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Fix NFA rings for bounded SVA delays (#8371)

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Signed-off-by: Artur Bieniek <[email protected]>
This commit is contained in:
Artur Bieniek
2026-09-16 17:52:28 -04:00
committed by GitHub
parent 08cadc034d
commit e2caf971de
13 changed files with 433 additions and 94 deletions
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src/V3AssertNfa.cpp
+51 -72
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@@ -335,6 +335,7 @@ class SvaNfaBuilder final {
bool m_isCoverSeq = false;
// Unsupported endpoint topology must reject, not ignore, or the wait hangs
bool m_isSeqEvent = false;
bool m_inSequencePrefix = false; // Prefix composition does not preserve every endpoint
struct RangeDelayRejectInfo final {
SvaStateVertex* startp = nullptr;
@@ -585,10 +586,13 @@ class SvaNfaBuilder final {
// Build NFA for an SExpr. finalCond = RHS (not yet added as a vertex).
// isTopLevelStep: marks outermost required boolean check as rejectOnFail.
// Apply a range delay `##[M:N]` to currentp. Returns true on success. On
// failure, sets outErrorEmitted per semantic-error policy and returns false.
bool applyRangeDelay(AstDelay* delayp, AstNodeExpr* rhsExprp, SvaStateVertex*& currentp,
std::vector<SvaStateVertex*>& midSources, FileLine* flp,
bool& outErrorEmitted, RangeDelayRejectInfo* rangeRejectInfop = nullptr) {
// failure, emits a diagnostic and returns false.
bool applyRangeDelay(AstSExpr* sexprp, SvaStateVertex*& currentp,
std::vector<SvaStateVertex*>& midSources,
RangeDelayRejectInfo* rangeRejectInfop = nullptr) {
FileLine* const flp = sexprp->fileline();
AstDelay* const delayp = VN_AS(sexprp->delayp(), Delay);
AstNodeExpr* const rhsExprp = sexprp->exprp();
const unsigned minDelay = getConstUInt(delayp->lhsp());
if (delayp->isUnbounded()) {
// `##[M:$]`: wait M cycles, then retain every matured attempt in a
@@ -613,62 +617,42 @@ class SvaNfaBuilder final {
}
const unsigned range = maxDelay - minDelay;
currentp = addDelayChain(currentp, minDelay, flp);
// kChainLimit bounds per-attempt unrolled vertices. Above this, a
// ring buffer (constant-size state) is used instead, so the vertex
// count is O(1) in range regardless of user input; no adversarial N
// blowup is possible.
constexpr unsigned kChainLimit = 256;
// IEEE 1800-2023 16.14.3: only a small bounded range before a plain
// boolean enumerates every end-of-match below. The large-range ring uses
// first-match clearing and the nested-sequence merge collapses ends, so
// reject those for a cover sequence rather than under-count.
if (m_isCoverSeq && (range > kChainLimit || VN_IS(rhsExprp, SExpr))) {
const bool multiCycleRhs = rhsExprp->isMultiCycleSva();
const AstNodeExpr* const preExprp = sexprp->preExprp();
// Prefix and nested-sequence composition can lose endpoints or their multiplicity
// required by cover sequence (IEEE 1800-2023 16.14.3), regardless of the range width.
if (m_isCoverSeq
&& (multiCycleRhs || m_inSequencePrefix
|| (preExprp && preExprp->isMultiCycleSva()))) {
warnEndpointUnsupported(flp, "this ranged cycle delay");
outErrorEmitted = true;
return false;
}
if (range > kChainLimit) {
currentp = addDelayChain(currentp, range + 1U, flp, false,
rhsExprp->isMultiCycleSva() ? nullptr : rhsExprp);
} else if (VN_IS(rhsExprp, SExpr)) {
// Nested-SExpr RHS: merge all [M,N] positions. Candidate-local misses
// are not assertion rejects while a later position can still match.
if (m_inSequencePrefix) {
flp->v3warn(E_UNSUPPORTED,
"Unsupported: Bounded ranged cycle delay in a sequence prefix");
return false;
}
if (multiCycleRhs) {
// Launch a candidate at every eligible tick. Candidate-local misses are not
// assertion rejects while a later position can still match.
if (rangeRejectInfop) {
const int rhsLen = fixedLength(rhsExprp);
if (rhsLen >= 0) *rangeRejectInfop = {currentp, range, rhsLen};
}
SvaStateVertex* const tailp = addDelayChain(currentp, range + 1U, flp, false);
SvaStateVertex* const mergeVtxp = scopedCreateVertex();
mergeVtxp->m_isUnbounded = true;
guardedLink(currentp, mergeVtxp, flp);
for (unsigned i = 0; i < range; ++i) {
SvaStateVertex* const nextVtxp = scopedCreateVertex();
guardedEdge(currentp, nextVtxp, flp);
guardedLink(nextVtxp, mergeVtxp, flp);
currentp = nextVtxp;
}
guardedLink(tailp, mergeVtxp, flp);
currentp = mergeVtxp;
m_inUnboundedScope = true;
} else {
// Pure boolean RHS: register chain. Each mid-position links to
// match (match-only); last position is the reject source.
// For cover_sequence (IEEE 1800-2023 16.14.3) the advance edge is
// unconditional so every (start, end) pair fires independently --
// dropping NOT(b) turns "first-match-wins" into "every end fires".
