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Optimize expanded constant pool words (#6979)

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Re-inline ConstPool entries in V3Subst that have been expanded into
word-wise accessed by V3Expand. This enables downstream constant folding
on the word-wise expressions.

As V3Subst now understands ConstPool entries, we can also omit expanding
straight assignments with a ConstPool entry on the RHS. This allows the
C++ compiler to see the memcpy directly.
This commit is contained in:
Geza Lore
2026-02-01 17:08:49 +00:00
committed by GitHub
parent 7ca113a84f
commit d3f608058f
6 changed files with 71 additions and 12 deletions
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src/V3Subst.cpp
+35 -8
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@@ -218,6 +218,8 @@ AstNodeExpr* SubstVarEntry::substRecord(SubstVarEntry::Record& record) {
class SubstVisitor final : public VNVisitor {
// NODE STATE - only Under AstCFunc
// AstVar::user1p -> SubstVarEntry* for assignment tracking. Also used by SubstValidVisitor
// AstVar::user2 -> bool. Is a constant pool variable
const VNUser2InUse m_user2InUse;
// STATE
std::deque<SubstVarEntry> m_entries; // Storage for SubstVarEntry instances
@@ -230,6 +232,7 @@ class SubstVisitor final : public VNVisitor {
size_t m_nWholeSubstituted = 0; // Number of whole variables substituted
size_t m_nWordAssignDeleted = 0; // Number of word assignments deleted
size_t m_nWholeAssignDeleted = 0; // Number of whole variable assignments deleted
size_t m_nConstWordsReinlined = 0; // Number of constant words substituted
static constexpr uint32_t SUBST_MAX_OPS_SUBST = 30; // Maximum number of ops to substitute in
static constexpr uint32_t SUBST_MAX_OPS_NA = 9999; // Not allowed to substitute
@@ -260,7 +263,7 @@ class SubstVisitor final : public VNVisitor {
bool isSubstitutable(AstVar* nodep) { return nodep->isStatementTemp() && !nodep->noSubst(); }
void substitute(AstNode* nodep, AstNodeExpr* substp) {
AstNodeExpr* newp = substp->cloneTreePure(true);
AstNodeExpr* newp = substp->backp() ? substp->cloneTreePure(true) : substp;
if (!nodep->isQuad() && newp->isQuad()) {
newp = new AstCCast{newp->fileline(), newp, nodep};
}
@@ -270,6 +273,16 @@ class SubstVisitor final : public VNVisitor {
}
// VISITORS
void visit(AstNetlist* nodep) override {
// Mark constant pool variables
for (AstNode* np = nodep->constPoolp()->modp()->stmtsp(); np; np = np->nextp()) {
if (VN_IS(np, Var)) np->user2(true);
}
iterateAndNextNull(nodep->modulesp());
}
void visit(AstCFunc* nodep) override {
UASSERT_OBJ(!m_funcp, nodep, "Should not nest");
UASSERT_OBJ(m_entries.empty(), nodep, "Should not visit outside functions");
@@ -357,8 +370,21 @@ class SubstVisitor final : public VNVisitor {
return;
}
// Otherwise it's a read, substitute it if possible
// Otherwise it's a read,
UASSERT_OBJ(refp->access().isReadOnly(), nodep, "Invalid access");
// If it's a constant pool variable, substiute with the constant word
AstVar* const varp = refp->varp();
if (varp->user2()) {
AstConst* const constp = VN_AS(varp->valuep(), Const);
const uint32_t value = constp->num().edataWord(word);
FileLine* const flp = nodep->fileline();
++m_nConstWordsReinlined;
substitute(nodep, new AstConst{flp, AstConst::SizedEData{}, value});
return;
}
// Substitute other variables if possible
if (isSubstitutable(refp->varp())) {
if (AstNodeExpr* const substp = entry.substWord(word)) {
++m_nWordSubstituted;
@@ -425,14 +451,15 @@ class SubstVisitor final : public VNVisitor {
public:
// CONSTRUCTORS
explicit SubstVisitor(AstNode* nodep) { iterate(nodep); }
explicit SubstVisitor(AstNetlist* nodep) { iterate(nodep); }
~SubstVisitor() override {
V3Stats::addStat("Optimizations, Substituted temps", m_nSubst);
V3Stats::addStat("Optimizations, Whole variable assignments deleted",
V3Stats::addStat("Optimizations, Subst, Substituted temps", m_nSubst);
V3Stats::addStat("Optimizations, Subst, Constant words reinlined", m_nConstWordsReinlined);
V3Stats::addStat("Optimizations, Subst, Whole variable assignments deleted",
m_nWholeAssignDeleted);
V3Stats::addStat("Optimizations, Whole variables substituted", m_nWholeSubstituted);
V3Stats::addStat("Optimizations, Word assignments deleted", m_nWordAssignDeleted);
V3Stats::addStat("Optimizations, Words substituted", m_nWordSubstituted);
V3Stats::addStat("Optimizations, Subst, Whole variables substituted", m_nWholeSubstituted);
V3Stats::addStat("Optimizations, Subst, Word assignments deleted", m_nWordAssignDeleted);
V3Stats::addStat("Optimizations, Subst, Words substituted", m_nWordSubstituted);
UASSERT(m_entries.empty(), "Should not visit outside functions");
}
};