[llvm] Add -bbsections-cold-text-prefix to emit cold clusters to a different section.

This change adds an option to basic block sections to allow cold
clusters to be assigned a custom text prefix. With a custom prefix such
as ".text.split." (D87840), lld can place them in a separate output section.
The benefits are -

* Empirically shown to improve icache and itlb metrics by 3-5%
(absolute) compared to placing split parts in .text.unlikely.
* Mitigates against poor profiles, eg samplePGO profiles used with the
machine function splitter. Optimizations such as hugepage remapping can
make different decisions at the section granularity.
* Enables section granularity hotness monitoring (checking on the
decisions made during compilation vs sample data from production).

Differential Revision: https://reviews.llvm.org/D87813

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 0c43ec8..84fe9df 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -21,6 +21,7 @@
 #include "llvm/BinaryFormat/Dwarf.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/BinaryFormat/MachO.h"
+#include "llvm/CodeGen/BasicBlockSectionUtils.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
@@ -833,7 +834,7 @@ MCSection *TargetLoweringObjectFileELF::getSectionForMachineBasicBlock(
   // name, or a unique ID for the section.
   SmallString<128> Name;
   if (MBB.getSectionID() == MBBSectionID::ColdSectionID) {
-    Name += ".text.unlikely.";
+    Name += BBSectionsColdTextPrefix;
     Name += MBB.getParent()->getName();
   } else if (MBB.getSectionID() == MBBSectionID::ExceptionSectionID) {
     Name += ".text.eh.";