[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.

Author: jdenny-ornl

llvm/docs/LangRef.rst

@@ -7840,6 +7840,54 @@ If a loop was successfully processed by the loop distribution pass,
 this metadata is added (i.e., has been distributed).  See
 :ref:`Transformation Metadata <transformation-metadata>` for details.
 
+'``llvm.loop.estimated_trip_count``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This metadata records an estimated trip count for the loop.  The first operand
+is the string ``llvm.loop.estimated_trip_count``.  The second operand is an
+integer constant of type ``i32`` or smaller specifying the estimate.  For
+example:
+
+.. code-block:: llvm
+
+   !0 = !{!"llvm.loop.estimated_trip_count", i32 8}
+
+Purpose
+"""""""
+
+A loop's estimated trip count is an estimate of the average number of loop
+iterations (specifically, the number of times the loop's header executes) each
+time execution reaches the loop.  It is usually only an estimate based on, for
+example, profile data.  The actual number of iterations might vary widely.
+
+The estimated trip count serves as a parameter for various loop transformations
+and typically helps estimate transformation cost.  For example, it can help
+determine how many iterations to peel or how aggressively to unroll.
+
+Initialization and Maintenance
+""""""""""""""""""""""""""""""
+
+Passes should interact with estimated trip counts always via
+``llvm::getLoopEstimatedTripCount`` and ``llvm::setLoopEstimatedTripCount``.
+
+When the ``llvm.loop.estimated_trip_count`` metadata is not present on a loop,
+``llvm::getLoopEstimatedTripCount`` estimates the loop's trip count from the
+loop's ``branch_weights`` metadata under the assumption that the latter still
+accurately encodes the program's original profile data.  However, as passes
+transform existing loops and create new loops, they must be free to update and
+create ``branch_weights`` metadata in a way that maintains accurate block
+frequencies.  Trip counts estimated from this new ``branch_weights`` metadata
+are not necessarily useful to the passes that consume estimated trip counts.
+
+For this reason, when a pass transforms or creates loops, the pass should
+separately estimate new trip counts based on the estimated trip counts that
+``llvm::getLoopEstimatedTripCount`` returns at the start of the pass, and the
+pass should record the new estimates by calling
+``llvm::setLoopEstimatedTripCount``, which creates or updates
+``llvm.loop.estimated_trip_count`` metadata.  Once this metadata is present on a
+loop, ``llvm::getLoopEstimatedTripCount`` returns its value instead of
+estimating the trip count from the loop's ``branch_weights`` metadata.
+
 '``llvm.licm.disable``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

llvm/include/llvm/IR/Metadata.h

@@ -919,8 +919,8 @@ class MDOperand {
 
   // Check if MDOperand is of type MDString and equals `Str`.
   bool equalsStr(StringRef Str) const {
-    return isa<MDString>(this->get()) &&
-           cast<MDString>(this->get())->getString() == Str;
+    return isa_and_nonnull<MDString>(get()) &&
+           cast<MDString>(get())->getString() == Str;
   }
 
   ~MDOperand() { untrack(); }

llvm/include/llvm/IR/ProfDataUtils.h

@@ -30,6 +30,10 @@ struct MDProfLabels {
   LLVM_ABI static const char *UnknownBranchWeightsMarker;
 };
 
+/// Profile-based loop metadata that should be accessed only by using
+/// \c llvm::getLoopEstimatedTripCount and \c llvm::setLoopEstimatedTripCount.
+LLVM_ABI extern const char *LLVMLoopEstimatedTripCount;
+
 /// Checks if an Instruction has MD_prof Metadata
 LLVM_ABI bool hasProfMD(const Instruction &I);
 

llvm/include/llvm/Transforms/Utils/LoopUtils.h

@@ -323,22 +323,48 @@ LLVM_ABI TransformationMode hasLICMVersioningTransformation(const Loop *L);
 LLVM_ABI void addStringMetadataToLoop(Loop *TheLoop, const char *MDString,
                                       unsigned V = 0);
 
