[PATCH] D131283: [BOLT] Disable -lite when split function is present

[Huan Nguyen via Phabricator via llvm-commits]

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In lite mode, BOLT only transforms a subset of functions, leave the
remaining functions intact.

For NoPIC, it is fine. BOLT can scan relocations and fix-up all refs
that point to any function body in the subset.

For no-split function PIC, it is fine. Since jump tables are intra-
procedural transfer, BOLT can find both the jump table base and the
target within same function. Thus, BOLT can update and/or move jump
tables.

However, it is wrong to process a subset of functions in split function
PIC. This is because BOLT does not know if functions in the subset are
isolated, i.e., cannot be accessed by functions out of the subset,
especially via split jump table.

For example, BOLT only process three functions A, B and C. Suppose that
A is reached via jump table from A.cold, which is not processed. When
A is moved (due to optimization), the jump table in A.cold is invalid.
We cannot fix-up this jump table since it is only recognized in A.cold,
which BOLT does not process.

Solution: Disable lite mode if split function is present.

Future improvement: In lite mode, if split function is found, BOLT
processes both functions in the subset and all of their sibling
fragments.

Test Plan:

  ninja check-bolt


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D131283

Files:
  bolt/lib/Rewrite/RewriteInstance.cpp


Index: bolt/lib/Rewrite/RewriteInstance.cpp
===================================================================
--- bolt/lib/Rewrite/RewriteInstance.cpp
+++ bolt/lib/Rewrite/RewriteInstance.cpp
@@ -1433,6 +1433,9 @@
                << *FragName << ". The support is limited in relocation mode.\n";
         PrintedWarning = true;
       }
+      errs() << "BOLT-WARNING: disable lite mode when split function is "
+             << "present.\n";
+      opts::Lite = false;
       Function.IsFragment = true;
     }
 


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