compiler bug : string parameter misbehaves within for loop

[gopherbot]

by kaushansky:

The following program doesn't compile, complains about unknown type 

func iterateFast(s string) int { 
  acc:=0; 
  for i:=0;i<10;i++ { 
    acc+=len(s); 
  } 
  return acc;  // fatal error: unknown etype 0/STRING 
} 

However, this variant compiles with no errors:

func iterateFast(s string) int { 
  s1:=s; 
  acc:=0; 
  for i:=0;i<10;i++ { 
    acc+=len(s1); 
  } 
  return acc;  // no problem
} 

I tried to localize the bug by removing for loop, but the following compiles just fine:
func iterateFast(s string) int { 
  acc:=0; 
  
  acc+=len(s); 
   
  return acc;  // no problem
}

[gopherbot]

Comment 1 by pshah.foss:

src/cmd/8g/reg.c line 1047

[gopherbot]

Comment 2 by [email protected]:

How is iterateFast being called? 
The zero value for string is supposed to be the empty string. But this looks like a real
0. Is there a pointer 
dereference involved? or a strange type cast?

[gopherbot]

Comment 3 by kaushansky:

But this is compiler error, not runtime error. It shows up even if no
one calls the function.
To eliminate a chance that inlining gets somehow involved, I compiled
this function by itself,  with the same result. Do you have problem reproducing it?

[robpike]

Comment 4:

Labels changed: added compilerbug.

Owner changed to [email protected].

Status changed to Accepted.

[rsc]

Comment 5:

This was fixed by 683ed10f78 (8g) and d9cbc65f0f (6g).

Status changed to Fixed.