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verilator/src/V3DfgDfgToAst.cpp

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// -*- mode: C++; c-file-style: "cc-mode" -*-
//*************************************************************************
// DESCRIPTION: Verilator: Convert DfgGraph to AstModule
//
// Code available from: https://verilator.org
//
//*************************************************************************
//
// Copyright 2003-2022 by Wilson Snyder. This program is free software; you
// can redistribute it and/or modify it under the terms of either the GNU
// Lesser General Public License Version 3 or the Perl Artistic License
// Version 2.0.
// SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
//
//*************************************************************************
//
// Convert DfgGraph back to AstModule. We recursively construct AstNodeMath expressions for each
// DfgVertex which represents a storage location (e.g.: DfgVarPacked), or has multiple sinks
// without driving a storage location (and hence needs a temporary variable to duplication). The
// recursion stops when we reach a DfgVertex representing a storage location (e.g.: DfgVarPacked),
// or a vertex that that has multiple sinks (as these nodes will have a [potentially new temporary]
// corresponding// storage location). Redundant variables (those whose source vertex drives
// multiple variables) are eliminated when possible. Vertices driving multiple variables are
// rendered once, driving an arbitrarily (but deterministically) chosen canonical variable, and the
// corresponding redundant variables are assigned from the canonical variable.
//
//*************************************************************************
#include "config_build.h"
#include "verilatedos.h"
#include "V3Dfg.h"
#include "V3DfgPasses.h"
#include "V3UniqueNames.h"
#include <algorithm>
#include <unordered_map>
VL_DEFINE_DEBUG_FUNCTIONS;
namespace {
// Create an AstNodeMath out of a DfgVertex. For most AstNodeMath subtypes, this can be done
// automatically. For the few special cases, we provide specializations below
template <typename Node, typename... Ops>
Node* makeNode(const DfgForAst<Node>* vtxp, Ops... ops) {
Node* const nodep = new Node{vtxp->fileline(), ops...};
UASSERT_OBJ(nodep->width() == static_cast<int>(vtxp->width()), vtxp,
"Incorrect width in AstNode created from DfgVertex "
<< vtxp->typeName() << ": " << nodep->width() << " vs " << vtxp->width());
return nodep;
}
//======================================================================
// Vertices needing special conversion
template <>
AstExtend* makeNode<AstExtend, AstNodeMath*>( //
const DfgExtend* vtxp, AstNodeMath* op1) {
return new AstExtend{vtxp->fileline(), op1, static_cast<int>(vtxp->width())};
}
template <>
AstExtendS* makeNode<AstExtendS, AstNodeMath*>( //
const DfgExtendS* vtxp, AstNodeMath* op1) {
return new AstExtendS{vtxp->fileline(), op1, static_cast<int>(vtxp->width())};
}
template <>
AstShiftL* makeNode<AstShiftL, AstNodeMath*, AstNodeMath*>( //
const DfgShiftL* vtxp, AstNodeMath* op1, AstNodeMath* op2) {
return new AstShiftL{vtxp->fileline(), op1, op2, static_cast<int>(vtxp->width())};
}
template <>
AstShiftR* makeNode<AstShiftR, AstNodeMath*, AstNodeMath*>( //
const DfgShiftR* vtxp, AstNodeMath* op1, AstNodeMath* op2) {
return new AstShiftR{vtxp->fileline(), op1, op2, static_cast<int>(vtxp->width())};
