[llvm] [Transforms][Utils] Add LoopSplit for iteration-space loop splitting (PR #217232)
Ashutosh Nema via llvm-commits
llvm-commits at lists.llvm.org
Fri Aug 28 02:47:13 PDT 2026
================
@@ -0,0 +1,542 @@
+//===- 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/ADT/STLExtras.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 (any_of(LatchCmp->operands(),
+ [&](Value *Op) { return Op == Induction || Op == 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(ScalarEvolution &SE, 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() && IndAR->hasNoUnsignedWrap()) {
+ const ConstantRange UR = SE.getUnsignedRange(IndAR);
+ const ConstantRange SR = SE.getSignedRange(IndAR);
+ if (UR.isFullSet() && SR.isFullSet()) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE
+ ": ambiguous iteration ordering with both nsw and nuw\n");
+ return std::nullopt;
+ }
+ if (!SR.isFullSet())
+ return true;
+ if (!UR.isFullSet())
+ return false;
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE
+ ": cannot prove iteration ordering signedness\n");
+ return std::nullopt;
+ }
+ if (IndAR->hasNoSignedWrap())
+ return true;
+ if (IndAR->hasNoUnsignedWrap())
+ return false;
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE
+ ": cannot prove iteration ordering signedness\n");
+ return std::nullopt;
+}
+
+// Prove \p Pred between \p LHS and \p RHS on entry to \p L, retrying with the
+// loop guards folded in so a bound fixed by a dominating condition is seen.
+static bool isEntryGuardedByCond(ScalarEvolution &SE, Loop *L,
+ ICmpInst::Predicate Pred, const SCEV *LHS,
+ const SCEV *RHS) {
+ if (SE.isLoopEntryGuardedByCond(L, Pred, LHS, RHS))
+ return true;
+ return SE.isLoopEntryGuardedByCond(L, Pred, SE.applyLoopGuards(LHS, L),
+ SE.applyLoopGuards(RHS, L));
+}
+
+// Check every structural precondition and record the induction analysis.
+bool LoopSplit::isLegal() {
+ // Require a bottom-tested single-exit loop in LCSSA form. Simplify form gives
+ // the preheader, single latch and dedicated exits; the rest pin the exit to
+ // the latch, so the latch compare can be rewritten per partition.
+ if (!L->isLoopSimplifyForm() || !L->isLCSSAForm(*DT) ||
+ L->getExitingBlock() != L->getLoopLatch() || !L->getExitBlock()) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE ": loop not in expected form\n");
+ return false;
+ }
+
+ // The latch compare must exist and reside in the latch: it is rewritten in
+ // place, once per partition.
+ ICmpInst *LatchCmp = L->getLatchCmpInst();
+ if (!LatchCmp || LatchCmp->getParent() != L->getLoopLatch()) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE ": latch compare not in the loop latch\n");
+ return false;
+ }
+
+ // Exit values are unsupported. Look for an LCSSA PHI and for a use outside
+ // the loop: a token-like type cannot appear in a PHI, so LCSSA can leave a
+ // value escaping with no PHI to find.
+ if (!L->getExitBlock()->phis().empty()) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE ": loop has exit values\n");
+ return false;
+ }
+ for (BasicBlock *BB : L->blocks())
+ for (Instruction &I : *BB)
+ for (User *U : I.users())
+ if (auto *UI = dyn_cast<Instruction>(U); UI && !L->contains(UI)) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE ": loop has exit values\n");
+ return false;
+ }
+
+ // Splitting a loop clones it, so cloning must be safe.
+ if (!L->isSafeToClone()) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE ": loop not safe to clone\n");
+ return false;
+ }
+
+ // A computable backedge-taken count fixes the iteration space we rebuild.
+ const SCEV *BTC = SE->getBackedgeTakenCount(L);
+ if (isa<SCEVCouldNotCompute>(BTC)) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE ": loop trip count uncomputable\n");
+ return false;
+ }
+
+ const SCEVAddRecExpr *IndAR = analyzeInduction(L, SE);
+ if (!IndAR) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE
+ ": no unique unit-step integer induction\n");
+ return false;
+ }
+
+ PHINode *Induction = L->getInductionVariable(*SE);
+
+ // Loop-carried values are unsupported: a later partition would have to resume
+ // the previous one's value, which needs SSA reconstruction. The induction is
+ // the exception, seeded per partition from its own start bound.
+ for (PHINode &HeaderPHI : L->getHeader()->phis())
+ if (&HeaderPHI != Induction) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE ": loop has carried values\n");
+ return false;
+ }
+
+ std::optional<bool> Signed = computeSignedness(*SE, L, IndAR);
+ if (!Signed)
+ return false;
+ InductionIsSigned = *Signed;
+ Descending =
+ cast<SCEVConstant>(IndAR->getStepRecurrence(*SE))->getAPInt().isAllOnes();
+
+ // Start and end must share the induction type; reject any width mismatch.
+ // evaluateAtIteration coerces to the start's type for an affine recurrence,
+ // so this is defensive rather than reachable.
+ InductionEnd = IndAR->evaluateAtIteration(BTC, *SE);
+ if (InductionEnd->getType() != IndAR->getStart()->getType()) {
+ LLVM_DEBUG(dbgs() << DEBUG_TYPE ": induction end/start type mismatch\n");
+ return false;
+ }
+
+ // Partition bounds and entry guards assume the space runs monotonically from
+ // start to end, so refuse one that wraps past the type extreme. A no-wrap
+ // flag on the recurrence asserts that directly.
+ bool NoWrap =
+ InductionIsSigned ? IndAR->hasNoSignedWrap() : IndAR->hasNoUnsignedWrap();
+ ICmpInst::Predicate Ordered =
+ Descending
+ ? (InductionIsSigned ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE)
+ : (InductionIsSigned ? ICmpInst::ICMP_SLE : ICmpInst::ICMP_ULE);
----------------
nema-ashutosh wrote:
Yes, same predicate. I'll reuse guardPredicate.
https://github.com/llvm/llvm-project/pull/217232
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