Update EIP-5450: Add JUMPF simplification (ethereum#6101)

* Update EIP-5450: Add JUMPF simplification change

* Apply stylistic suggestions and typo fixes from review

Co-authored-by: lightclient <[email protected]>

Co-authored-by: lightclient <[email protected]>

EIPS/eip-5450.md

@@ -50,6 +50,20 @@ Given new deploy-time guarantees, EVM implementation is not required anymore to
 
 Stack overflow check, on the other hand, is still required at run-time, because function execution can start at arbitrary (i.e. known only at run-time) stack height at `CALLF` instruction of a caller (i.e. each execution can be in arbitrary inner call frame). Verification algorithm examines only stack height changes relative to starting stack height of the function.
 
+#### JUMPF changes
+
+In case a function pushed more items to the stack than is required as inputs by the jumped-to function, the previously defined `JUMPF` instruction behaviour was to remove the extra items:
+
+> 3. If data stack has more than `caller_stack_height + type[code_section_index].inputs` items, discards the items between `caller_stack_height` and top `type[code_section_index].inputs`, so that there are exactly `caller_stack_height +  type[code_section_index].items` items left.
+
+Given the new deploy-time guarantee of no function underflowing its stack frame, `JUMPF` instruction can check only that there is enough items for input arguments of the callee on the stack, without making sure there is exactly `inputs` items and not more. 
+
+Therefore, the previously defined behavior of discarding extra items is removed, and only the check for enough inputs is done:
+
+> 2. If data stack has less than `caller_stack_height + type[code_section_index].inputs` items, execution results in exceptional halt.
+
+With this change `JUMPF` operation complexity does not depend on `ouputs` value and is constant-time, therefore the price of `JUMPF` is lowered to 3 gas.
+
 ## Rationale
 
 ### Stack overflow check only in CALLF