}
template <>
AstShiftRS* makeNode<AstShiftRS, AstNodeMath*, AstNodeMath*>( //
const DfgShiftRS* vtxp, AstNodeMath* op1, AstNodeMath* op2) {
return new AstShiftRS{vtxp->fileline(), op1, op2, static_cast<int>(vtxp->width())};
}
//======================================================================
// Currently unhandled nodes - see corresponding AstToDfg functions
// LCOV_EXCL_START
template <>
AstCCast* makeNode<AstCCast, AstNodeMath*>(const DfgCCast* vtxp, AstNodeMath*) {
vtxp->v3fatal("not implemented");
}
template <>
AstAtoN* makeNode<AstAtoN, AstNodeMath*>(const DfgAtoN* vtxp, AstNodeMath*) {
vtxp->v3fatal("not implemented");
}
template <>
AstCompareNN* makeNode<AstCompareNN, AstNodeMath*, AstNodeMath*>(const DfgCompareNN* vtxp,
AstNodeMath*, AstNodeMath*) {
vtxp->v3fatal("not implemented");
}
template <>
AstSliceSel* makeNode<AstSliceSel, AstNodeMath*, AstNodeMath*, AstNodeMath*>(
const DfgSliceSel* vtxp, AstNodeMath*, AstNodeMath*, AstNodeMath*) {
vtxp->v3fatal("not implemented");
}
// LCOV_EXCL_STOP
} // namespace
class DfgToAstVisitor final : DfgVisitor {
// NODE STATE
// AstVar::user1() bool: this is a temporary we are introducing
const VNUser1InUse m_inuser1;
// STATE
AstModule* const m_modp; // The parent/result module
V3DfgOptimizationContext& m_ctx; // The optimization context for stats
AstNodeMath* m_resultp = nullptr; // The result node of the current traversal
// Map from DfgVertex to the AstVar holding the value of that DfgVertex after conversion
std::unordered_map<const DfgVertex*, AstVar*> m_resultVars;
// Map from an AstVar, to the canonical AstVar that can be substituted for that AstVar
std::unordered_map<AstVar*, AstVar*> m_canonVars;
V3UniqueNames m_tmpNames{"_VdfgTmp"}; // For generating temporary names
DfgVertex::HashCache m_hashCache; // For caching hashes
// METHODS
// Given a DfgVarPacked, return the canonical AstVar that can be used for this DfgVarPacked.
// Also builds the m_canonVars map as a side effect.
AstVar* getCanonicalVar(const DfgVarPacked* vtxp) {
// If variable driven (at least partially) outside the DFG, then we have no choice
if (!vtxp->isDrivenFullyByDfg()) return vtxp->varp();
// Look up map
const auto it = m_canonVars.find(vtxp->varp());
if (it != m_canonVars.end()) return it->second;
// Not known yet, compute it (for all vars driven fully from the same driver)
std::vector<const DfgVarPacked*> varps;
vtxp->source(0)->forEachSink([&](const DfgVertex& vtx) {
if (const DfgVarPacked* const varVtxp = vtx.cast<DfgVarPacked>()) {
if (varVtxp->isDrivenFullyByDfg()) varps.push_back(varVtxp);
}
});
UASSERT_OBJ(!varps.empty(), vtxp, "The input vtxp is always available");
std::stable_sort(varps.begin(), varps.end(),
[](const DfgVarPacked* ap, const DfgVarPacked* bp) {
if (ap->hasExtRefs() != bp->hasExtRefs()) return ap->hasExtRefs();
const FileLine& aFl = *(ap->fileline());
const FileLine& bFl = *(bp->fileline());
if (const int cmp = aFl.operatorCompare(bFl)) return cmp < 0;
return ap->varp()->name() < bp->varp()->name();
});
AstVar* const canonVarp = varps.front()->varp();
// Add results to map
for (const DfgVarPacked* const varp : varps) m_canonVars.emplace(varp->varp(), canonVarp);
// Return it
return canonVarp;
}
// Given a DfgVertex, return an AstVar that will hold the value of the given DfgVertex once we
// are done with converting this Dfg into Ast form.
