[ThinLTO] Ensure prevailing linkonce emitted as weak in ThinLTO backends

Summary:
Since IR files are all compiled into separate independent object files
in ThinLTO mode, the prevailing linkonce symbols must be emitted in its
object file even if it is no longer referenced there, e.g. if no
references remain in the module after inlining, since it may be
referenced by another ThinLTO compiled object file. This is done by
changing LDPR_PREVAILING_DEF_IRONLY* symbols to LDPR_PREVAILING_DEF,
which converts the prevailing linkonce to weak. We also don't need the
other prevailing IRONLY handling for internalization, which is not
currently performed for ThinLTO.

Test case included.

Reviewers: davidxl, rafael

Subscribers: rafael, joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D16173

llvm-svn: 262727

llvm/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
+}

llvm/test/tools/gold/X86/thinlto_linkonceresolution.ll

@@ -0,0 +1,32 @@
+; 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 ensures that for ThinLTO the prevailing copy of a
+; linkonce symbol is changed to weak to ensure it is not eliminated.
+; 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:     --plugin-opt=save-temps \
+; RUN:     -o %t3.o %t2.o %t.o
+; RUN: llvm-nm %t3.o | FileCheck %s
+; RUN: llvm-dis %t.o.opt.bc -o - | FileCheck --check-prefix=OPT %s
+; RUN: llvm-dis %t2.o.opt.bc -o - | FileCheck --check-prefix=OPT2 %s
+
+; Ensure that f() is defined in resulting object file, and also
+; confirm the weak linkage directly in the saved opt bitcode files.
+; CHECK-NOT: U f
+; OPT: declare extern_weak hidden void @f()
+; OPT2: define weak_odr hidden void @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
+}

llvm/tools/gold/gold-plugin.cpp

@@ -709,6 +709,16 @@ getModuleForFile(LLVMContext &Context, claimed_file &F, const void *View,
     if (Resolution == LDPR_PREVAILING_DEF_IRONLY_EXP && !Res.IsLinkonceOdr)
       Resolution = LDPR_PREVAILING_DEF;
 
+    // In ThinLTO mode change all prevailing resolutions to LDPR_PREVAILING_DEF.
+    // For ThinLTO the IR files are compiled through the backend independently,
+    // so we need to ensure that any prevailing linkonce copy will be emitted
+    // into the object file by making it weak. Additionally, we can skip the
+    // IRONLY handling for internalization, which isn't performed in ThinLTO
+    // mode currently anyway.
+    if (options::thinlto && (Resolution == LDPR_PREVAILING_DEF_IRONLY_EXP ||
+                             Resolution == LDPR_PREVAILING_DEF_IRONLY))
+      Resolution = LDPR_PREVAILING_DEF;
+
     GV->setUnnamedAddr(Res.UnnamedAddr);
     GV->setVisibility(Res.Visibility);