic block based on live-in registers of its successors. It appears to me that, if I perform a depth-first traversal through the control flow graph of a function, calling LivePhysRegs::computeLiveIns on every machine basic block, live-in/live-out physical registers are computed for every basic block. Is this true? 
>> 
>>
>>Also, I would like to know whether APSR is dead or not on entry and exit of every basic block. Is the register treated by LLVM specially or in the same way as other general-purpose registers? >>>Thank you!  >>>>Ming Zhang  >