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[DependenceAnalysis] Extending SIV to handle fusable loops #128782

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Sep 19, 2025
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[DependenceAnalysis] Extending SIV to handle fusable loops #128782

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188 changes: 140 additions & 48 deletions llvm/include/llvm/Analysis/DependenceAnalysis.h
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Original file line number Diff line number Diff line change
Expand Up @@ -82,6 +82,16 @@ class LLVM_ABI Dependence {
/// Dependence::DVEntry - Each level in the distance/direction vector
/// has a direction (or perhaps a union of several directions), and
/// perhaps a distance.
/// The dependency information could be across a single loop level or across
/// two separate levels that have the same trip count and nesting depth,
/// which helps to provide information for loop fusion candidation.
/// For example, loops b and c have the same iteration count and depth:
/// for (a = ...) {
/// for (b = 0; b < 10; b++) {
/// }
/// for (c = 0; c < 10; c++) {
/// }
/// }
struct DVEntry {
enum : unsigned char {
NONE = 0,
Expand Down Expand Up @@ -144,12 +154,25 @@ class LLVM_ABI Dependence {
/// source and destination of the dependence.
virtual unsigned getLevels() const { return 0; }

/// getDirection - Returns the direction associated with a particular level.
virtual unsigned getDirection(unsigned Level) const { return DVEntry::ALL; }
/// getSameSDLevels - Returns the number of separate SameSD loops surrounding
/// the source and destination of the dependence.
virtual unsigned getSameSDLevels() const { return 0; }

/// getDistance - Returns the distance (or NULL) associated with a particular
/// level.
virtual const SCEV *getDistance(unsigned Level) const { return nullptr; }
/// getDVEntry - Returns the DV entry associated with a regular or a
/// SameSD level
DVEntry getDVEntry(unsigned Level, bool isSameSD) const;

/// getDirection - Returns the direction associated with a particular
/// common or SameSD level.
virtual unsigned getDirection(unsigned Level, bool SameSD = false) const {
return DVEntry::ALL;
}

/// getDistance - Returns the distance (or NULL) associated with a
/// particular common or SameSD level.
virtual const SCEV *getDistance(unsigned Level, bool SameSD = false) const {
return nullptr;
}

/// Check if the direction vector is negative. A negative direction
/// vector means Src and Dst are reversed in the actual program.
Expand All @@ -162,21 +185,32 @@ class LLVM_ABI Dependence {
virtual bool normalize(ScalarEvolution *SE) { return false; }

/// isPeelFirst - Returns true if peeling the first iteration from
/// this loop will break this dependence.
virtual bool isPeelFirst(unsigned Level) const { return false; }
/// this regular or SameSD loop level will break this dependence.
virtual bool isPeelFirst(unsigned Level, bool SameSD = false) const {
return false;
}

/// isPeelLast - Returns true if peeling the last iteration from
/// this loop will break this dependence.
virtual bool isPeelLast(unsigned Level) const { return false; }
/// this regular or SameSD loop level will break this dependence.
virtual bool isPeelLast(unsigned Level, bool SameSD = false) const {
return false;
}

/// isSplitable - Returns true if splitting this loop will break the
/// dependence.
virtual bool isSplitable(unsigned Level) const { return false; }
/// isSplitable - Returns true if splitting the loop will break
/// the dependence.
virtual bool isSplitable(unsigned Level, bool SameSD = false) const {
return false;
}

/// isScalar - Returns true if a particular level is scalar; that is,
/// if no subscript in the source or destination mention the induction
/// variable associated with the loop at this level.
virtual bool isScalar(unsigned Level) const;
/// inSameSDLoops - Returns true if this level is an SameSD level, i.e.,
/// performed across two separate loop nests that have the Same Iteration and
/// Depth.
virtual bool inSameSDLoops(unsigned Level) const { return false; }

/// isScalar - Returns true if a particular regular or SameSD level is
/// scalar; that is, if no subscript in the source or destination mention
/// the induction variable associated with the loop at this level.
virtual bool isScalar(unsigned Level, bool SameSD = false) const;

/// getNextPredecessor - Returns the value of the NextPredecessor field.
const Dependence *getNextPredecessor() const { return NextPredecessor; }
Expand All @@ -198,6 +232,10 @@ class LLVM_ABI Dependence {
/// dump - For debugging purposes, dumps a dependence to OS.
void dump(raw_ostream &OS) const;

/// dumpImp - For debugging purposes. Dumps a dependence to OS with or
/// without considering the SameSD levels.
void dumpImp(raw_ostream &OS, bool SameSD = false) const;

protected:
Instruction *Src, *Dst;

Expand Down Expand Up @@ -238,13 +276,30 @@ class LLVM_ABI FullDependence final : public Dependence {
/// source and destination of the dependence.
unsigned getLevels() const override { return Levels; }

