Reland r327041: [ThinLTO] Keep available_externally symbols live

Summary:
This change fixes PR36483. The bug was originally introduced by a change
that marked non-prevailing symbols dead. This broke LowerTypeTests
handling of available_externally functions, which are non-prevailing.
LowerTypeTests uses liveness information to avoid emitting thunks for
unused functions.

Marking available_externally functions dead is incorrect, the functions
are used though the function definitions are not. This change keeps them
live, and lets the EliminateAvailableExternally/GlobalDCE passes remove
them later instead.

(Reland with a suspected fix for a unit test failure I haven't been able
to reproduce locally)

Reviewers: pcc, tejohnson

Reviewed By: tejohnson

Subscribers: grimar, mehdi_amini, inglorion, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D43690

llvm-svn: 327360

1 files changed, 20 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index f759474e40f..d021b4a59a8 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -610,9 +610,26 @@ void llvm::computeDeadSymbols(
       if (S->isLive())
         return;
 
-    // We do not keep live symbols that are known to be non-prevailing.
-    if (isPrevailing(VI.getGUID()) == PrevailingType::No)
-      return;
+    // We only keep live symbols that are known to be non-prevailing if any are
+    // available_externally. Those symbols are discarded later in the
+    // EliminateAvailableExternally pass and setting them to not-live breaks
+    // downstreams users of liveness information (PR36483).
+    if (isPrevailing(VI.getGUID()) == PrevailingType::No) {
+      bool AvailableExternally = false;
+      bool Interposable = false;
+      for (auto &S : VI.getSummaryList()) {
+        if (S->linkage() == GlobalValue::AvailableExternallyLinkage)
+          AvailableExternally = true;
+        else if (GlobalValue::isInterposableLinkage(S->linkage()))
+          Interposable = true;
+      }
+
+      if (!AvailableExternally)
+        return;
+
+      if (Interposable)
+        report_fatal_error("Interposable and available_externally symbol");
+    }
 
     for (auto &S : VI.getSummaryList())
       S->setLive(true);