[InstrProf] Support conditional counter updates

This patch adds support of conditional counter updates
in single byte counters mode to reduce the write contention
by first checking whether the counter is set before
overwriting it.

Author: gulfemsavrun

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

@@ -1213,8 +1213,20 @@ Value *InstrLowerer::getBitmapAddress(InstrProfMCDCTVBitmapUpdate *I) {
 void InstrLowerer::lowerCover(InstrProfCoverInst *CoverInstruction) {
   auto *Addr = getCounterAddress(CoverInstruction);
   IRBuilder<> Builder(CoverInstruction);
-  // We store zero to represent that this block is covered.
-  Builder.CreateStore(Builder.getInt8(0), Addr);
+  if (ConditionalCounterUpdate) {
+    auto &Ctx = CoverInstruction->getParent()->getContext();
+    auto *Int8Ty = llvm::Type::getInt8Ty(Ctx);
+    Value *Load = Builder.CreateLoad(Int8Ty, Addr, "pgocount");
+    Value *Cmp = Builder.CreateICmpNE(Load, ConstantInt::get(Int8Ty, 0),
+                                      "pgocount.ifnonzero");
+    Value *Sel =
+        Builder.CreateSelect(Cmp, Builder.getInt8(0), Load, "pgocount.select");
+    Builder.CreateStore(Sel, Addr);
+  } else {
+    // We store zero to represent that this block is covered.
+    Builder.CreateStore(Builder.getInt8(0), Addr);
+  }
+
   CoverInstruction->eraseFromParent();
 }
 

llvm/test/Instrumentation/InstrProfiling/conditional-counter-updates.ll

@@ -0,0 +1,15 @@
+; RUN: opt < %s -S -passes=instrprof -conditional-counter-update | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+@__profn_foo = private constant [3 x i8] c"foo"
+
+; CHECK-LABEL: define void @foo
+; CHECK-NEXT: %pgocount = load i8, ptr @__profc_foo, align 1
+; CHECK-NEXT: %pgocount.ifnonzero = icmp ne i8 %pgocount, 0
+; CHECK-NEXT: %pgocount.select = select i1 %pgocount.ifnonzero, i8 0, i8 %pgocount
+; CHECK-NEXT: store i8 %pgocount.select, ptr @__profc_foo, align 1
+define void @foo() {
+  call void @llvm.instrprof.cover(ptr @__profn_foo, i64 0, i32 1, i32 0)
+  ret void
+}