From 6660cec568504df47d9becb0c552c20577880df8 Mon Sep 17 00:00:00 2001
From: Petr Hosek <phosek@google.com>
Date: Tue, 3 Aug 2021 10:39:24 -0700
Subject: [PATCH] [InstrProfiling] Emit bias variable eagerly

Rather than emitting the bias variable lazily as needed, emit it
eagerly. This allows profile runtime to refer to this variable
unconditionally without having to use the weak reference. The bias
variable is in a COMDAT so there'll never be more than one instance,
and if it's not needed, linker should be able to GC it, so the overhead
should be minimal.

Differential Revision: https://reviews.llvm.org/D107377
---
 .../Transforms/Instrumentation/InstrProfiling.h    |  3 ++
 .../Transforms/Instrumentation/InstrProfiling.cpp  | 43 ++++++++++++++--------
 .../Instrumentation/InstrProfiling/bias-var.ll     |  6 +++
 3 files changed, 36 insertions(+), 16 deletions(-)
 create mode 100644 llvm/test/Instrumentation/InstrProfiling/bias-var.ll

diff --git a/llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h b/llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
index 94b156f..b1764b6 100644
--- a/llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
+++ b/llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
@@ -121,6 +121,9 @@ private:
   /// Create a static initializer for our data, on platforms that need it,
   /// and for any profile output file that was specified.
   void emitInitialization();
+
+  // Emit the variable used for runtime counter relocation.
+  bool emitBiasVar();
 };
 
 } // end namespace llvm
diff --git a/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp b/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
index 6ec7fde..39f863f 100644
--- a/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
+++ b/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
@@ -551,6 +551,10 @@ bool InstrProfiling::run(
   // Emit the runtime hook even if no counters are present.
   bool MadeChange = emitRuntimeHook();
 
+  // Emit the bias variable in each module when counter relocation is enabled.
+  if (isRuntimeCounterRelocationEnabled())
+    MadeChange |= emitBiasVar();
+
   // Improve compile time by avoiding linear scans when there is no work.
   GlobalVariable *CoverageNamesVar =
       M.getNamedGlobal(getCoverageUnusedNamesVarName());
@@ -692,23 +696,7 @@ void InstrProfiling::lowerIncrement(InstrProfIncrementInst *Inc) {
     LoadInst *LI = dyn_cast<LoadInst>(&I);
     if (!LI) {
       IRBuilder<> Builder(&I);
-      Type *Int64Ty = Type::getInt64Ty(M->getContext());
       GlobalVariable *Bias = M->getGlobalVariable(getInstrProfCounterBiasVarName());
-      if (!Bias) {
-        // Compiler must define this variable when runtime counter relocation
-        // is being used. Runtime has a weak external reference that is used
-        // to check whether that's the case or not.
-        Bias = new GlobalVariable(*M, Int64Ty, false, GlobalValue::LinkOnceODRLinkage,
-                                  Constant::getNullValue(Int64Ty),
-                                  getInstrProfCounterBiasVarName());
-        Bias->setVisibility(GlobalVariable::HiddenVisibility);
-        // A definition that's weak (linkonce_odr) without being in a COMDAT
-        // section wouldn't lead to link errors, but it would lead to a dead
-        // data word from every TU but one. Putting it in COMDAT ensures there
-        // will be exactly one data slot in the link.
-        if (TT.supportsCOMDAT())
-          Bias->setComdat(M->getOrInsertComdat(Bias->getName()));
-      }
       LI = Builder.CreateLoad(Int64Ty, Bias);
     }
     auto *Add = Builder.CreateAdd(Builder.CreatePtrToInt(Addr, Int64Ty), LI);
@@ -1195,3 +1183,26 @@ void InstrProfiling::emitInitialization() {
 
   appendToGlobalCtors(*M, F, 0);
 }
+
+bool InstrProfiling::emitBiasVar() {
+  // Module already provided its own variable, nothin to do.
+  if (M->getGlobalVariable(getInstrProfCounterBiasVarName()))
+    return false;
+
+  // Compiler must define this variable when runtime counter relocation
+  // is being used. Runtime has a weak external reference that is used
+  // to check whether that's the case or not.
+  Type *Int64Ty = Type::getInt64Ty(M->getContext());
+  auto *Bias = new GlobalVariable(*M, Int64Ty, false, GlobalValue::LinkOnceODRLinkage,
+                            Constant::getNullValue(Int64Ty),
+                            getInstrProfCounterBiasVarName());
+  Bias->setVisibility(GlobalVariable::HiddenVisibility);
+  // A definition that's weak (linkonce_odr) without being in a COMDAT
+  // section wouldn't lead to link errors, but it would lead to a dead
+  // data word from every TU but one. Putting it in COMDAT ensures there
+  // will be exactly one data slot in the link.
+  if (TT.supportsCOMDAT())
+    Bias->setComdat(M->getOrInsertComdat(Bias->getName()));
+
+  return true;
+}
diff --git a/llvm/test/Instrumentation/InstrProfiling/bias-var.ll b/llvm/test/Instrumentation/InstrProfiling/bias-var.ll
new file mode 100644
index 0000000..354751e
--- /dev/null
+++ b/llvm/test/Instrumentation/InstrProfiling/bias-var.ll
@@ -0,0 +1,6 @@
+; RUN: opt < %s -S -instrprof -runtime-counter-relocation | FileCheck -check-prefixes=RELOC %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+; RELOC: $__llvm_profile_counter_bias = comdat any
+; RELOC: @__llvm_profile_counter_bias = linkonce_odr hidden global i64 0, comdat
-- 
2.7.4