[llvm-dev] Basic Jump indirect

[Gschwandtner Georg - S1310567008 via llvm-dev]

Hallo,


I am trying to build a backend for a simple 16-bit CPU with a very small instruction set of only 24 instructions called PROL16 is used to teach CPU architecture and chip design.


I am using Fraser Cormack and Pierre-Andre Saulais LEG CPU (https://github.com/frasercrmck/llvm-leg) as starting point.


I do struggle with the concept of indirect loads and jumps where the address to jump has to be save in an register first.


So my jump looks like this:

jump r3

where register r3 is containing the address to jump to.


When I change the  InstrInfo.td from a direct jump like this:


let isTerminator = 1, isBranch = 1, isBarrier = 1 in {
  def B : InstLEG<(outs), (ins b_target:$dst),
                  "b $dst", [(br bb:$dst)]> {
    bits<24> dst;
    let Inst{31-28} = 0b0000;
    let Inst{27-24} = 0b1010;
    let Inst{23-0} = dst;
  }
}


to register based like:


def SDT_LEGJmpLink      : SDTypeProfile<0, 1, [SDTCisVT<0, iPTR>]>;
def LEGJmpLink : SDNode<"LEG::B",SDT_LEGJmpLink,
                         [SDNPHasChain, SDNPOutGlue, SDNPOptInGlue,
                          SDNPVariadic]>;

let isTerminator = 1, isBranch = 1, isBarrier = 1 in {
  def B : InstLEG<(outs), (ins GRRegs:$Ra, variable_ops),
                  "JUMP", [(LEGJmpLink  GRRegs:$Ra)]> {
    bits<24> dst;
    let Inst{31-28} = 0b0000;
    let Inst{27-24} = 0b1010;
    //let Inst{23-0} = dst;
  }
}

def : Pat<(LEGJmpLink (i32 tglobaladdr:$dst)),
          (B tglobaladdr:$dst)>;
def : Pat<(LEGJmpLink (i32 texternalsym:$dst)),
          (B texternalsym:$dst)>;


This is used in CPU0, MIPS and LEG for call.


When I translate my test IR code LLVM is unable to match the Branch.


LLVM ERROR: Cannot select: 0x9f32948: ch = br 0x9f32504, 0x9f328ac [ORD=13] [ID=20]
In function: _Z17test_add_overflowv


I clearly do not understand how the concept for loading an address to a register works in LLVM.


For some insights I would be very thankful.


BR

Georg
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160905/cc72b9fc/attachment.html>