[InstrProfiling] Use nobits as __llvm_prf_cnts section type in ELF

This can reduce the binary size because counters will no longer occupy
space in the binary, instead they will be allocated by dynamic linker.

Differential Revision: https://reviews.llvm.org/D97110

Author: petrhosek

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -440,6 +440,10 @@ static SectionKind getELFKindForNamedSection(StringRef Name, SectionKind K) {
       Name == ".llvmbc" || Name == ".llvmcmd")
     return SectionKind::getMetadata();
 
+  if (Name == getInstrProfSectionName(IPSK_cnts, Triple::ELF,
+                                      /*AddSegmentInfo=*/false))
+    return SectionKind::getBSS();
+
   if (Name.empty() || Name[0] != '.') return K;
 
   // Default implementation based on some magic section names.

llvm/test/Instrumentation/InstrProfiling/X86/nobits.ll

@@ -0,0 +1,15 @@
+;; Ensure that SHT_NOBITS section type is set for __llvm_prf_cnts in ELF.
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+@__profc_foo = hidden global [1 x i64] zeroinitializer, section "__llvm_prf_cnts", align 8
+
+define void @foo() {
+  %pgocount = load i64, i64* getelementptr inbounds ([1 x i64], [1 x i64]* @__profc_foo, i64 0, i64 0), align 4
+  %1 = add i64 %pgocount, 1
+  store i64 %1, i64* getelementptr inbounds ([1 x i64], [1 x i64]* @__profc_foo, i64 0, i64 0), align 4
+  ret void
+}
+
+declare void @llvm.instrprof.increment(i8*, i64, i32, i32)
+
+; CHECK: .section __llvm_prf_cnts,"aw",@nobits