[PATCH] D107377: [InstrProfiling] Emit bias variable eagerly

[Petr Hosek via Phabricator via llvm-commits]

phosek created this revision.
phosek added reviewers: mcgrathr, davidxl.
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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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D107377

Files:
  llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
  llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
  llvm/test/Instrumentation/InstrProfiling/bias-var.ll


Index: llvm/test/Instrumentation/InstrProfiling/bias-var.ll
===================================================================
--- /dev/null
+++ 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
Index: llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
+++ llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
@@ -551,6 +551,10 @@
   // 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 @@
     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 @@
 
   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;
+}
Index: llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
===================================================================
--- llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
+++ llvm/include/llvm/Transforms/Instrumentation/InstrProfiling.h
@@ -121,6 +121,9 @@
   /// 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


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