llvm dev - Aug 2007 - [LLVMdev] Destination register needs to be valid after callee saved register restore when tail calling

Hello list,

i am currently trying to implement tail call optimization in the X86  
backend , so far i have it working for cases (modulo many unknown  
bugs :) where the tail called function is a destination within the  
source file and frame pointer elimination is performed. i implemented  
it as a dagcombiner transformation running in post legalized phase  
within the X86TargetLowering::PerformDAGCombine(SDNode *N,  
DAGCombinerInfo &DCI). now to my problem:

When tail calling a function pointer the register the function  
pointer was loaded to might be invalidated by the restored callee  
saved registers.

  ...
  movl 12(%esp), %esi    << 12(%esp) contains the function pointer
  movl %eax, 44(%esp)
  movl 32(%esp), %esi     <<  *!# %esi no longer contains the  
function pointer to be called
  addl $44, %esp
   jmp *%esi  # TAIL CALL jmpr


Is there a way to indicate that the register the tail call  
instruction uses as destination needs to be valid after the callee  
saved registers have been restored? (some X86InstrInfo.td foo magic  
maybe ?)
Or do i have to insert code into PEI::saveCalleeSavedRegisters to  
detect that there is a tail called function that uses a callee saved  
register and move it to another (EAX).

that's how the TAILJMP instruction looks:
let isCall = 1, isTerminator = 1, isReturn = 1, isBarrier = 1,  
noResults = 1 in
   def TAILJMPr : I<0xFF, MRM4r, (ops GR32:$dst), "jmp {*}$dst  #  
TAIL CALL jmpr",
                  []>;

btw. i am currently working with llvm-2.0 codebase.

thanks for any help

regards arnold

On Aug 8, 2007, at 6:12 AM, Arnold Schwaighofer wrote:
> When tail calling a function pointer the register the function
> pointer was loaded to might be invalidated by the restored callee
> saved registers.
Inserting a pseudo before your tail call that defines all the callee- 
saved
registers should work.  See FP_REG_KILL.