Editing Threaded code (section)

===Subroutine threading===
So-called "subroutine-threaded code" (also "call-threaded code") consists of a series of machine-language "call" instructions (or addresses of functions to "call", as opposed to direct threading's use of "jump"). Early compilers for [[ALGOL]], Fortran, Cobol and some Forth systems often produced subroutine-threaded code. The code in many of these systems operated on a last-in-first-out (LIFO) stack of operands, for which compiler theory was well-developed. Most modern processors have special hardware support for subroutine "call" and "return" instructions, so the overhead of one extra machine instruction per dispatch is somewhat diminished.

Anton Ertl, the [[Gforth]] compiler's co-creator, stated that "in contrast to popular myths, subroutine threading is usually slower than direct threading".<ref>{{cite web| url=http://www.complang.tuwien.ac.at/forth/threaded-code.html#what| title=What is Threaded Code?| first=Anton| last=Ertl}}</ref> However, Ertl's most recent tests<ref name="tuwien1"/> show that subroutine threading is faster than direct threading in 15 out of 25 test cases. More specifically, he found that direct threading is the fastest threading model on Xeon, Opteron, and Athlon processors, indirect threading is fastest on Pentium M processors, and subroutine threading is fastest on Pentium 4, Pentium III, and PPC processors.

As an example of call threading for "push A, push B, add":

<syntaxhighlight lang="c">
thread:
  call pushA
  call pushB
  call add
  ret
pushA:
  *sp++ = A
  ret
pushB:
  *sp++ = B
  ret
add:
  addend1 = *--sp
  addend2 = *--sp
  *sp++ = addend1 + addend2
  ret
</syntaxhighlight>