Subroutines

EIPS/eip-2315.md

@@ -17,7 +17,7 @@ This proposal introduces three opcodes to support subroutines:  `BEGINSUB`, `JUM
 
 The EVM does not provide subroutines as a primitive.  Instead, calls can be synthesized by fetching and pushing the current program counter on the data stack and jumping to the subroutine address; returns can be synthesized by contriving to get the return address back to the top of stack and jumping back to it.  Complex calling conventions are then needed to use the same stack for computation and control flow.  Code becomes harder to read and write, and tools may need to pattern-match the conventions to identify the use of subroutines.  Complex calling conventions like these can be avoided using memory, but regardless, it costs a lot of gas.
 
-Having opcodes to directly support subroutines can eliminate this complexity and cost, just as for other machines and interpreters going back at least 60 years.
+Having opcodes to directly support subroutines can eliminate this complexity and cost, just as for other physical and virtual machines going back at least 50 years.
 
 In the Appendix we show example solc output for a simple program that uses over three times as much gas just calling and returning from subroutines as comparable code using these opcodes.
 
@@ -49,7 +49,7 @@ offset step opcode    stack stack
 0      0    PUSH1 3   []    []
 1      1    JUMPSUB   [3]   [1]
 2      4    STOP      []    [1]
-3      2    JUMPDEST  []    [1]
+3      2    BEGINSUB  []    [1]
 4      3    RETURNSUB []    []
 ```
 The above code should terminate after 4 steps with an empty stack.
@@ -58,7 +58,7 @@ The above code should terminate after 4 steps with an empty stack.
 offset step opcode    stack stack
 0      0    PUSH1 2   []    []
 1      1    JUMPSUB   [2]   [1]
-2      2    JUMPDEST  []    [1]
+2      2    BEGINSUB  []    [1]
 3      3    RETURNSUB []    []
 ```
 The above code should terminate after 4 steps with an empty stack.
@@ -68,11 +68,11 @@ offset step opcode    stack stack
 0      0    PUSH1 3   []    []
 1      1    JUMPSUB   [3]   [1]
 2      8    STOP      []    []
-3      2    JUMPDEST  []    [1]
+3      2    BEGINSUB  []    [1]
 4      3    PUSH1 7   []    [1]
 5      4    JUMPSUB   [7]   [1,5]
 6      7    RETURNSUB []    [1]
-7      5    JUMPDEST  []    [1]
+7      5    BEGINSUB  []    [1]
 8      6    RETURNSUB []    [1]
 ```
 The above code should terminate after 8 steps with an empty stack.  The above code should terminate after 8 steps with an empty stack.
@@ -118,7 +118,7 @@ We suggest that the cost of `BEGINSUB` be _base_, `JUMPSUB` be _low_, and `RETUR
 ```
 ## Security Considerations
 
-Program flow analysis frameworks will need to be updated to allow for a new type of branch - `JUMPSUB` - and new type of branching - `RETURNSUB` - which will cause a jump to a destination which is a `JUMPSUB`, not a `JUMPDEST`.
+Program flow analysis frameworks will need to be updated to allow for a new type of branch - `JUMPSUB` - which will cause a jump to a destination which is a `BEGINSUB`, not a `JUMPDEST`.
 
 ## Appendix: Comparative costs.
 
@@ -138,28 +138,32 @@ contract fun {
 ```
 Here is solc 0.6.3 assembly code with labeled destinations.
 ```
-   TEST:
+TEST:
+     jumpdest
      0x00
      RTN
      0x02
      0x03
      TEST_MUL
      jump
-   TEST_MUL:
+TEST_MUL:
+     jumpdest
      0x00
      RTN
      dup4
      dup4
      MULTIPLY
      jump
-   RTN:
+RTN:
+     jumpdest
      swap4
      swap3
      pop
      pop
      pop
      jump
-   MULTIPLY:   
+MULTIPLY:   
+     jumpdest
      mul
      swap1
      jump
@@ -168,17 +172,20 @@ solc does a good job with the multiply() function, which is a leaf.  Non-leaf fu
 
   This is the same code written using `jumpsub` and `returnsub`.  Calling `fun.test()` will cost _34 gas_ (plus 5).
 ```
-   TEST:
+TEST:
+     beginsub
      0x02
      0x03
      TEST_MUL
      jumpsub
      returnsub
-   TEST_MUL:
+TEST_MUL:
+     beginsub
      MULTIPLY
      jumpsub
      returnsub
-   MULTIPLY:
+MULTIPLY:
+     beginsub
      mul
      returnsub
 ```