Commits on Apr 12, 2022

1. [Bitcode] materialize Functions early when BlockAddress taken …

   IRLinker builds a work list of functions to materialize, then moves them
   from a source module to a destination module one at a time.
   
   This is a problem for blockaddress Constants, since they need not refer
   to the function they are used in; IPSCCP is quite good at sinking these
   constants deep into other functions when passed as arguments.
   
   This would lead to curious errors during LTO:
     ld.lld: error: Never resolved function from blockaddress ...
   based on the ordering of function definitions in IR.
   
   The problem was that IRLinker would basically do:
   
     for function f in worklist:
       materialize f
       splice f from source module to destination module
   
   in one pass, with Functions being lazily added to the running worklist.
   This confuses BitcodeReader, which cannot disambiguate whether a
   blockaddress is referring to a function which has not yet been parsed
   ("materialized") or is simply empty because its body was spliced out.
   This causes BitcodeReader to insert Functions into its BasicBlockFwdRefs
   list incorrectly, as it will never re-materialize an already
   materialized (but spliced out) function.
   
   Because of the possibility that blockaddress Constants may appear in
   Functions other than the ones they reference, this patch adds a new
   bitcode function code FUNC_CODE_BLOCKADDR_USERS that is a simple list of
   Functions that contain BlockAddress Constants that refer back to this
   Function, rather then the Function they are scoped in. We then
   materialize those functions when materializing `f` from the example loop
   above. This might over-materialize Functions should the user of
   BitcodeReader ultimately decide not to link those Functions, but we can
   at least now we can avoid this ordering related issue with blockaddresses.
   
   Fixes: llvm#52787
   Fixes: ClangBuiltLinux/linux#1215
   
   Reviewed By: dexonsmith
   
   Differential Revision: https://reviews.llvm.org/D120781
   
   (cherry picked from commit 23ec578)

   

   nickdesaulniers authored and llvmbot committed Apr 12, 2022
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