Necessity of J vs. JAL (and JR vs. JALR) in MIPS a

Question

I signed up because I've been googling forever for an answer to this question and can't find one.

I'd like to know if the jump instructions WITHOUT linking are strictly necessary in MIPS?

I can imagine for example that using "AL" versions when not required would incur some power penalty, but is there any situation (that's not completely contrived or could be coded around relatively simply) where ONLY J/JR would work?

Thank you!

Answer 1

Formalizing the comments into an answer

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J/JR can be emulated with JAL/JALR as the latter performs a super-set of the operations of the former.

As @Jester pointed out, routines (functions in C jargon) must be careful to preserve their return address present in $ra.
Unless the routine is a leaf routine (one that doesn't do any call) $ra must be saved somewhere anyway.

Actually both JAL/JALR and J/JR can be implemented one in terms of the other:

• Emulate JAL/JALR with J/JR

  Original               Emulated
  
  jal foo                la $ra, ret_label
                         j foo
                        ret_label:
  

• Emulate J/JR with JAL/JALR

  Original                Emulated
  
  j foo                   prolog:
                            addi $sp, $sp, -4
                            sw $ra, ($sp)   
  
                            jal foo
  
                          epilog:
                            lw $ra, ($sp)
                            addi $sp, $sp, 4
  

  For this to work, the code must return to epilog. It is assumed that $ra is mostly preserved in routines (hence the names of the labels). Many thanks to @EOF for point out a mistake in this snippet.

As @Peter pointed out, the access to the $pc leads to an easier (for humans) emulation of JAL/JALR.

As @EOF pointed out, some RISC machine actually have only one instruction for JAL/JALR and J/JR given their innate entanglement.

Considering that jumps and call happen very often in a typical program, being able to implement easily (and executing them fast) is mandatory for any successful ISA.