Place the "cold" code piece into the same section as the original function

[void]

Summary

This cropped up in the Linux kernel where cold code was placed in an
incompatible section.

[vsk]

Thanks, LGTM.

llvm/lib/Transforms/IPO/HotColdSplitting.cpp

if (Function *OutF = CE.extractCodeRegion(CEAC)) {
     CallSite CS(CI);
     NumColdRegionsOutlined++;
     if (TTI.useColdCCForColdCall(*OutF)) {
       OutF->setCallingConv(CallingConv::Cold);
       CS.setCallingConv(CallingConv::Cold);
     }
     CI->setIsNoInline();
 
+    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 "
              << ore::NV("Split", OutF);

llvm/test/Transforms/HotColdSplit/retain-section.ll

+; RUN: opt -hotcoldsplit -hotcoldsplit-threshold=0 -S < %s | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.14.0"
+
+; Retain section after cold code is split off.
+
+; CHECK-LABEL: @fun
+; CHECK: call void @fun.cold.1{{.*}} section ".text.cold"
+define void @fun() section ".text.cold" {
+entry:
+  br i1 undef, label %if.then, label %if.else
+
+if.then:
+  ret void
+
+if.else:
+  call void @sink()
+  ret void
+}
+
+; CHECK: define {{.*}} @foo{{.*}}#[[outlined_func_attr:[0-9]+]]
+define void @foo() cold {
+  ret void
+}
+
+declare void @sink() cold
+
+; CHECK: define {{.*}} @fun.cold.1{{.*}}#[[outlined_func_attr]]
+
+; CHECK: attributes #[[outlined_func_attr]] = {
+; CHECK-SAME: cold
+; CHECK-SAME: minsize