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V3Gate: Defer constant folding until required 

Rather than constant folding each logic block after every substitution,
only constant fold updated blocks when re-analysed, or at the end. This
removes a lot of invocations of V3Const on large blocks that can be
optimized well, and should yield the same result.

This speeds up V3Gate by ~4x on a design where V3Gate dominates.
This commit is contained in:
Geza Lore 2022-07-31 19:28:54 +01:00
parent 682a60e325
commit 0d2bf23d82
1 changed files with 24 additions and 12 deletions
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36
src/V3Gate.cpp
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@ -312,7 +312,7 @@ private:
// NODE STATE
// Entire netlist:
// AstVarScope::user1p -> GateVarVertex* for usage var, 0=not set yet
// {statement}Node::user1p -> GateLogicVertex* for this statement
// {logic}Node::user1 -> bool: Some signals were optimized, hence needs constant folding
// AstVarScope::user2 -> bool: Signal used in SenItem in *this* always statement
// AstVar::user2 -> bool: Warned about SYNCASYNCNET
// AstNodeVarRef::user2 -> bool: ConcatOffset visited
@ -328,6 +328,8 @@ private:
bool m_activeReducible = true; // Is activation block reducible?
bool m_inSenItem = false; // Underneath AstSenItem; any varrefs are clocks
bool m_inSlow = false; // Inside a slow structure
std::vector<AstNode*> m_optimized; // Logic blocks optimized
VDouble0 m_statSigs; // Statistic tracking
VDouble0 m_statRefs; // Statistic tracking
VDouble0 m_statDedupLogic; // Statistic tracking
@ -378,16 +380,8 @@ private:
void optimizeElimVar(AstVarScope* varscp, AstNode* substp, AstNode* consumerp) {
if (debug() >= 5) consumerp->dumpTree(cout, " elimUsePre: ");
const bool replaced = eliminate(consumerp, varscp, substp, nullptr);
if (!replaced) return;
if (debug() >= 9) consumerp->dumpTree(cout, " elimUseCns: ");
// Caution: Can't let V3Const change our handle to consumerp, such as by
// optimizing away this assignment, etc.
consumerp = V3Const::constifyEdit(consumerp);
if (debug() >= 5) consumerp->dumpTree(cout, " elimUseDne: ");
// Some previous input edges may have disappeared, perhaps all of them.
// If we remove the edges we can further optimize
// See e.g t_var_overzero.v.
if (replaced && !consumerp->user1()) m_optimized.push_back(consumerp);
consumerp->user1(2); // Added to m_optimized and needs folding
}
void optimizeSignals(bool allowMultiIn);
@ -417,6 +411,17 @@ private:
optimizeSignals(false);
// Then propagate more complicated equations
optimizeSignals(true);
// Constant fold optimized logic
for (AstNode* const logicp : m_optimized) {
// Ignore if already simplified
if (logicp->user1() != 2) continue;
AstNode* const foldedp = V3Const::constifyEdit(logicp);
// Caution: Can't let V3Const change our handle to consumerp, such as by
// optimizing away this assignment, etc.
UASSERT_OBJ(foldedp == logicp, foldedp, "should not remove node");
}
// Remove redundant logic
if (v3Global.opt.fDedupe()) {
dedupe();
@ -577,8 +582,15 @@ void GateVisitor::optimizeSignals(bool allowMultiIn) {
= static_cast<GateLogicVertex*>(vvertexp->inBeginp()->fromp());
if (!logicVertexp->reducible()) continue;
// Can we eliminate?
// Constant fold driving logic if itself has been optimized, but not yet folded
AstNode* const logicp = logicVertexp->nodep();
if (logicp->user1() == 2) {
logicp->user1(1); // Added to m_optimized but already folded
AstNode* const foldedp = V3Const::constifyEdit(logicp);
UASSERT_OBJ(foldedp == logicp, foldedp, "Should not remove whole logic");
}
// Can we eliminate?
const GateOkVisitor okVisitor{logicp, vvertexp->isClock(), false};
// Was it ok?