From ce3c5dae06566eb3ae31e44a459cc8c9cb9e060d Mon Sep 17 00:00:00 2001
From: Fangrui Song <i@maskray.me>
Date: Thu, 22 Oct 2020 15:26:52 -0700
Subject: [PATCH] [ELF] --warn-backrefs: save the referenced InputFile *

For a diagnostic `A refers to B` where B refers to a bitcode file, if the
symbol gets optimized out, the user may see `A refers to <internal>`; if the
symbol is retained, the user may see `A refers to lto.tmp`.

Save the reference InputFile * in the DenseMap so that the original filename is
available in reportBackrefs().
---
 lld/ELF/Symbols.cpp               | 11 +++++++----
 lld/ELF/Symbols.h                 |  4 +++-
 lld/test/ELF/lto/warn-backrefs.ll | 30 ++++++++++++++++++++++++++++++
 3 files changed, 40 insertions(+), 5 deletions(-)
 create mode 100644 lld/test/ELF/lto/warn-backrefs.ll

diff --git a/lld/ELF/Symbols.cpp b/lld/ELF/Symbols.cpp
index 8f2f55418df5..eed0251c82e3 100644
--- a/lld/ELF/Symbols.cpp
+++ b/lld/ELF/Symbols.cpp
@@ -64,7 +64,8 @@ Defined *ElfSym::relaIpltStart;
 Defined *ElfSym::relaIpltEnd;
 Defined *ElfSym::riscvGlobalPointer;
 Defined *ElfSym::tlsModuleBase;
-DenseMap<const Symbol *, const InputFile *> elf::backwardReferences;
+DenseMap<const Symbol *, std::pair<const InputFile *, const InputFile *>>
+    elf::backwardReferences;
 
 static uint64_t getSymVA(const Symbol &sym, int64_t &addend) {
   switch (sym.kind()) {
@@ -377,7 +378,8 @@ void elf::reportBackrefs() {
   for (auto &it : backwardReferences) {
     const Symbol &sym = *it.first;
     warn("backward reference detected: " + sym.getName() + " in " +
-         toString(it.second) + " refers to " + toString(sym.file));
+         toString(it.second.first) + " refers to " +
+         toString(it.second.second));
   }
 }
 
@@ -532,9 +534,10 @@ void Symbol::resolveUndefined(const Undefined &other) {
     // A traditional linker does not error for -ldef1 -lref -ldef2 (linking
     // sandwich), where def2 may or may not be the same as def1. We don't want
     // to warn for this case, so dismiss the warning if we see a subsequent lazy
-    // definition.
+    // definition. this->file needs to be saved because in the case of LTO it
+    // may be reset to nullptr or be replaced with a file named lto.tmp.
     if (backref && !isWeak())
-      backwardReferences.try_emplace(this, other.file);
+      backwardReferences.try_emplace(this, std::make_pair(other.file, file));
     return;
   }
 
diff --git a/lld/ELF/Symbols.h b/lld/ELF/Symbols.h
index b69d263153d2..a8d056a32f98 100644
--- a/lld/ELF/Symbols.h
+++ b/lld/ELF/Symbols.h
@@ -567,7 +567,9 @@ void reportBackrefs();
 
 // A mapping from a symbol to an InputFile referencing it backward. Used by
 // --warn-backrefs.
-extern llvm::DenseMap<const Symbol *, const InputFile *> backwardReferences;
+extern llvm::DenseMap<const Symbol *,
+                      std::pair<const InputFile *, const InputFile *>>
+    backwardReferences;
 
 } // namespace elf
 } // namespace lld
diff --git a/lld/test/ELF/lto/warn-backrefs.ll b/lld/test/ELF/lto/warn-backrefs.ll
new file mode 100644
index 000000000000..0de73673c0e5
--- /dev/null
+++ b/lld/test/ELF/lto/warn-backrefs.ll
@@ -0,0 +1,30 @@
+; REQUIRES: x86
+;; Test that the referenced filename is correct (not <internal> or lto.tmp).
+
+; RUN: split-file %s %t
+; RUN: llvm-as %t/a.ll -o %ta.o
+; RUN: llvm-as %t/b.ll -o %tb.o
+; RUN: ld.lld --warn-backrefs --start-lib %tb.o --end-lib %ta.o 2>&1 | FileCheck %s
+
+; CHECK: warning: backward reference detected: f in {{.*}}a.o refers to {{.*}}b.o
+
+;--- a.ll
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @f()
+
+define void @_start() {
+entry:
+  call void () @f()
+  ret void
+}
+
+;--- b.ll
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @f() {
+entry:
+  ret void
+}