__attribute__((cleanup)) on variable declaration in for loop initialization doesn't work correctly in c89/90

[ahatanak]

clang has an extension that allows variable declarations in initialization statements of for loops before C99. The variable doesn't go out of scope at the end of the for loop.

// This compiles without errors with `-std=c89`.
void foo() {
  for (int i = 1; 0;)
    ;
  printf("i=%d\n", i);
}

However, when the declared variable has a cleanup, the cleanup function is run at the end of the for loop rather than at the end of function foo.

$ cat test.c

#include <stdio.h>

void cleaner(int *a) { printf("run cleanup\n"); }

void foo() {
  for (__attribute__((cleanup(cleaner))) int i = 1; 0;)
    ;
  printf("i=%d\n", i);
}

int main() {
  foo();
  return 0;
}

$ clang test.c -std=c89
$ ./a.out
run cleanup
i=1

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Akira Hatanaka (ahatanak)

clang has an extension that allows variable declarations in initialization statements of for loops before C99. The variable doesn't go out of scope at the end of the for loop.

// This compiles without errors with `-std=c89`.
void foo() {
  for (int i = 1; 0;)
    ;
  printf("i=%d\n", i);
}

However, when the declared variable has a cleanup, the cleanup function is run at the end of the for loop rather than at the end of function `foo`.

$ cat test.c

#include <stdio.h>

void cleaner(int *a) { printf("run cleanup\n"); }

void foo() {
for (attribute((cleanup(cleaner))) int i = 1; 0;)
;
printf("i=%d\n", i);
}

int main() {
foo();
return 0;
}

$ clang test.c -std=c89
$ ./a.out
run cleanup
i=1
</details>

[pinskia]

Note GCC had a similar extension but only for C++ and only with -fno-for-scope but that support was removed for GCC 9. GCC never support for loop initializers for C90 mode either.

I also don't see init-statement in for documented as an extension backported either: https://clang.llvm.org/docs/LanguageExtensions.html .

[llvmbot]

@llvm/issue-subscribers-c

Author: Akira Hatanaka (ahatanak)

clang has an extension that allows variable declarations in initialization statements of for loops before C99. The variable doesn't go out of scope at the end of the for loop.

// This compiles without errors with `-std=c89`.
void foo() {
  for (int i = 1; 0;)
    ;
  printf("i=%d\n", i);
}

However, when the declared variable has a cleanup, the cleanup function is run at the end of the for loop rather than at the end of function foo.

$ cat test.c

#include <stdio.h>

void cleaner(int *a) { printf("run cleanup\n"); }

void foo() {
  for (__attribute__((cleanup(cleaner))) int i = 1; 0;)
    ;
  printf("i=%d\n", i);
}

int main() {
  foo();
  return 0;
}

$ clang test.c -std=c89
$ ./a.out
run cleanup
i=1

[AaronBallman]

Clang emits a GCC compat diagnostic there: warning: GCC does not allow variable declarations in for loop initializers before C99 so the divergence is expected; we should still update the documentation though, so thank you @pinskia!

So yeah, I think this is a Clang bug -- we should emit the call to cleanup at the end of the function not the end of the for loop.

CC @efriedma-quic for second opinions

[AaronBallman]

I put up #154744 to address the documentation issues.

[Sirraide]

So yeah, I think this is a Clang bug -- we should emit the call to cleanup at the end of the function not the end of the for loop.

It might have something to do w/ how we set up scopes when we parse loops; I recall seeing that we do some things differently in C89 mode specifically.

[AaronBallman]

I think we're setting the scope flags properly when parsing (lookup behavior is what I'd expect it to be). I think the issue is with handling the cleanup attribute itself. My guess is the issue is closer to CodeGenFunction::EmitAutoVarCleanups()