Is "for(;;)" idiom for infinite loop correctly attributed to PDP-11 C compiler?

Question

Recently I found this article that claims that the idea to prefer for(;;) over while(1) for infinite loop came because the C compiler originally available on PDP-11 generated an extra machine instruction for while(1).

Btw now even Visual C++ warnings tend to favor the former.

How realistic is such attribution of for(;;) idiom?

Answer 1

Here's what the V7 Unix compiler cc produces (using SIMH and an image from TUHS):

$ cat>a.c
main(){
 while(1);
}
$ cat>b.c
main(){
 for(;;);
}
$ cc -S a.c
$ cc -S b.c

a.c (while) compiles to:

.globl  _main
.text
_main:
~~main:
jsr     r5,csv
jbr     L1
L2:L4:tst       $1
jeq     L5
jbr     L4
L5:L3:jmp       cret
L1:jbr  L2
.globl
.data

While b.c (for) becomes:

.globl  _main
.text
_main:
~~main:
jsr     r5,csv
jbr     L1
L2:L4:jbr       L4
L5:L3:jmp       cret
L1:jbr  L2
.globl
.data

So it's at least true that for(;;) compiled to fewer instructions when not using optimization. However, when compiling with -O, both programs produce exactly the same assembly:

.globl  _main
.text
_main:
~~main:
jsr     r5,csv
L4:jbr  L4
.globl
.data

and when I add a loop body of printf("Hello");, the programs are still the same.

So, it might be that the idiom has its origins in PDP-11 machine language, but by 1979 the difference was already largely irrelevant.