From: tkoenig@netcologne.de   
      
   Robert Finch  schrieb:   
   > Contemplating having conditional branch instructions branch to a target   
   > value in a register instead of using a displacement.   
   >   
   > I think this has about the same code density as having a branch to a   
   > displacement from the IP.   
      
   Should be possible.  A question is if you want to have a special   
   register for that (like POWER's link register), tell the CPU   
   what the target is (like VEC in My66000) or just use a general   
   purpose register with a general-purpose instruction.   
      
   > Using a fused compare-and-branch instruction for Qupls4   
      
   Is that the name of your architecture, or an instruction?  (That   
   may have been mentioned upthread, in that case I don't remember).   
      
   > there is not   
   > enough room in the instruction for a large branch displacement (10   
   > bits). So, my thought is to branch to a register value instead.   
   > There is already an add-to-instruction-pointer instruction that can be   
   > used to generate relative addresses.   
      
   That makes sense.   
      
   > By moving the register load outside of a loop, the dynamic instruction   
   > count can be reduced. I think this solution is a bit better than having   
   > compare and branch as two separate instructions, or having an extended   
   > constant added to the branch instruction.   
      
   Are you talking about a normal loop condition or a jump out of   
   a loop?   
      
   > One gotcha may be that the branch target needs to be predicted as it   
   > cannot be calculated earlier in the pipeline.   
      
   If you use a link register or a special instruction, the CPU could   
   do that.   
      
   > The 10-bit displacement format could also be supported, but it is yet   
   > another branch instruction format. I may leave holes in the instruction   
   > set for future support, but I think it is best to start with just a   
   > single format.   
   >   
   > Code:   
   > AIPSI R3,1234	; add displacement to IP and store in R3 (hoist-able)   
   > BLT R1,R2,R3		; branch to R3 if R1 < R2   
   >   
   > Versus:   
   > CMP R3,R1,R2   
   > BLT R3,displacement   
      
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