[LLVMdev] Question about appending linkage

Talin viridia at gmail.com
Wed Apr 23 19:25:51 PDT 2008

My apologies for opening up an old thread, but I've been thinking about 
this quite a bit lately, and it seems to me that "appending linkage" 
would be a lot more useful in general if there was some way to determine 
the size of the appended array without having to write a custom pass to 
do it. In other words, using the standard llvm tools (llvm-link and 
friends), one ought to be able to determine the starting and ending 
address of the array after it has been concatenated together.

While it is true that I can solve the problem for my own special case 
using the steps outlined below, it seems to me that anyone using 
appending linkage for anything is going to run up against a similar 
problem - how to know where the end of the array is. Thus it makes sense 
to me to try and solve the general case instead of just my own 
particular need.

Unfortunately I can't think of any reasonable way to do it that doesn't 
involve polluting the IR language with a lot of special cases. Here are 
some ideas that I thought of and discarded:

*  Have a special "PostAppendingLinkage" type which acts just like 
"AppendingLinkage", except that it is guaranteed to come after all 
"AppendingLinkage" sections have been appended. Thus, you could use this 
to nail a sentinel value at the end of the list. Rejected because the 
linker has no control over the order in which modules are processed; It 
would have to keep the two linkage types separate until the last 
possible step.

*  Have some way to define a global symbol representing the ending 
address, rather than the starting address, of a declaration. Sort of 
like GetElementPtr(1), except evaluated at link time. Still, it feels 
ugly and hard to implement.

-- Talin

John Criswell wrote:
> Talin wrote:
>> I'm trying to figure out how to do static initialization (like static
>> constructors in C++ or Java). I figured I would use appending linkage -
>> that is, for each module I'd generate a function that did all of the
>> static initialization for that module, and then I'd put a pointer to
>> that function in an array with appending linkage. Then my
>> compiler-generated startup code would simply iterate through the array
>> and call each function in turn, before calling my "main" method.
>> Only problem is - how do I know when I've reached the end of the array?
> There are several ways to do this:
> 1) Link in a bytecode file at the end that has an array that ends with a 
> NULL function pointer.  The last time I checked, appending arrays 
> appended arrays in link order, so the NULL will be at the end.  LLVM 
> does not define this behavior, so it may disappear in future releases.
> 2) Write a transform that either puts a NULL function pointer at the end 
> or creates a global variable with the array size.
> 3) Put the array into its own section.  Write a GNU ld linker script 
> that defines symbols at the beginning and end of the section.  You then 
> write your code so that it loops until it hits the variable at the end 
> of the section (the Linux 2.4 linker does this).
> My personal preference would be option 2.  The transform is trivial to 
> write and does not rely on undefined behavior.
> -- John T.
>> -- Talin
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