llvm dev - Feb 2009 - [LLVMdev] what's correct behavior for struct forward declarations?

hi all,

i'm trying to use LLVM to compile some linux kernel code, and i noticed a
mismatch with gcc.  here is a simplified test case:

struct foo {
  int a;
  int b;
  int c;
};

static struct foo x; // 'forward' declaration?

int bar() {
  printf("a: %d, b: %d, c: %d\n", x.a, x.b, x.c);
}

static struct foo x = {
  .a = 1, .b = 2, .c = 3,
};

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


when this code is compiled with gcc and run, stdout prints "a: 1, b: 2, c:
3", which means that it takes the true declaration of x, initialized to 1,
2, 3.  however, when it's compiled with llvm, llvm emits the following code
for x:

@x = internal global %struct.foo zeroinitializer    ; <%struct.foo*>
[#uses=3]

which seems to me like it's taking the first declaration of x, which is a
forward declaration.  is that the correct behavior?  i believe that the
kernel developers intended for the second (real declaration) of x to be
visible, even in bar(), but that's not what's happening with llvm.  is
there
an easy workaround where i can get llvm to emit code initializing x to
{1,2,3}?  thanks!

Philip
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On Feb 18, 2009, at 8:20 PM, Philip Guo wrote:
> hi all,
>
> i'm trying to use LLVM to compile some linux kernel code, and i  
> noticed a mismatch with gcc.  here is a simplified test case:
This definitely looks like a bug, but I can't reproduce it with  
mainline.  Are you sure this is not fixed with SVN?

-Chris
>
>
> struct foo {
>   int a;
>   int b;
>   int c;
> };
>
> static struct foo x; // 'forward' declaration?
>
> int bar() {
>   printf("a: %d, b: %d, c: %d\n", x.a, x.b, x.c);
> }
>
> static struct foo x = {
>   .a = 1, .b = 2, .c = 3,
> };
>
> int main() {
>   bar();
>   return 0;
> }
>
>
> when this code is compiled with gcc and run, stdout prints "a: 1, b:  
> 2, c: 3", which means that it takes the true declaration of x,  
> initialized to 1, 2, 3.  however, when it's compiled with llvm, llvm  
> emits the following code for x:
>
> @x = internal global %struct.foo zeroinitializer    ; <%struct.foo*>
> [#uses=3]
>
> which seems to me like it's taking the first declaration of x, which  
> is a forward declaration.  is that the correct behavior?  i believe  
> that the kernel developers intended for the second (real  
> declaration) of x to be visible, even in bar(), but that's not  
> what's happening with llvm.  is there an easy workaround where i can  
> get llvm to emit code initializing x to {1,2,3}?  thanks!
>
> Philip
>
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