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

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 strict

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.