[HotColdSplit] Add options for splitting cold functions in separate section

[rjf]

Summary

Add support for (if enabled) splitting cold functions into a separate section
in order to further boost locality of hot code.

[rjf]

Will add test case soon.

[rcorcs]

Minor suggestions.

llvm/lib/Transforms/IPO/HotColdSplitting.cpp

90–91

Suggested description:
Enable the placement of extracted cold functions into a separate section after hot-cold splitting.

96

Suggested description:
Name for the section containing cold functions extracted by hot-cold splitting.

[rjf]

Add tests

[rjf]

Incorporate @rcorcs 's suggestions.

[rjf]

@rcorcs Your comments are addressed. Thanks!

[rjf]

The tests are modified from split-cold-2.ll in the same folder. Add comments to note this.

llvm/lib/Transforms/IPO/HotColdSplitting.cpp

 static cl::opt<bool> EnableStaticAnalyis("hot-cold-static-analysis",
                               cl::init(true), cl::Hidden);
 
 static cl::opt<int>
     SplittingThreshold("hotcoldsplit-threshold", cl::init(2), cl::Hidden,
                        cl::desc("Base penalty for splitting cold code (as a "
                                 "multiple of TCC_Basic)"));
 
+static cl::opt<bool>
+    EnableColdSection("enable-cold-section", cl::init(false), cl::Hidden,
+                      cl::desc("Set to true for splitting cold functions into"
+                               " separate cold region."));
+
+static cl::opt<StringRef>
+    ColdSectionName("hotcoldsplit-cold-section-name", cl::init("__llvm_cold"),
+                    cl::Hidden,
+                    cl::desc("Cold section name for section splitting"));
+
 namespace {
 // Same as blockEndsInUnreachable in CodeGen/BranchFolding.cpp. Do not modify
 // this function unless you modify the MBB version as well.
 //
 /// A no successor, non-return block probably ends in unreachable and is cold.
 /// Also consider a block that ends in an indirect branch to be a return block,
 /// since many targets use plain indirect branches to return.
 bool blockEndsInUnreachable(const BasicBlock &BB) {
if (Function *OutF = CE.extractCodeRegion(CEAC)) {
     CallInst *CI = cast<CallInst>(U);
     NumColdRegionsOutlined++;
     if (TTI.useColdCCForColdCall(*OutF)) {
       OutF->setCallingConv(CallingConv::Cold);
       CI->setCallingConv(CallingConv::Cold);
     }
     CI->setIsNoInline();
 
+    if (EnableColdSection)
+      OutF->setSection(ColdSectionName);
+    else {
       if (OrigF->hasSection())
         OutF->setSection(OrigF->getSection());
+    }
 
     markFunctionCold(*OutF, BFI != nullptr);
 
     LLVM_DEBUG(llvm::dbgs() << "Outlined Region: " << *OutF);
     ORE.emit([&]() {
       return OptimizationRemark(DEBUG_TYPE, "HotColdSplit",
                                 &*Region[0]->begin())
              << ore::NV("Original", OrigF) << " split cold code into "