-/// Returns a loop's estimated trip count based on branch weight metadata.
-/// In addition if \p EstimatedLoopInvocationWeight is not null it is
-/// initialized with weight of loop's latch leading to the exit.
-/// Returns a valid positive trip count, saturated at UINT_MAX, or std::nullopt
-/// when a meaningful estimate cannot be made.
+/// Return either:
+/// - \c std::nullopt, if the implementation is unable to handle the loop form
+///   of \p L (e.g., \p L must have a latch block that controls the loop exit).
+/// - The value of \c llvm.loop.estimated_trip_count from the loop metadata of
+///   \p L, if that metadata is present.
+/// - Else, a new estimate of the trip count from the latch branch weights of
+///   \p L.
+///
+/// An estimated trip count is always a valid positive trip count, saturated at
+/// \c UINT_MAX.
+///
+/// In addition, if \p EstimatedLoopInvocationWeight, then either:
+/// - Set \c *EstimatedLoopInvocationWeight to the weight of the latch's branch
+///   to the loop exit.
+/// - Do not set it, and return \c std::nullopt, if the current implementation
+///   cannot compute that weight (e.g., if \p L does not have a latch block that
+///   controls the loop exit) or the weight is zero (because zero cannot be
+///   used to compute new branch weights that reflect the estimated trip count).
+///
+/// TODO: Eventually, once all passes have migrated away from setting branch
+/// weights to indicate estimated trip counts, this function will drop the
+/// \p EstimatedLoopInvocationWeight parameter.
 LLVM_ABI std::optional<unsigned>
 getLoopEstimatedTripCount(Loop *L,
                           unsigned *EstimatedLoopInvocationWeight = nullptr);
 
-/// Set a loop's branch weight metadata to reflect that loop has \p
-/// EstimatedTripCount iterations and \p EstimatedLoopInvocationWeight exits
-/// through latch. Returns true if metadata is successfully updated, false
-/// otherwise. Note that loop must have a latch block which controls loop exit
-/// in order to succeed.
-LLVM_ABI bool setLoopEstimatedTripCount(Loop *L, unsigned EstimatedTripCount,
-                                        unsigned EstimatedLoopInvocationWeight);
+/// Set \c llvm.loop.estimated_trip_count with the value \p EstimatedTripCount
+/// in the loop metadata of \p L.  Return false if the implementation is unable
+/// to handle the loop form of \p L (e.g., \p L must have a latch block that
+/// controls the loop exit).  Otherwise, return true.
+///
+/// In addition, if \p EstimatedLoopInvocationWeight, set the branch weight
+/// metadata of \p L to reflect that \p L has an estimated
+/// \p EstimatedTripCount iterations and has \c *EstimatedLoopInvocationWeight
+/// exit weight through the loop's latch.
+///
+/// TODO: Eventually, once all passes have migrated away from setting branch
+/// weights to indicate estimated trip counts, this function will drop the
+/// \p EstimatedLoopInvocationWeight parameter.
+LLVM_ABI bool setLoopEstimatedTripCount(
+    Loop *L, unsigned EstimatedTripCount,
+    std::optional<unsigned> EstimatedLoopInvocationWeight = std::nullopt);
 
 /// Check inner loop (L) backedge count is known to be invariant on all
 /// iterations of its outer loop. If the loop has no parent, this is trivially

llvm/lib/IR/ProfDataUtils.cpp

@@ -95,6 +95,7 @@ const char *MDProfLabels::FunctionEntryCount = "function_entry_count";
 const char *MDProfLabels::SyntheticFunctionEntryCount =
     "synthetic_function_entry_count";
 const char *MDProfLabels::UnknownBranchWeightsMarker = "unknown";
+const char *LLVMLoopEstimatedTripCount = "llvm.loop.estimated_trip_count";
 
 bool hasProfMD(const Instruction &I) {
   return I.hasMetadata(LLVMContext::MD_prof);

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);

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -804,26 +804,51 @@ static BranchInst *getExpectedExitLoopLatchBranch(Loop *L) {
   return LatchBR;
 }
 
