[PATCH] D16173: [ThinLTO] Do not mark linkonce symbols as preempted in ThinLTO backends

[Teresa Johnson via llvm-commits]

tejohnson created this revision.
tejohnson added reviewers: rafael, davidxl.
tejohnson added a subscriber: llvm-commits.
Herald added a subscriber: joker.eph.

Since IR files are all compiled into separate independent object files
in ThinLTO mode, linkonce symbols must not be treated as preempted even
if gold has marked them LDPR_PREEMPTED_IR. There is no guarantee that
they will be emitted in the object file with the copy gold marked as
prevailing, e.g. if no references remain after inlining. For ThinLTO
linkonce symbols mark preempted symbols as LDPR_PREVAILING_DEF_IRONLY.

Test case included.

http://reviews.llvm.org/D16173

Files:
  test/tools/gold/X86/Inputs/thinlto_linkonceresolution.ll
  test/tools/gold/X86/thinlto_linkonceresolution.ll
  tools/gold/gold-plugin.cpp

Index: tools/gold/gold-plugin.cpp
===================================================================
--- tools/gold/gold-plugin.cpp
+++ tools/gold/gold-plugin.cpp
@@ -712,6 +712,14 @@
     if (Resolution == LDPR_PREVAILING_DEF_IRONLY_EXP && !Res.IsLinkonceOdr)
       Resolution = LDPR_PREVAILING_DEF;
 
+    // For ThinLTO the IR files are compiled through the backend independently,
+    // so gold's preemption decisions must be ignored for linkonce symbols.
+    // There is no guarantee that the symbol will be emitted into the object
+    // file with the prevailing copy.
+    if (options::thinlto && Res.IsLinkonceOdr &&
+        Resolution == LDPR_PREEMPTED_IR)
+      Resolution = LDPR_PREVAILING_DEF_IRONLY;
+
     GV->setUnnamedAddr(Res.UnnamedAddr);
     GV->setVisibility(Res.Visibility);
 
Index: test/tools/gold/X86/thinlto_linkonceresolution.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/thinlto_linkonceresolution.ll
@@ -0,0 +1,27 @@
+; RUN: llvm-as -function-summary %s -o %t.o
+; RUN: llvm-as -function-summary %p/Inputs/thinlto_linkonceresolution.ll -o %t2.o
+
+; Ensure the plugin changes the symbol resolution for linkonce_odr
+; f() to LDPR_PREVAILING_DEF_IRONLY instead of LDPR_PREEMPTED_IR
+; for ThinLTO, since it isn't a full LTO link and the unreferenced
+; f() will be deleted in the other module. Note that gold picks the
+; first copy of f() as the prevailing one, so listing %t2.o first
+; is sufficient to ensure that this copy is preempted. Also,
+; set the import-instr-limit to 0 to prevent f() from being imported
+; from %t2.o which hides the problem.
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:     --plugin-opt=thinlto \
+; RUN:     --plugin-opt=-import-instr-limit=0 \
+; RUN:     -o %t3.o %t2.o %t.o
+; RUN: llvm-nm %t3.o | FileCheck %s
+
+; CHECK-NOT: U f
+
+target triple = "x86_64-unknown-linux-gnu"
+define i32 @g() {
+  call void @f()
+  ret i32 0
+}
+define linkonce_odr hidden void @f() {
+  ret void
+}
Index: test/tools/gold/X86/Inputs/thinlto_linkonceresolution.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/Inputs/thinlto_linkonceresolution.ll
@@ -0,0 +1,4 @@
+target triple = "x86_64-unknown-linux-gnu"
+define linkonce_odr hidden void @f() {
+  ret void
+}


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