AstVar* getResultVar(const DfgVertex* vtxp) {
const auto pair = m_resultVars.emplace(vtxp, nullptr);
AstVar*& varp = pair.first->second;
if (pair.second) {
// If this vertex is a DfgVarPacked, then we know the variable. If this node is not a
// DfgVarPacked, then first we try to find a DfgVarPacked driven by this node, and use
// that, otherwise we create a temporary
if (const DfgVarPacked* const thisDfgVarPackedp = vtxp->cast<DfgVarPacked>()) {
// This is a DfgVarPacked
varp = getCanonicalVar(thisDfgVarPackedp);
} else if (const DfgVarArray* const thisDfgVarArrayp = vtxp->cast<DfgVarArray>()) {
// This is a DfgVarArray
varp = thisDfgVarArrayp->varp();
} else if (const DfgVarPacked* const sinkDfgVarPackedp = vtxp->findSink<DfgVarPacked>(
[](const DfgVarPacked& var) { return var.isDrivenFullyByDfg(); })) {
// We found a DfgVarPacked driven fully by this node
varp = getCanonicalVar(sinkDfgVarPackedp);
} else {
// No DfgVarPacked driven fully by this node. Create a temporary.
// TODO: should we reuse parts when the AstVar is used as an rvalue?
const string name = m_tmpNames.get(vtxp->hash(m_hashCache).toString());
// Note: It is ok for these temporary variables to be always unsigned. They are
// read only by other expressions within the graph and all expressions interpret
// their operands based on the expression type, not the operand type.
AstNodeDType* const dtypep = v3Global.rootp()->findBitDType(
vtxp->width(), vtxp->width(), VSigning::UNSIGNED);
varp = new AstVar{vtxp->fileline(), VVarType::MODULETEMP, name, dtypep};
varp->user1(true); // Mark as temporary
// Add temporary AstVar to containing module
m_modp->addStmtsp(varp);
}
// Add to map
}
return varp;
}
AstNodeMath* convertDfgVertexToAstNodeMath(DfgVertex* vtxp) {
UASSERT_OBJ(!m_resultp, vtxp, "Result already computed");
iterate(vtxp);
UASSERT_OBJ(m_resultp, vtxp, "Missing result");
AstNodeMath* const resultp = m_resultp;
m_resultp = nullptr;
return resultp;
}
bool inlineVertex(DfgVertex& vtx) {
// Inline vertices that drive only a single node, or are special
if (!vtx.hasMultipleSinks()) return true;
if (vtx.is<DfgConst>()) return true;
if (vtx.is<DfgVarPacked>()) return true;
if (vtx.is<DfgVarArray>()) return true;
if (const DfgArraySel* const selp = vtx.cast<DfgArraySel>()) {
return selp->bitp()->is<DfgConst>();
}
return false;
}
AstNodeMath* convertSource(DfgVertex* vtxp) {
if (inlineVertex(*vtxp)) {
// Inlined vertices are simply recursively converted
UASSERT_OBJ(vtxp->hasSinks(), vtxp, "Must have one sink: " << vtxp->typeName());
return convertDfgVertexToAstNodeMath(vtxp);
} else {
// Vertices that are not inlined need a variable, just return a reference
return new AstVarRef{vtxp->fileline(), getResultVar(vtxp), VAccess::READ};
}
}
void convertCanonicalVarDriver(const DfgVarPacked* dfgVarp) {
const auto wRef = [dfgVarp]() {
return new AstVarRef{dfgVarp->fileline(), dfgVarp->varp(), VAccess::WRITE};
};
if (dfgVarp->isDrivenFullyByDfg()) {
// Whole variable is driven. Render driver and assign directly to whole variable.
