[LLVMdev] problem trying to write an LLVM register-allocation pass

Susan Horwitz horwitz at cs.wisc.edu
Mon Dec 3 07:11:36 PST 2012

Aha, yes, this is definitely helpful!

Is there documentation on all this somewhere?  I still have errors for 
some large test inputs, and it would be nice if I didn't have to keep 
bugging you for information.



On 12/2/2012 10:55 PM, Lang Hames wrote:
> Hi Susan,
> Thanks for the clarification, and the test case. I think I know what 
> the problem is now. Saving and restoring RBP is the job of the PEI 
> (PrologEpilogInsertion) pass, which runs after register allocation. To 
> determine which callee-saved physregs actually need to be saved it 
> checks MachineRegisterInfo::isPhysRegOrOverlapUsed(unsigned reg). Your 
> register allocator needs to notify MachineRegisterInfo about the 
> physical registers that have been assigned by calling 
> MachineRegisterInfo::setPhysRegUsed(unsigned reg).
> You only need to call setPhysRegUsed for the physregs that you 
> actually use. You do not need to specify the aliasing registers.
> Hope this helps!
> Regards,
> Lang.
> On Sat, Dec 1, 2012 at 9:31 AM, Susan Horwitz <horwitz at cs.wisc.edu 
> <mailto:horwitz at cs.wisc.edu>> wrote:
>     On 11/30/2012 6:36 PM, Lang Hames wrote:
>>     RBP is used as the frame pointer on x86 (hence its automatic
>>     appearance in your code), and shouldn't be allocated to any vreg
>>     in function bar. Loading/saving RBP should be managed by the
>>     stack frame setup/teardown code.
>>     If it doesn't already, your allocator should filter out reserved
>>     registers (See MachineRegisterInfo::isReserved(unsigned preg))
>>     when assigning physregs.
>     I AM filtering out reserved registers.
>     I am not allocating RBP in function bar, I am allocating EBP,
>     because it is NOT in the list of reserved registers for function bar.
>     Neither register RBP nor register EBP is saved/restored across the
>     call from foo to bar, either by the code for the call or the code
>     for entry to bar.
>     The input C file that is causing this problem is flex.c
>     (attached).  The calling function is "yyparse" and the called
>     function is "scinstal".
>     Here are the reserved registers for yyparse: { 7 44 54 106 111 114
>     118 }
>     Here are the reserved registers for scinstal: { 54 111 114 }
>     Register EBP is preg 44, which is NOT in the reserved list for
>     scinstal (nor is it an alias of any of those reserved registers;
>     the aliases are  { 50 64 117 118 <tel:50%2064%20117%20118> }).  I
>     don;t know which preg corresponds to RBP.
>     You say that RBP should be saved/restored across the call.  I
>     tried to generate that code, but, as I said in my previous mail, 
>     I don't know how to get the appropriate TargetRegisterClass
>     (needed to call CreateSpillStackObject).   Should I instead be
>     generating code to save register EBP at the start of scinstal,
>     restoring it at the end of that function?
>     Susan
>>     ArrayRef<MCPhysReg> pregs = TRC->getRawAllocationOrder(&MF);
>>     for (int i = 0; i < pregs.size(); ++i) {
>>       if (MRI->isReserved(pregs[i]))
>>         continue;
>>       // use preg...
>>     }
>>     You could also use the AllocationOrder class to simplify the task
>>     of finding valid pregs, though it does require you to use
>>     VirtRegMap.
>>     If you are already checking the reserved regs then I've
>>     misdiagnosed the problem. I'd be happy to dig further if you can
>>     point me to a copy of your allocator and a test case.
>>     Cheers,
>>     Lang.
>>     On Thu, Nov 29, 2012 at 3:49 PM, Susan Horwitz
>>     <horwitz at cs.wisc.edu <mailto:horwitz at cs.wisc.edu>> wrote:
>>         I have a new problem: Register RBP is used in a function foo.
>>          (I am not allocating RBP to any virtual register, the
>>         instances of RBP in function foo are in the machine code when
>>         my register allocator starts.)
>>         Function foo calls function bar.  Register RBP is not saved
>>         across the call, though it is live after the call.  Function
>>         bar includes a virtual register.  The code that I'm using to
>>         find the registers available to be allocated to that virtual
>>         register includes EBP in that available-preg set.  This is a
>>         disaster, since writing into EBP clobbers RBP.
>>         I tried to add code to save all live physical registers
>>         across calls, but I don't know how to get the appropriate
>>         TargetRegisterClass (needed to call CreateSpillStackObject).
>>         Is there a different way to save/restore RBP across calls?
>>          Or a way to get its TargetRegisterClass?
>>         Susan

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