[LLVMdev] what's correct behavior for struct forward declarations?
Chris Lattner
clattner at apple.com
Wed Feb 18 20:30:31 PST 2009
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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