AstNodeMath* const rhsp = convertDfgVertexToAstNodeMath(dfgVarp->source(0));
addResultEquation(dfgVarp->driverFileLine(0), wRef(), rhsp);
} else {
// Variable is driven partially. Render each driver as a separate assignment.
dfgVarp->forEachSourceEdge([&](const DfgEdge& edge, size_t idx) {
UASSERT_OBJ(edge.sourcep(), dfgVarp, "Should have removed undriven sources");
// Render the rhs expression
AstNodeMath* const rhsp = convertDfgVertexToAstNodeMath(edge.sourcep());
// Create select LValue
FileLine* const flp = dfgVarp->driverFileLine(idx);
AstConst* const lsbp = new AstConst{flp, dfgVarp->driverLsb(idx)};
AstConst* const widthp = new AstConst{flp, edge.sourcep()->width()};
AstSel* const lhsp = new AstSel{flp, wRef(), lsbp, widthp};
// Add assignment of the value to the selected bits
addResultEquation(flp, lhsp, rhsp);
});
}
}
void convertDuplicateVarDriver(const DfgVarPacked* dfgVarp, AstVar* canonVarp) {
const auto rRef = [canonVarp]() {
return new AstVarRef{canonVarp->fileline(), canonVarp, VAccess::READ};
};
const auto wRef = [dfgVarp]() {
return new AstVarRef{dfgVarp->fileline(), dfgVarp->varp(), VAccess::WRITE};
};
if (dfgVarp->isDrivenFullyByDfg()) {
// Whole variable is driven. Just assign from the canonical variable.
addResultEquation(dfgVarp->driverFileLine(0), wRef(), rRef());
} else {
// Variable is driven partially. Asign from parts of the canonical var.
dfgVarp->forEachSourceEdge([&](const DfgEdge& edge, size_t idx) {
UASSERT_OBJ(edge.sourcep(), dfgVarp, "Should have removed undriven sources");
// Create select LValue
FileLine* const flp = dfgVarp->driverFileLine(idx);
AstConst* const lsbp = new AstConst{flp, dfgVarp->driverLsb(idx)};
AstConst* const widthp = new AstConst{flp, edge.sourcep()->width()};
AstSel* const rhsp = new AstSel{flp, rRef(), lsbp, widthp->cloneTree(false)};
AstSel* const lhsp = new AstSel{flp, wRef(), lsbp->cloneTree(false), widthp};
// Add assignment of the value to the selected bits
addResultEquation(flp, lhsp, rhsp);
});
}
}
void convertArrayDiver(const DfgVarArray* dfgVarp) {
// Variable is driven partially. Asign from parts of the canonical var.
dfgVarp->forEachSourceEdge([&](const DfgEdge& edge, size_t idx) {
UASSERT_OBJ(edge.sourcep(), dfgVarp, "Should have removed undriven sources");
// Render the rhs expression
AstNodeMath* const rhsp = convertDfgVertexToAstNodeMath(edge.sourcep());
// Create select LValue
FileLine* const flp = dfgVarp->driverFileLine(idx);
AstVarRef* const refp = new AstVarRef{flp, dfgVarp->varp(), VAccess::WRITE};
AstConst* const idxp = new AstConst{flp, dfgVarp->driverIndex(idx)};
AstArraySel* const lhsp = new AstArraySel{flp, refp, idxp};
// Add assignment of the value to the selected bits
addResultEquation(flp, lhsp, rhsp);
});
}
void addResultEquation(FileLine* flp, AstNode* lhsp, AstNode* rhsp) {
m_modp->addStmtsp(new AstAssignW{flp, lhsp, rhsp});
++m_ctx.m_resultEquations;
}
// VISITORS
void visit(DfgVertex* vtxp) override { // LCOV_EXCL_START
vtxp->v3fatal("Unhandled DfgVertex: " << vtxp->typeName());
} // LCOV_EXCL_STOP
void visit(DfgVarPacked* vtxp) override {
m_resultp = new AstVarRef{vtxp->fileline(), getCanonicalVar(vtxp), VAccess::READ};
}
void visit(DfgVarArray* vtxp) override {
m_resultp = new AstVarRef{vtxp->fileline(), vtxp->varp(), VAccess::READ};
}
void visit(DfgConst* vtxp) override { //
m_resultp = vtxp->constp()->cloneTree(false);
}
// The rest of the 'visit' methods are generated by 'astgen'
