[llvm-dev] distinct DISubprograms hindering sharing inlined subprogram descriptions

[Mehdi Amini via llvm-dev]

> On Dec 15, 2016, at 10:54 AM, David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Branching off from a discussion of improvements to DIGlobalVariable representations that Adrian's working on - got me thinking about related changes that have already been made to DISubprogram.
> 
> To reduce duplicate debug info when things like linkonce_odr functions were deduplicated in LTO linking, the relationship between a CU and DISubprogram was inverted (instead of a CU maintaining a list of subprograms, subprograms specify which CU they come from - and the llvm::Function references the DISubprogram, so if the llvm::Function goes away, so does the associated DISubprogram)
> 
> I'm not sure if this caused a regression, but at least seems to miss a possible improvement:
> 
> During IR linking (for LTO, ThinLTO, etc) these distinct DISubprogram definitions (& their CU link, even if they weren't marked 'distinct', the CU link would cause them to effectively be so) remain separate - this means that inlined versions in one CU don't refer to an existing subprogram definition in another CU.
> 
> To demonstrate:
> inl.h:
> void f1();
> inline __attribute__((always_inline)) void f2() {
>   f1();
> }
> inl1.cpp:
> #include "inl.h"
> void c1() {
>   f2();
> }
> inl2.cpp:
> #include "inl.h"
> void c2() {
>   f2();
> }
> 
> Compile to IR, llvm-link the result. The DWARF you get is basically the same as the DWARF you'd get without linking:
> 
> DW_TAG_compile_unit
>   DW_AT_name "inl1.cpp"
>   DW_TAG_subprogram #0
>     DW_AT_name "f2"
>   DW_TAG_subprogram
>     DW_AT_name "c1"
>     DW_TAG_inlined_subroutine
>       DW_TAG_abstract_origin #0 "f2"
> DW_TAG_compile_unit
>   DW_AT_name "inl2.cpp"
>   DW_TAG_subprogram #1
>     DW_AT_name "f2"
>   DW_TAG_subprogram
>     DW_AT_name "c2"
>     DW_TAG_inlined_subroutine
>       DW_TAG_abstract_origin #1 "f2"
> 
> Instead of something more like this:
> 
> DW_TAG_compile_unit
>   DW_AT_name "inl1.cpp"
>   DW_TAG_subprogram #0
>     DW_AT_name "f2"
>   DW_TAG_subprogram
>     DW_AT_name "c1"
>     DW_TAG_inlined_subroutine
>       DW_TAG_abstract_origin #0 "f2"
> DW_TAG_compile_unit
>   DW_AT_name "inl2.cpp"
>   DW_TAG_subprogram
>     DW_AT_name "c2"
>     DW_TAG_inlined_subroutine
>       DW_TAG_abstract_origin #0 "f2"
> 
> (note that only one abstract definition of f2 is produced here)
> 
> Any thoughts? I imagine this is probably worth a reasonable amount of savings in an optimized build. Not huge, but not nothing. (probably not the top of anyone's list though, I realize)

I should see the IR metadata in both cases to really know how it worked before, but it seems that to be able to merge the two subprogram definitions when linking, we’d need to be able to have a list of CU per subprogram instead of a single one, right?

> Should we remove the CU link from a non-internal linkage subprogram (& this may have an effect on the GV representation issue originally being discussed) and just emit it into whichever CU happens to need it first?

Looks like this should work as well, but I don’t enough about the way we emit Dwarf...

— 
Mehdi