/// getSameSDLevels - Returns the number of separate SameSD loops surrounding
/// the source and destination of the dependence.
unsigned getSameSDLevels() const override { return SameSDLevels; }

/// getDVEntry - Returns the DV entry associated with a regular or a
/// SameSD level.
DVEntry getDVEntry(unsigned Level, bool isSameSD) const {
if (!isSameSD) {
assert(0 < Level && Level <= Levels && "Level out of range");
return DV[Level - 1];
} else {
assert(Levels < Level && Level <= Levels + SameSDLevels &&
"isSameSD level out of range");
return DVSameSD[Level - Levels - 1];
}
}

/// getDirection - Returns the direction associated with a particular
/// level.
unsigned getDirection(unsigned Level) const override;
/// common or SameSD level.
unsigned getDirection(unsigned Level, bool SameSD = false) const override;

/// getDistance - Returns the distance (or NULL) associated with a
/// particular level.
const SCEV *getDistance(unsigned Level) const override;
/// particular common or SameSD level.
const SCEV *getDistance(unsigned Level, bool SameSD = false) const override;

/// Check if the direction vector is negative. A negative direction
/// vector means Src and Dst are reversed in the actual program.
Expand All @@ -257,27 +312,34 @@ class LLVM_ABI FullDependence final : public Dependence {
bool normalize(ScalarEvolution *SE) override;

/// isPeelFirst - Returns true if peeling the first iteration from
/// this loop will break this dependence.
bool isPeelFirst(unsigned Level) const override;
/// this regular or SameSD loop level will break this dependence.
bool isPeelFirst(unsigned Level, bool SameSD = false) const override;

/// isPeelLast - Returns true if peeling the last iteration from
/// this loop will break this dependence.
bool isPeelLast(unsigned Level) const override;
/// this regular or SameSD loop level will break this dependence.
bool isPeelLast(unsigned Level, bool SameSD = false) const override;

/// isSplitable - Returns true if splitting the loop will break
/// the dependence.
bool isSplitable(unsigned Level) const override;
bool isSplitable(unsigned Level, bool SameSD = false) const override;

/// inSameSDLoops - Returns true if this level is an SameSD level, i.e.,
/// performed across two separate loop nests that have the Same Iteration and
/// Depth.
bool inSameSDLoops(unsigned Level) const override;

/// isScalar - Returns true if a particular level is scalar; that is,
/// if no subscript in the source or destination mention the induction
/// variable associated with the loop at this level.
bool isScalar(unsigned Level) const override;
/// isScalar - Returns true if a particular regular or SameSD level is
/// scalar; that is, if no subscript in the source or destination mention
/// the induction variable associated with the loop at this level.
bool isScalar(unsigned Level, bool SameSD = false) const override;

private:
unsigned short Levels;
unsigned short SameSDLevels;
bool LoopIndependent;
bool Consistent; // Init to true, then refine.
std::unique_ptr<DVEntry[]> DV;
std::unique_ptr<DVEntry[]> DVSameSD; // DV entries on SameSD levels
friend class DependenceInfo;
};

Expand Down Expand Up @@ -406,7 +468,8 @@ class DependenceInfo {
const SCEV *A;
const SCEV *B;
const SCEV *C;
const Loop *AssociatedLoop;
const Loop *AssociatedSrcLoop;
const Loop *AssociatedDstLoop;

public:
/// isEmpty - Return true if the constraint is of kind Empty.
Expand Down Expand Up @@ -450,19 +513,27 @@ class DependenceInfo {
/// Otherwise assert.
LLVM_ABI const SCEV *getD() const;

/// getAssociatedLoop - Returns the loop associated with this constraint.
LLVM_ABI const Loop *getAssociatedLoop() const;
/// getAssociatedSrcLoop - Returns the source loop associated with this
/// constraint.
LLVM_ABI const Loop *getAssociatedSrcLoop() const;

/// getAssociatedDstLoop - Returns the destination loop associated with
/// this constraint.
LLVM_ABI const Loop *getAssociatedDstLoop() const;

/// setPoint - Change a constraint to Point.
LLVM_ABI void setPoint(const SCEV *X, const SCEV *Y,
const Loop *CurrentLoop);
const Loop *CurrentSrcLoop,
const Loop *CurrentDstLoop);

/// setLine - Change a constraint to Line.
LLVM_ABI void setLine(const SCEV *A, const SCEV *B, const SCEV *C,
const Loop *CurrentLoop);
const Loop *CurrentSrcLoop,
const Loop *CurrentDstLoop);

/// setDistance - Change a constraint to Distance.
LLVM_ABI void setDistance(const SCEV *D, const Loop *CurrentLoop);
LLVM_ABI void setDistance(const SCEV *D, const Loop *CurrentSrcLoop,
const Loop *CurrentDstLoop);

/// setEmpty - Change a constraint to Empty.
LLVM_ABI void setEmpty();
Expand All @@ -475,6 +546,12 @@ class DependenceInfo {
LLVM_ABI void dump(raw_ostream &OS) const;
};

