[LLVMdev] debug metadata incomplete for array arguments to functions?

[Devang Patel]

On Jul 13, 2011, at 9:56 PM, Eli Bendersky wrote:

> Hello,
> 
> Consider the following two functions:
> 
> void foo(int* arg_ptr) {
> ...
> }
> 
> void bar(int arg_arr[42]) {
> ...
> }
> 
> ------
> 
> According to the C standard, both arguments will be passed to the function as pointers. However, in the debug information metadata generated in LLVM, it appears that they are also equivalent.
> 
> More specifically, for both arg_ptr and arg_arr I get a derived type descriptor with the DW_TAG_pointer_type tag, referencing 'int' as the type derived from, in argument 8 of the relevant MDNode. There is no way to know arg_arr is in the user's eyes an array.
> 
> This reflects the compiler's view of things correctly, but is problematic for a debugger. The debugger should know that arg_arr refers to a 42-element array and isn't just a pointer into a buffer of unspecified length. This is something the user would expect.
> On the other hand, the debugger should also get the information that arg_arr is actually a pointer, to be able to get to its values correctly. 
> 
> Is there a way out?

The only way out is to figure out how to distinguish between these two in front-end AST nodes while emitting LLVM IR. The part of front end that emits debug info for an argument is seeing arg_arr as a pointer to int. 

If you manually patch metadata in llvm IR then you'll get debug info as you expect.
-
Devang