[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, 9 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/BasicBlockSections.cpp b/llvm/lib/CodeGen/BasicBlockSections.cpp
index 421c1d8..9692e9b 100644
--- a/llvm/lib/CodeGen/BasicBlockSections.cpp
+++ b/llvm/lib/CodeGen/BasicBlockSections.cpp
@@ -79,6 +79,15 @@ using llvm::StringMap;
 using llvm::StringRef;
 using namespace llvm;
 
+// Placing the cold clusters in a separate section mitigates against poor
+// profiles and allows optimizations such as hugepage mapping to be applied at a
+// section granularity. Where necessary, users should set this to ".text.split."
+// which is recognized by lld via the `-z keep-text-section-prefix` flag.
+cl::opt<std::string> llvm::BBSectionsColdTextPrefix(
+    "bbsections-cold-text-prefix",
+    cl::desc("The text prefix to use for cold basic block clusters"),
+    cl::init(".text.unlikely."), cl::Hidden);
+
 namespace {
 
 // This struct represents the cluster information for a machine basic block.