/// Returns true if two loops have the Same iteration Space and Depth. To be
/// more specific, two loops have SameSD if they are in the same nesting
/// depth and have the same backedge count. SameSD stands for Same iteration
/// Space and Depth.
bool haveSameSD(const Loop *SrcLoop, const Loop *DstLoop) const;

/// establishNestingLevels - Examines the loop nesting of the Src and Dst
/// instructions and establishes their shared loops. Sets the variables
/// CommonLevels, SrcLevels, and MaxLevels.
Expand Down Expand Up @@ -525,9 +602,21 @@ class DependenceInfo {
/// e - 5
/// f - 6
/// g - 7 = MaxLevels
/// SameSDLevels counts the number of levels after common levels that are
/// not common but have the same iteration space and depth. Internally this
/// is checked using haveSameSD. Assume that in this code fragment, levels c
/// and e have the same iteration space and depth, but levels d and f does
/// not. Then SameSDLevels is set to 1. In that case the level numbers for the
/// previous code look like
/// a - 1
/// b - 2
/// c,e - 3 = CommonLevels
/// d - 4 = SrcLevels
/// f - 5
/// g - 6 = MaxLevels
void establishNestingLevels(const Instruction *Src, const Instruction *Dst);

unsigned CommonLevels, SrcLevels, MaxLevels;
unsigned CommonLevels, SrcLevels, MaxLevels, SameSDLevels;

/// mapSrcLoop - Given one of the loops containing the source, return
/// its level index in our numbering scheme.
Expand Down Expand Up @@ -652,9 +741,9 @@ class DependenceInfo {
/// If there might be a dependence, returns false.
/// Sets appropriate direction and distance.
bool strongSIVtest(const SCEV *Coeff, const SCEV *SrcConst,
const SCEV *DstConst, const Loop *CurrentLoop,
unsigned Level, FullDependence &Result,
Constraint &NewConstraint) const;
const SCEV *DstConst, const Loop *CurrentSrcLoop,
const Loop *CurrentDstLoop, unsigned Level,
FullDependence &Result, Constraint &NewConstraint) const;

/// weakCrossingSIVtest - Tests the weak-crossing SIV subscript pair
/// (Src and Dst) for dependence.
Expand All @@ -667,9 +756,9 @@ class DependenceInfo {
/// Set consistent to false.
/// Marks the dependence as splitable.
bool weakCrossingSIVtest(const SCEV *SrcCoeff, const SCEV *SrcConst,
const SCEV *DstConst, const Loop *CurrentLoop,
unsigned Level, FullDependence &Result,
Constraint &NewConstraint,
const SCEV *DstConst, const Loop *CurrentSrcLoop,
const Loop *CurrentDstLoop, unsigned Level,
FullDependence &Result, Constraint &NewConstraint,
const SCEV *&SplitIter) const;

/// ExactSIVtest - Tests the SIV subscript pair
Expand All @@ -683,8 +772,9 @@ class DependenceInfo {
/// Set consistent to false.
bool exactSIVtest(const SCEV *SrcCoeff, const SCEV *DstCoeff,
const SCEV *SrcConst, const SCEV *DstConst,
const Loop *CurrentLoop, unsigned Level,
FullDependence &Result, Constraint &NewConstraint) const;
const Loop *CurrentSrcLoop, const Loop *CurrentDstLoop,
unsigned Level, FullDependence &Result,
Constraint &NewConstraint) const;

/// weakZeroSrcSIVtest - Tests the weak-zero SIV subscript pair
/// (Src and Dst) for dependence.
Expand All @@ -697,8 +787,9 @@ class DependenceInfo {
/// Set consistent to false.
/// If loop peeling will break the dependence, mark appropriately.
bool weakZeroSrcSIVtest(const SCEV *DstCoeff, const SCEV *SrcConst,
const SCEV *DstConst, const Loop *CurrentLoop,
unsigned Level, FullDependence &Result,
const SCEV *DstConst, const Loop *CurrentSrcLoop,
const Loop *CurrentDstLoop, unsigned Level,
FullDependence &Result,
Constraint &NewConstraint) const;

/// weakZeroDstSIVtest - Tests the weak-zero SIV subscript pair
Expand All @@ -712,8 +803,9 @@ class DependenceInfo {
/// Set consistent to false.
/// If loop peeling will break the dependence, mark appropriately.
bool weakZeroDstSIVtest(const SCEV *SrcCoeff, const SCEV *SrcConst,
const SCEV *DstConst, const Loop *CurrentLoop,
unsigned Level, FullDependence &Result,
const SCEV *DstConst, const Loop *CurrentSrcLoop,
const Loop *CurrentDstLoop, unsigned Level,
FullDependence &Result,
Constraint &NewConstraint) const;

/// exactRDIVtest - Tests the RDIV subscript pair for dependence.
Expand Down
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