[llvm] [Transforms][Utils] Add LoopSplit for iteration-space loop splitting (PR #217232)

[Ramkumar Ramachandra via llvm-commits]

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+//===- LoopSplit.cpp - Split a loop's iteration space ---------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Splits a counted loop's iteration space into a chain of per-partition
+// sub-loops. See LoopSplit.h for the high-level usage guidelines.
+//
+// Structure produced for partitions [S0,E0], [S1,E1], ... where E is the loop's
+// last iteration and each clamped end sel_i is min(E_i, E), or max descending:
+//
+//   guard0:                            ; every S_i and sel_i is computed here
+//     if (S0 <= sel0) goto preheader0 else goto guard1
+//   loop0: ...                         ; latch iterates while i < sel0
+//   exit0 -> guard1
+//   guard1:
+//     if (S1 <= sel1) goto preheader1 else goto guard2
+//   loop1: ...                         ; latch iterates while i < sel1
+//   exit1 -> guard2
+//     ...
+//   final.exit:
+//
+// Each guard holds the "S_i <= sel_i" check and skips an empty partition by
+// falling through to the next guard. All S_i/sel_i are materialized once in
+// guard0, and the end clamp keeps the "runs at least once" iteration in the
+// right partition.
+//
+// The latch keeps iterating while the value the next iteration would use is
+// still in the partition. That is written as the strict "i < sel_i" on the
+// induction PHI rather than "i + 1 <= sel_i" on the step value; the two agree
+// because isLegal() has established that the space does not wrap, and the
+// strict form never forms i + 1, so it remains a real test even when sel_i is
+// the last value of the type, where the inclusive one would be a tautology and
+// the partition would never exit.
+//
+// A descending (step -1) loop uses the same structure mirrored: partitions run
+// high-to-low and the clamp and predicates flip (>=/>).
+//
+// Usage guidelines:
+//  - Caller bounds must not wrap the induction type. The clamp absorbs a bound
+//    past the runtime trip count, and isLegal() reserves the one step past the
+//    induction start that an empty partition needs, but a bound reaching any
+//    further wraps in the bound arithmetic and cannot be repaired here.
+//  - Bounds must be loop-invariant: they are expanded in guard0, so a bound
+//    depending on a value defined inside the loop cannot be placed.
+//  - The partitions must tile the original iteration space exactly -- same
+//    iterations, same order -- so the split preserves program behaviour.
+//
+// The transform is structural: inside a partition it only seeds the induction
+// PHI with that partition's start and replaces the latch test. It never
+// rebuilds a value that flows between partitions, so no SSA reconstruction is
+// needed.
+//
+// Not yet supported, and rejected by isLegal(): loop-carried values, values
+// that escape the loop (exit values), non-unit and non-integer inductions,
+// top-tested loops, and multiple exits. Also rejected is an induction start at
+// the extreme of the iteration direction, which leaves nowhere to put a
+// boundary. An induction *end* at that extreme is fine, because the latch stays
+// strict.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/LoopSplit.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
+#include "llvm/Analysis/ScalarEvolutionPatternMatch.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/ProfDataUtils.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
+#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
+#include <optional>
+
+using namespace llvm;
+using namespace llvm::SCEVPatternMatch;
+
+#define DEBUG_TYPE "loop-split"
+
+//===----------------------------------------------------------------------===//
+// LoopSplit - construction, partition list, induction analysis
+//===----------------------------------------------------------------------===//
+
+/// Per-split() scratch shared by the phase helpers; lives for one split() call.
+/// Everything derived from the induction lives on LoopSplit itself, filled in
+/// by isLegal(); this holds only what the transform creates.
+struct LoopSplit::SplitState {
+  // Partition 0 reuses the original loop's preheader, exit, and entry guard;
+  // those blocks live in Partitions[0] rather than being duplicated here.
+  BasicBlock *FinalExit = nullptr; // where the partition chain converges.
+  Loop *OuterLoop = nullptr;       // parent of the new blocks, if any.
+  PHINode *Induction = nullptr;    // the loop's induction variable.
+};
+
+// Record a new partition with the given inclusive iteration range.
+void LoopSplit::addPartition(const SCEV *Start, const SCEV *End) {
+  assert(InductionEnd && "addPartition() requires a successful isLegal()");
+  // The bounds are combined with the induction end and expanded in its type. A
+  // mismatch would otherwise surface either as a bare "Operand types don't
+  // match!" from inside ScalarEvolution, or worse, as a silent cast.
+  assert(Start->getType() == InductionEnd->getType() &&
+         End->getType() == InductionEnd->getType() &&
+         "partition bounds must have the induction type");
+  Partitions.emplace_back(Start, End);
+}
+
+// Return the induction's add-recurrence, or null unless the induction is an
+// integer with a unit step that the latch compares.
+static const SCEVAddRecExpr *analyzeInduction(Loop *L, ScalarEvolution *SE) {
+  ICmpInst *LatchCmp = L->getLatchCmpInst();
+
+  // SCEV's induction variable, restricted to a unit-step affine recurrence.
+  PHINode *Induction = L->getInductionVariable(*SE);
+  if (!Induction)
+    return nullptr;
+  // Partition bounds are integer arithmetic on the induction type, so a loop
+  // whose only induction is a pointer is out of scope.
+  if (!Induction->getType()->isIntegerTy())
+    return nullptr;
+  const SCEV *IndSCEV = SE->getSCEV(Induction);
+  // Match an affine add-recurrence and capture its constant step; accept a unit
+  // step in either direction: +1 (ascending) or -1 (descending).
+  const APInt *Step;
+  if (!match(IndSCEV, m_scev_AffineAddRec(m_SCEV(), m_scev_APInt(Step))))
+    return nullptr;
+  if (!Step->isOne() && !Step->isAllOnes())
+    return nullptr;
+  const auto *AR = cast<SCEVAddRecExpr>(IndSCEV);
+
+  // The induction's "next" value (i + 1), produced in the latch.
+  auto *StepInst = dyn_cast<Instruction>(
+      Induction->getIncomingValueForBlock(L->getLoopLatch()));
+  if (!StepInst)
+    return nullptr;
+
+  // One compare operand must be the induction, either the PHI or its step. The
+  // rebuilt latch always compares the PHI, so which operand it was is not used.
+  if (LatchCmp->getOperand(0) == Induction ||
+      LatchCmp->getOperand(0) == StepInst ||
+      LatchCmp->getOperand(1) == Induction ||
+      LatchCmp->getOperand(1) == StepInst)
+    return AR;
+  return nullptr;
+}
+
+// Decide whether the iteration ordering is signed or unsigned; returns the
+// signedness, or nullopt if it cannot be proven.
+static std::optional<bool> computeSignedness(Loop *L,
+                                             const SCEVAddRecExpr *IndAR) {
+  ICmpInst::Predicate P = L->getLatchCmpInst()->getPredicate();
+  // A relational predicate gives the ordering directly; for eq/ne fall back to
+  // the recurrence's no-wrap flags.
+  if (ICmpInst::isRelational(P))
+    return ICmpInst::isSigned(P);
+  if (IndAR->hasNoSignedWrap())
+    return true;
+  if (IndAR->hasNoUnsignedWrap())
+    return false;
----------------
artagnon wrote:

I think this is incorrect, as IndAR can have nuw as well as nsw?

https://github.com/llvm/llvm-project/pull/217232