From: user5857@newsgrouper.org.invalid   
      
   Robert Finch  posted:   
      
   > Tradeoffs bypassing r0 causing more ISA tweaks.   
   >   
   > It is expensive to bypass r0. To truly bypass it, it needs to be   
   > bypassed in a couple of dozen places which really drives up the LUT   
   > count. Removing the bypassing of r0 from the register file shaved 1000   
   > LUTs off the design. This is no real loss as most instructions can   
   > substitute small constants for register values.   
      
   Often the use of R0 as an operand causes the calculation to be degenerate.   
   That is, R0 is not needed at all.   
        ADD   R9,R7,R0        // is a MOV instruction   
        AND   R9,R7,R0        // is a CLR instruction   
      
   So, you don't have to treat R0 in bypassing, but as Operand processing.   
      
   > Decided to go PowerPC style with bypassing of r0 to zero. R0 is bypassed   
   > to zero only in the agen units. So, the bypass is only in a couple of   
   > places. Otherwise r0 can be used as an ordinary register. Load / store   
   > instructions cannot use r0 as a GPR then, but it works for the PowerPC.   
      
   AGEN Rbase ==R0 implies Rbase  = IP   
   AGEN Rindex==R0 implies Rindex = 0   
      
   > I hit this trying to decide where to bypass another register code to   
   > represent the instruction pointer. In that case I think it may be better   
   > to go RISCV style and just add an instruction to add the IP to a   
   > constant and place it in a register. The alternative might be to   
   > sacrifice a bit of displacement to indicate IP relative addressing.   
   >   
   > Anyone got a summary of bypassing r0 in different architectures?   
      
   These are some of the reasons I went with   
   a) universal constants   
   b) R0 is just another GPR   
   So, R0, gets forwarded just as often (or lack thereof) as any joe-random   
   register.   
      
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