Re: [PATCH dwarves 0/3] add option to merge more dwarf cu's into

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Mar 24, 2021 at 11:53:16PM -0700, Yonghong Song escreveu:
> For vmlinux built with clang thin-lto or lto for latest bpf-next,
> there exist cross cu debuginfo type references. For example,
>       compile unit 1:
>          tag 10:  type A
>       compile unit 2:
>          ...
>            refer to type A (tag 10 in compile unit 1)
> I only checked a few but have seen type A may be a simple type
> like "unsigned char" or a complex type like an array of base types.
> I am using latest llvm trunk and bpf-next. I suspect llvm12 or
> linus tree >= 5.12 rc2 should be able to exhibit the issue as well.
> Both thin-lto and lto have the same issues.
> 
> Current pahole cannot handle this. It will report types cannot
> be found error. Bill Wendling has attempted to fix the issue
> with [1] by permitting all tags/types are hashed to the same
> hash table and then process cu's one by one. This does not
> really work. The reason is that each cu resolves types locally
> so for the above example we may have
>   compile unit 1:
>     type A : type_id = 10
>   compile unit 2:
>     refer to type A : type A will be resolved as type id = 10
> But id 10 refers to compile unit 1, we will get either out
> of bound type id or incorrect one.
> 
> This patch set is a continuation of Bill's work. We still
> increase the hashtable size and traverse all cu's before
> recoding and finalization. But instead of creating one-to-one
> mapping between debuginfo cu and pahole cu, we just create
> one pahole cu, which should solve the above incorrect type
> id issue.
> 
> Patches #1 and #2 are refactoring the existing code
> and Patch #3 added an option "merge_cus" to permit
> merging all debuginfo cu's into one pahole cu.
> For vmlinux built, it can be detected that if LTO or Thin-LTO
> is enabled, "merge_cus" can be added into pahole
> command line.
> 
>   [1] https://www.spinics.net/lists/dwarves/msg00999.html

Thanks for working on this, I'll look at it today.

- Arnaldo