#include "V3Dfg__gen_dfg_to_ast.h"
// Constructor
explicit DfgToAstVisitor(DfgGraph& dfg, V3DfgOptimizationContext& ctx)
: m_modp{dfg.modulep()}
, m_ctx{ctx} {
// We can eliminate some variables completely
std::vector<AstVar*> redundantVarps;
// Convert vertices back to assignments
dfg.forEachVertex([&](DfgVertex& vtx) {
// Render packed variable assignments
if (const DfgVarPacked* const dfgVarp = vtx.cast<DfgVarPacked>()) {
// DfgVarPacked instances (these might be driving the given AstVar variable)
// If there is no driver (i.e.: this DfgVarPacked is an input to the Dfg), then
// nothing to do
if (!dfgVarp->isDrivenByDfg()) return;
// The driver of this DfgVarPacked might drive multiple variables. Only emit one
// assignment from the driver to an arbitrarily chosen canonical variable, and
// assign the other variables from that canonical variable
AstVar* const canonVarp = getCanonicalVar(dfgVarp);
if (canonVarp == dfgVarp->varp()) {
// This is the canonical variable, so render the driver
convertCanonicalVarDriver(dfgVarp);
} else if (dfgVarp->keep()) {
// Not the canonical variable but it must be kept
convertDuplicateVarDriver(dfgVarp, canonVarp);
} else {
// Not a canonical var, and it can be removed. We will replace all references
// to it with the canonical variable, and hence this can be removed.
redundantVarps.push_back(dfgVarp->varp());
++m_ctx.m_replacedVars;
}
return;
}
// Render array variable assignments
if (const DfgVarArray* dfgVarp = vtx.cast<DfgVarArray>()) {
// If there is no driver, then there is nothing to do
if (!dfgVarp->isDrivenByDfg()) return;
// We don't canonicalize arrays, so just render the drivers
convertArrayDiver(dfgVarp);
//
return;
}
// If the vertex is known to be inlined, then nothing else to do
if (inlineVertex(vtx)) return;
// Check if this uses a temporary, vs one of the vars rendered above
AstVar* const resultVarp = getResultVar(&vtx);
if (resultVarp->user1()) {
// We introduced a temporary for this DfgVertex
++m_ctx.m_intermediateVars;
FileLine* const flp = vtx.fileline();
// Just render the logic
AstNodeMath* const rhsp = convertDfgVertexToAstNodeMath(&vtx);
// The lhs is a temporary
AstNodeMath* const lhsp = new AstVarRef{flp, resultVarp, VAccess::WRITE};
// Add assignment of the value to the variable
addResultEquation(flp, lhsp, rhsp);
}
});
// Remap all references to point to the canonical variables, if one exists
VNDeleter deleter;
m_modp->foreach<AstVarRef>([&](AstVarRef* refp) {
// Any variable that is written outside the DFG will have itself as the canonical
// var, so need not be replaced, furthermore, if a variable is traced, we don't
// want to update the write ref we just created above, so we only replace read only
// references.
if (!refp->access().isReadOnly()) return;
const auto it = m_canonVars.find(refp->varp());
if (it == m_canonVars.end()) return;
if (it->second == refp->varp()) return;
refp->replaceWith(new AstVarRef{refp->fileline(), it->second, refp->access()});
deleter.pushDeletep(refp);
});
// Remove redundant variables
for (AstVar* const varp : redundantVarps) varp->unlinkFrBack()->deleteTree();
}
public:
static AstModule* apply(DfgGraph& dfg, V3DfgOptimizationContext& ctx) {
return DfgToAstVisitor{dfg, ctx}.m_modp;
}
};
AstModule* V3DfgPasses::dfgToAst(DfgGraph& dfg, V3DfgOptimizationContext& ctx) {
return DfgToAstVisitor::apply(dfg, ctx);
}
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