[LLVMdev] The live interval of write-only registers

[Tzu-Chien Chiu]

2005/12/13, Chris Lattner <sabre at nondot.org>:
> On Tue, 13 Dec 2005, Tzu-Chien Chiu wrote:
>
> > In my ISA, some registers are write-only. These registers serve as
> > output ports, writing to these registers will output the values to an
> > external device. They cannot be read. So their live intervals should
> > not be joined with any other registers.
>
> Ok.  Since they are not really registers in the sense that LLVM is used
> to, I wouldn't suggest even exposing them as registers.  You may as well
> just tell LLVM they are immediates or something, and have the asm printer
> print them however you like.  As an example, the X86 has I/O ports which
> can be read and written to.  These are not thought of as registers, but
> they have the same property.  They are accessed with in/out instructions,
> not with "register writes".

If the output register is represented by an immediate operand or
belongs to a different register class, the following sequence:

  add %reg1024, %reg1025, %reg1026
  out %o1, %reg1024
  // out is an intrinsic, there is no such machine instruction
  // o1 is a write-only output register, mapping to a physical output port

cannot be changed to:

  add %o1, %reg1025, %reg1026

because LiveIntervals::joinIntervalsInMachineBB() requires one of the
operands of a 'move' instruction (the 'out' intrinsic in this example)
to be a virtual register and both must be in the same register class.

Note this operation cannot be performed in the instruction selector
because %reg1024 must live after the 'out' intrinsic.

--
Tzu-Chien Chiu - XGI Technology, Inc.
URL: http://www.csie.nctu.edu.tw/~jwchiu/