[LLVMdev] Variable declarations vs. definitions

Dustin Laurence dllaurence at dslextreme.com
Sat Jan 9 21:31:12 PST 2010

On 01/09/2010 01:12 PM, Chris Lattner wrote:

> The equivalent of "extern int G;" is:
> @G = external global i32

Hmm.  Is it really?  This

@foo = external global i32

@foo = global i32 5

define i32
@main(i32 %argc, i8 **%argv)
    %fooVal = load i32* @foo

    ret i32 %fooVal

produces a "redefinition of global '@foo'" error.  But this

extern int x;

int x = 5;

int *y = &x;

compiles to

@x = global i32 5		; <i32*> [#uses=1]
@y = global i32* @x		; <i32**> [#uses=0]

The difference is crucial, because I want to put

"@foo = external global i32"

in a header file that is then #included in every module where it is
used, *including the defining module* (for a consistency check, and
because otherwise I'd have to create extra headers that are only
#included by the outside world but not the defining module).  It appears
that the front end is supposed to decide between external and global.
In my case I actually could maintain all declarations by hand for one
global word used to return exception information, but that wouldn't work
for a more involved case.

Is there no way to get the same effect as with define/declare for
functions?  There I have no problem #includeing a declaration into the
same file as a definition.

The alternative appears to be asking the linker to do it.  The docs for
"linkonce" say "This is typically used to implement inline functions,
templates, or other code which must be generated in each translation
unit that uses it."  That sounds like my case, and it compiles, but I
don't know if that's gong to get me into trouble.


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