1//===- ScalarEvolutionNormalization.cpp - See below -----------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements utilities for working with "normalized" expressions.
10// See the comments at the top of ScalarEvolutionNormalization.h for details.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/Analysis/ScalarEvolutionNormalization.h"
15#include "llvm/Analysis/LoopInfo.h"
16#include "llvm/Analysis/ScalarEvolution.h"
17#include "llvm/Analysis/ScalarEvolutionExpressions.h"
18using namespace llvm;
19
20/// TransformKind - Different types of transformations that
21/// TransformForPostIncUse can do.
22enum TransformKind {
23 /// Normalize - Normalize according to the given loops.
24 Normalize,
25 /// Denormalize - Perform the inverse transform on the expression with the
26 /// given loop set.
27 Denormalize
28};
29
30namespace {
31struct NormalizeDenormalizeRewriter
32 : public SCEVRewriteVisitor<NormalizeDenormalizeRewriter> {
33 const TransformKind Kind;
34
35 // NB! Pred is a function_ref. Storing it here is okay only because
36 // we're careful about the lifetime of NormalizeDenormalizeRewriter.
37 const NormalizePredTy Pred;
38
39 NormalizeDenormalizeRewriter(TransformKind Kind, NormalizePredTy Pred,
40 ScalarEvolution &SE)
41 : SCEVRewriteVisitor<NormalizeDenormalizeRewriter>(SE), Kind(Kind),
42 Pred(Pred) {}
43 const SCEV *visitAddRecExpr(const SCEVAddRecExpr *Expr);
44};
45} // namespace
46
47const SCEV *
48NormalizeDenormalizeRewriter::visitAddRecExpr(const SCEVAddRecExpr *AR) {
49 SmallVector<const SCEV *, 8> Operands;
50
51 transform(Range: AR->operands(), d_first: std::back_inserter(x&: Operands),
52 F: [&](const SCEV *Op) { return visit(S: Op); });
53
54 if (!Pred(AR))
55 return SE.getAddRecExpr(Operands, L: AR->getLoop(), Flags: SCEV::FlagAnyWrap);
56
57 // Normalization and denormalization are fancy names for decrementing and
58 // incrementing a SCEV expression with respect to a set of loops. Since
59 // Pred(AR) has returned true, we know we need to normalize or denormalize AR
60 // with respect to its loop.
61
62 if (Kind == Denormalize) {
63 // Denormalization / "partial increment" is essentially the same as \c
64 // SCEVAddRecExpr::getPostIncExpr. Here we use an explicit loop to make the
65 // symmetry with Normalization clear.
66 for (int i = 0, e = Operands.size() - 1; i < e; i++)
67 Operands[i] = SE.getAddExpr(LHS: Operands[i], RHS: Operands[i + 1]);
68 } else {
69 assert(Kind == Normalize && "Only two possibilities!");
70
71 // Normalization / "partial decrement" is a bit more subtle. Since
72 // incrementing a SCEV expression (in general) changes the step of the SCEV
73 // expression as well, we cannot use the step of the current expression.
74 // Instead, we have to use the step of the very expression we're trying to
75 // compute!
76 //
77 // We solve the issue by recursively building up the result, starting from
78 // the "least significant" operand in the add recurrence:
79 //
80 // Base case:
81 // Single operand add recurrence. It's its own normalization.
82 //
83 // N-operand case:
84 // {S_{N-1},+,S_{N-2},+,...,+,S_0} = S
85 //
86 // Since the step recurrence of S is {S_{N-2},+,...,+,S_0}, we know its
87 // normalization by induction. We subtract the normalized step
88 // recurrence from S_{N-1} to get the normalization of S.
89
90 for (int i = Operands.size() - 2; i >= 0; i--)
91 Operands[i] = SE.getMinusSCEV(LHS: Operands[i], RHS: Operands[i + 1]);
92 }
93
94 return SE.getAddRecExpr(Operands, L: AR->getLoop(), Flags: SCEV::FlagAnyWrap);
95}
96
97const SCEV *llvm::normalizeForPostIncUse(const SCEV *S,
98 const PostIncLoopSet &Loops,
99 ScalarEvolution &SE,
100 bool CheckInvertible) {
101 if (Loops.empty())
102 return S;
103 auto Pred = [&](const SCEVAddRecExpr *AR) {
104 return Loops.count(Ptr: AR->getLoop());
105 };
106 const SCEV *Normalized =
107 NormalizeDenormalizeRewriter(Normalize, Pred, SE).visit(S);
108 const SCEV *Denormalized = denormalizeForPostIncUse(S: Normalized, Loops, SE);
109 // If the normalized expression isn't invertible.
110 if (CheckInvertible && Denormalized != S)
111 return nullptr;
112 return Normalized;
113}
114
115const SCEV *llvm::normalizeForPostIncUseIf(const SCEV *S, NormalizePredTy Pred,
116 ScalarEvolution &SE) {
117 return NormalizeDenormalizeRewriter(Normalize, Pred, SE).visit(S);
118}
119
120const SCEV *llvm::denormalizeForPostIncUse(const SCEV *S,
121 const PostIncLoopSet &Loops,
122 ScalarEvolution &SE) {
123 if (Loops.empty())
124 return S;
125 auto Pred = [&](const SCEVAddRecExpr *AR) {
126 return Loops.count(Ptr: AR->getLoop());
127 };
128 return NormalizeDenormalizeRewriter(Denormalize, Pred, SE).visit(S);
129}
130