[PGO] Add llvm.loop.estimated_trip_count metadata (#152775)

This patch implements the `llvm.loop.estimated_trip_count` metadata
discussed in [[RFC] Fix Loop Transformations to Preserve Block
Frequencies](https://discourse.llvm.org/t/rfc-fix-loop-transformations-to-preserve-block-frequencies/85785).
As the RFC explains, that metadata enables future patches, such as PR
#128785, to fix block frequency issues without losing estimated trip
counts.

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index 7d362ce..c06b60f 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -1076,6 +1076,18 @@ void Verifier::visitMDNode(const MDNode &MD, AreDebugLocsAllowed AllowLocs) {
     }
   }
 
+  // Check llvm.loop.estimated_trip_count.
+  if (MD.getNumOperands() > 0 &&
+      MD.getOperand(0).equalsStr(LLVMLoopEstimatedTripCount)) {
+    Check(MD.getNumOperands() == 2, "Expected two operands", &MD);
+    auto *Count = dyn_cast_or_null<ConstantAsMetadata>(MD.getOperand(1));
+    Check(Count && Count->getType()->isIntegerTy() &&
+              cast<IntegerType>(Count->getType())->getBitWidth() <= 32,
+          "Expected second operand to be an integer constant of type i32 or "
+          "smaller",
+          &MD);
+  }
+
   // Check these last, so we diagnose problems in operands first.
   Check(!MD.isTemporary(), "Expected no forward declarations!", &MD);
   Check(MD.isResolved(), "All nodes should be resolved!", &MD);