-/// Return the estimated trip count for any exiting branch which dominates
-/// the loop latch.
-static std::optional<unsigned> getEstimatedTripCount(BranchInst *ExitingBranch,
-                                                     Loop *L,
-                                                     uint64_t &OrigExitWeight) {
+struct DbgLoop {
+  const Loop *L;
+  explicit DbgLoop(const Loop *L) : L(L) {}
+};
+
+#ifndef NDEBUG
+static inline raw_ostream &operator<<(raw_ostream &OS, DbgLoop D) {
+  OS << "function ";
+  D.L->getHeader()->getParent()->printAsOperand(OS, /*PrintType=*/false);
+  return OS << " " << *D.L;
+}
+#endif // NDEBUG
+
+static std::optional<unsigned> estimateLoopTripCount(Loop *L) {
+  // Currently we take the estimate exit count only from the loop latch,
+  // ignoring other exiting blocks.  This can overestimate the trip count
+  // if we exit through another exit, but can never underestimate it.
+  // TODO: incorporate information from other exits
+  BranchInst *ExitingBranch = getExpectedExitLoopLatchBranch(L);
+  if (!ExitingBranch) {
+    LLVM_DEBUG(dbgs() << "estimateLoopTripCount: Failed to find exiting "
+                      << "latch branch of required form in " << DbgLoop(L)
+                      << "\n");
+    return std::nullopt;
+  }
+
   // To estimate the number of times the loop body was executed, we want to
   // know the number of times the backedge was taken, vs. the number of times
   // we exited the loop.
   uint64_t LoopWeight, ExitWeight;
-  if (!extractBranchWeights(*ExitingBranch, LoopWeight, ExitWeight))
+  if (!extractBranchWeights(*ExitingBranch, LoopWeight, ExitWeight)) {
+    LLVM_DEBUG(dbgs() << "estimateLoopTripCount: Failed to extract branch "
+                      << "weights for " << DbgLoop(L) << "\n");
     return std::nullopt;
+  }
 
   if (L->contains(ExitingBranch->getSuccessor(1)))
     std::swap(LoopWeight, ExitWeight);
 
-  if (!ExitWeight)
+  if (!ExitWeight) {
     // Don't have a way to return predicated infinite
+    LLVM_DEBUG(dbgs() << "estimateLoopTripCount: Failed because of zero exit "
+                      << "probability for " << DbgLoop(L) << "\n");
     return std::nullopt;
-
-  OrigExitWeight = ExitWeight;
+  }
 
   // Estimated exit count is a ratio of the loop weight by the weight of the
   // edge exiting the loop, rounded to nearest.
@@ -834,43 +859,102 @@ static std::optional<unsigned> getEstimatedTripCount(BranchInst *ExitingBranch,
     return std::numeric_limits<unsigned>::max();
 
   // Estimated trip count is one plus estimated exit count.
-  return ExitCount + 1;
+  uint64_t TC = ExitCount + 1;
+  LLVM_DEBUG(dbgs() << "estimateLoopTripCount: Estimated trip count of " << TC
+                    << " for " << DbgLoop(L) << "\n");
+  return TC;
 }
 
