[clang] [llvm] Add option to generate additional debug info for expression dereferencing pointer to pointers. (PR #81545)

[David Blaikie via cfe-commits]

dwblaikie wrote:

> > Reading LLVM IR lit CHECK lines from clang codegen is a bit difficult - could you include some simple examples (perhaps from the new clang tests in this patch) showing the DWARF output just as comments in this review for something more easily glanceable?
> 
> Attached is the output of the following command
> 
> `clang ~/llvm-project/clang/test/CodeGenCXX/debug-info-ptr-to-ptr.cpp -fdebug-info-for-pointer-type -g2 -S -O3 -o /tmp/debug-info-ptr-to-ptr.txt`
> 
> [debug-info-ptr-to-ptr.txt](https://github.com/llvm/llvm-project/files/14659111/debug-info-ptr-to-ptr.txt)

Thanks - OK, so this only applies to intermediate structures and arrays (is it useful for arrays? You can't really reorder them - learning that the hot part of an array is the 5th-10th element might be of limited (or at least sufficiently different from the struct layout stuff) value - and it's a more dynamic property/has a runtime parameter component that might be harder to use?)

Do you have size impact numbers for this? I wouldn't put this under a (possibly cc1) flag for now unless we've got some pretty compelling data that this doesn't substantially change debug info size, which would be a bit surprising to me - I'd assume this would be quite expensive, but it's just a guess.

https://github.com/llvm/llvm-project/pull/81545