[PGO] Turn off comdat renaming in IR PGO by default

Summary:
In IR PGO we append the function hash to comdat functions to avoid the
potential hash mismatch. This turns out not legal in some cases: if the comdat
function is address-taken and used in comparison. Renaming changes the semantic.

This patch turns off comdat renaming by default.

To alleviate the hash mismatch issue, we now rename the profile variable
for comdat functions. Profile allows co-existing multiple versions of profiles
with different hash value. The inlined copy will always has the correct profile
counter. The out-of-line copy might not have the correct count. But we will
not have the bogus mismatch warning.

Reviewers: davidxl

Subscribers: llvm-commits, xur

Differential Revision: https://reviews.llvm.org/D28416

llvm-svn: 291588

1 files changed, 43 insertions, 0 deletions

diff --git a/llvm/lib/ProfileData/InstrProf.cpp b/llvm/lib/ProfileData/InstrProf.cpp
index 77c6ffc9c25..74acd9e5e20 100644
--- a/llvm/lib/ProfileData/InstrProf.cpp
+++ b/llvm/lib/ProfileData/InstrProf.cpp
@@ -811,4 +811,47 @@ bool needsComdatForCounter(const Function &F, const Module &M) {
 
   return true;
 }
+
+// Check if INSTR_PROF_RAW_VERSION_VAR is defined.
+bool isIRPGOFlagSet(const Module *M) {
+  auto IRInstrVar =
+      M->getNamedGlobal(INSTR_PROF_QUOTE(INSTR_PROF_RAW_VERSION_VAR));
+  if (!IRInstrVar || IRInstrVar->isDeclaration() ||
+      IRInstrVar->hasLocalLinkage())
+    return false;
+
+  // Check if the flag is set.
+  if (!IRInstrVar->hasInitializer())
+    return false;
+
+  const Constant *InitVal = IRInstrVar->getInitializer();
+  if (!InitVal)
+    return false;
+
+  return (dyn_cast<ConstantInt>(InitVal)->getZExtValue() &
+          VARIANT_MASK_IR_PROF) != 0;
+}
+
+// Check if we can safely rename this Comdat function.
+bool canRenameComdatFunc(const Function &F, bool CheckAddressTaken) {
+  if (F.getName().empty())
+    return false;
+  if (!needsComdatForCounter(F, *(F.getParent())))
+    return false;
+  // Unsafe to rename the address-taken function (which can be used in
+  // function comparison).
+  if (CheckAddressTaken && F.hasAddressTaken())
+    return false;
+  // Only safe to do if this function may be discarded if it is not used
+  // in the compilation unit.
+  if (!GlobalValue::isDiscardableIfUnused(F.getLinkage()))
+    return false;
+
+  // For AvailableExternallyLinkage functions.
+  if (!F.hasComdat()) {
+    assert(F.getLinkage() == GlobalValue::AvailableExternallyLinkage);
+    return true;
+  }
+  return true;
+}
 } // end namespace llvm