From 4fbd8ea38e4463e4184c9552f47da0c32d38434d Mon Sep 17 00:00:00 2001
From: Teresa Johnson <tejohnson@google.com>
Date: Fri, 4 Mar 2016 17:48:35 +0000
Subject: [PATCH] [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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262727 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../X86/Inputs/thinlto_linkonceresolution.ll  |  4 +++
 .../gold/X86/thinlto_linkonceresolution.ll    | 32 +++++++++++++++++++
 tools/gold/gold-plugin.cpp                    | 10 ++++++
 3 files changed, 46 insertions(+)
 create mode 100644 test/tools/gold/X86/Inputs/thinlto_linkonceresolution.ll
 create mode 100644 test/tools/gold/X86/thinlto_linkonceresolution.ll

diff --git a/test/tools/gold/X86/Inputs/thinlto_linkonceresolution.ll b/test/tools/gold/X86/Inputs/thinlto_linkonceresolution.ll
new file mode 100644
index 000000000000..2f7153268aca
--- /dev/null
+++ b/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
+}
diff --git a/test/tools/gold/X86/thinlto_linkonceresolution.ll b/test/tools/gold/X86/thinlto_linkonceresolution.ll
new file mode 100644
index 000000000000..044087b98090
--- /dev/null
+++ b/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
+}
diff --git a/tools/gold/gold-plugin.cpp b/tools/gold/gold-plugin.cpp
index 83640bac6c1f..ae5c41a173b5 100644
--- a/tools/gold/gold-plugin.cpp
+++ b/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);