[PATCH] D96931: [LTO] Discard non-prevailing defined symbols in module-level assembly

[Peter Collingbourne via Phabricator via llvm-commits]

pcc added a comment.

In D96931#2612552 <https://reviews.llvm.org/D96931#2612552>, @ychen wrote:

> In D96931#2612521 <https://reviews.llvm.org/D96931#2612521>, @pcc wrote:
>
>> This is just solving part of the problem, no? Imagine that you have a weak definition and a global definition in two object files. Then you will have the same problem.
>
> I would think the global one is chosen by the linker?

I thought that the problem was with full LTO where the module inline asm blobs are simply concatenated. So if you have one file with

  .global foo
  foo:
  .byte 1

and another with

  .weak foo
  foo:
  .byte 2

then the assembler will see both `foo` labels and complain about multiple definitions.

>> I think that some kind of marker that we can use to mark blocks of asm as conceptually coming from different object files would be best.
>
> Encoded into metadata: the `map<inlineasm string offset, vector<nonprevail symbols>>`  Then in the MC layer, discard the non-prevail symbols.

I mean you would be able to say something like

  .new_module
  .global foo
  foo:
  .byte 1
  
  .new_module
  .weak foo
  foo:
  .byte 2

and that would be equivalent to

  .global foo
  foo:
  .byte 1
  .byte 2

Similarly if the second file did not provide a definition of `foo` the assembler would ignore the `.weak` directive.


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