 std::optional<unsigned>
 llvm::getLoopEstimatedTripCount(Loop *L,
                                 unsigned *EstimatedLoopInvocationWeight) {
-  // Currently we take the estimate exit count only from the loop latch,
-  // ignoring other exiting blocks.  This can overestimate the trip count
-  // if we exit through another exit, but can never underestimate it.
-  // TODO: incorporate information from other exits
-  if (BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L)) {
-    uint64_t ExitWeight;
-    if (std::optional<uint64_t> EstTripCount =
-            getEstimatedTripCount(LatchBranch, L, ExitWeight)) {
-      if (EstimatedLoopInvocationWeight)
-        *EstimatedLoopInvocationWeight = ExitWeight;
-      return *EstTripCount;
-    }
+  // If EstimatedLoopInvocationWeight, we do not support this loop if
+  // getExpectedExitLoopLatchBranch returns nullptr.
+  //
+  // FIXME: Also, this is a stop-gap solution for nested loops.  It avoids
+  // mistaking LLVMLoopEstimatedTripCount metadata to be for an outer loop when
+  // it was created for an inner loop.  The problem is that loop metadata is
+  // attached to the branch instruction in the loop latch block, but that can be
+  // shared by the loops.  A solution is to attach loop metadata to loop headers
+  // instead, but that would be a large change to LLVM.
+  //
+  // Until that happens, we work around the problem as follows.
+  // getExpectedExitLoopLatchBranch (which also guards
+  // setLoopEstimatedTripCount) returns nullptr for a loop unless the loop has
+  // one latch and that latch has exactly two successors one of which is an exit
+  // from the loop.  If the latch is shared by nested loops, then that condition
+  // might hold for the inner loop but cannot hold for the outer loop:
+  // - Because the latch is shared, it must have at least two successors: the
+  //   inner loop header and the outer loop header, which is also an exit for
+  //   the inner loop.  That satisifies the condition for the inner loop.
+  // - To satsify the condition for the outer loop, the latch must have a third
+  //   successor that is an exit for the outer loop.  But that violates the
+  //   condition for both loops.
+  BranchInst *ExitingBranch = getExpectedExitLoopLatchBranch(L);
+  if (!ExitingBranch)
+    return std::nullopt;
+
+  // If requested, either compute *EstimatedLoopInvocationWeight or return
+  // nullopt if cannot.
+  //
+  // TODO: Eventually, once all passes have migrated away from setting branch
+  // weights to indicate estimated trip counts, this function will drop the
+  // EstimatedLoopInvocationWeight parameter.
+  if (EstimatedLoopInvocationWeight) {
+    uint64_t LoopWeight = 0, ExitWeight = 0; // Inits expected to be unused.
+    if (!extractBranchWeights(*ExitingBranch, LoopWeight, ExitWeight))
+      return std::nullopt;
+    if (L->contains(ExitingBranch->getSuccessor(1)))
+      std::swap(LoopWeight, ExitWeight);
+    if (!ExitWeight)
+      return std::nullopt;
+    *EstimatedLoopInvocationWeight = ExitWeight;
   }
-  return std::nullopt;
+
+  // Return the estimated trip count from metadata unless the metadata is
+  // missing or has no value.
+  if (auto TC = getOptionalIntLoopAttribute(L, LLVMLoopEstimatedTripCount)) {
+    LLVM_DEBUG(dbgs() << "getLoopEstimatedTripCount: "
+                      << LLVMLoopEstimatedTripCount << " metadata has trip "
+                      << "count of " << *TC << " for " << DbgLoop(L) << "\n");
+    return TC;
+  }
+
+  // Estimate the trip count from latch branch weights.
+  return estimateLoopTripCount(L);
 }
 
-bool llvm::setLoopEstimatedTripCount(Loop *L, unsigned EstimatedTripCount,
-                                     unsigned EstimatedloopInvocationWeight) {
-  // At the moment, we currently support changing the estimate trip count of
-  // the latch branch only.  We could extend this API to manipulate estimated
-  // trip counts for any exit.
+bool llvm::setLoopEstimatedTripCount(
+    Loop *L, unsigned EstimatedTripCount,
+    std::optional<unsigned> EstimatedloopInvocationWeight) {
+  // If EstimatedLoopInvocationWeight, we do not support this loop if
+  // getExpectedExitLoopLatchBranch returns nullptr.
+  //
+  // FIXME: See comments in getLoopEstimatedTripCount for why this is required
+  // here regardless of EstimatedLoopInvocationWeight.
   BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L);
   if (!LatchBranch)
     return false;
 
+  // Set the metadata.
+  addStringMetadataToLoop(L, LLVMLoopEstimatedTripCount, EstimatedTripCount);
+
+  // At the moment, we currently support changing the estimated trip count in
+  // the latch branch's branch weights only.  We could extend this API to
+  // manipulate estimated trip counts for any exit.
+  //
+  // TODO: Eventually, once all passes have migrated away from setting branch
+  // weights to indicate estimated trip counts, we will not set branch weights
+  // here at all.
+  if (!EstimatedloopInvocationWeight)
+    return true;
+
   // Calculate taken and exit weights.
   unsigned LatchExitWeight = 0;
   unsigned BackedgeTakenWeight = 0;
 
-  if (EstimatedTripCount > 0) {
-    LatchExitWeight = EstimatedloopInvocationWeight;
+  if (EstimatedTripCount != 0) {
+    LatchExitWeight = *EstimatedloopInvocationWeight;
     BackedgeTakenWeight = (EstimatedTripCount - 1) * LatchExitWeight;
   }