AstVar* const hoistVarp
= m_isCoverSeq ? nullptr : tryHoistSampled(rhsExprp, flp, range);
// The first eligible tick is explicit. The ring retains attempts for the
// remaining ticks and exposes the final tick for rejection. Only properties
// clear on success, since cover sequence counts every endpoint.
midSources.push_back(currentp);
for (unsigned i = 0; i < range; ++i) {
SvaStateVertex* const nextVtxp = scopedCreateVertex();
if (m_isCoverSeq) {
guardedEdge(currentp, nextVtxp, flp);
} else {
AstNodeExpr* const notExprp
= new AstLogNot{flp, sampledRefOrClone(hoistVarp, rhsExprp, flp)};
guardedEdge(currentp, nextVtxp, notExprp, flp);
}
if (i < range - 1) midSources.push_back(nextVtxp);
currentp = nextVtxp;
}
currentp = addDelayChain(currentp, range + 1U, flp, false,
m_isCoverSeq ? nullptr : rhsExprp);
}
return true;
}
@@ -687,14 +671,11 @@ class SvaNfaBuilder final {
= result.finalCondp ? sampled(result.finalCondp->cloneTreePure(false)) : nullptr;
SvaStateVertex* const successNowp = scopedCreateVertex();
guardedLink(srcp, successNowp, condp, flp);
SvaStateVertex* stagep = successNowp;
guardedLink(stagep, expiryMatchp, flp);
for (unsigned i = 0; i < info.range; ++i) {
SvaStateVertex* const nextp = scopedCreateVertex();
guardedEdge(stagep, nextp, flp);
stagep = nextp;
guardedLink(stagep, expiryMatchp, flp);
}
// Retain a success through the latest deadline it can satisfy.
SvaStateVertex* const historyp
= addDelayChain(successNowp, info.range + 1U, flp, false);
guardedLink(successNowp, expiryMatchp, flp);
guardedLink(historyp, expiryMatchp, flp);
}
SvaStateVertex* const sinkVtxp = m_graph.createStateVertex();
@@ -715,6 +696,8 @@ class SvaNfaBuilder final {
// Handle LHS (preExpr)
SvaStateVertex* currentp = entryVtxp;
if (AstNodeExpr* const preExprp = sexprp->preExprp()) {
VL_RESTORER(m_inSequencePrefix);
m_inSequencePrefix = true;
const BuildResult pre = buildExpr(preExprp, currentp, isTopLevelStep);
if (!pre.valid()) return BuildResult::fail(pre.errorEmitted); // LCOV_EXCL_LINE
if (pre.finalCondp) {
@@ -737,10 +720,9 @@ class SvaNfaBuilder final {
RangeDelayRejectInfo rangeRejectInfo;
const bool addRangeReject = isTopLevelStep && !m_inUnboundedScope;
if (delayp->isRangeDelay()) {
bool errorEmitted = false;
if (!applyRangeDelay(delayp, sexprp->exprp(), currentp, rangeMidSources, flp,
errorEmitted, addRangeReject ? &rangeRejectInfo : nullptr)) {
return BuildResult::fail(errorEmitted);
if (!applyRangeDelay(sexprp, currentp, rangeMidSources,
addRangeReject ? &rangeRejectInfo : nullptr)) {
return BuildResult::failWithError();
}
} else {
const unsigned delayCycles = getConstUInt(delayp->lhsp());
@@ -3343,10 +3325,10 @@ class AssertNfaVisitor final : public VNVisitor {
// Recursively walk a consequent. Returns cycle length consumed and
// substitutes each VarRef to a captured local var with $past(rhs, K)
// (or rhs inline when K == 0). Reports E_UNSUPPORTED on non-constant
// delays or composite sequence operators.
int walkSubstituteMatchItems(AstNodeExpr* nodep, unsigned K,
const std::unordered_map<const AstVar*, AstNodeExpr*>& matchItems,
bool& errorEmitted) {
// delays or composite sequence operators and returns -1 on failure.
int
walkSubstituteMatchItems(AstNodeExpr* nodep, unsigned K,
const std::unordered_map<const AstVar*, AstNodeExpr*>& matchItems) {
if (AstSExpr* const sexprp = VN_CAST(nodep, SExpr)) {
// IEEE 1800-2023 16.9.2: cycle_delay's lhsp is a constant_expression
// and the delay form in a sequence is always `##N`, folded by
@@ -3359,25 +3341,23 @@ class AssertNfaVisitor final : public VNVisitor {
sexprp->v3warn(E_UNSUPPORTED, "Unsupported: property local variable used across "
"non-constant cycle delay in consequent"
" (IEEE 1800-2023 16.10)");
errorEmitted = true;
return -1;
}
const unsigned delayCycles = VN_AS(delayp->lhsp(), Const)->toUInt();
int preLen = 0;
if (AstNodeExpr* const prep = sexprp->preExprp()) {
preLen = walkSubstituteMatchItems(prep, K, matchItems, errorEmitted);
if (errorEmitted) return -1;
preLen = walkSubstituteMatchItems(prep, K, matchItems);
if (preLen < 0) return -1;
}
const int bodyLen = walkSubstituteMatchItems(sexprp->exprp(), K + preLen + delayCycles,
matchItems, errorEmitted);
if (errorEmitted) return -1;
const int bodyLen
= walkSubstituteMatchItems(sexprp->exprp(), K + preLen + delayCycles, matchItems);
if (bodyLen < 0) return -1;
return preLen + delayCycles + bodyLen;
}
if (nodep->isMultiCycleSva()) {
nodep->v3warn(E_UNSUPPORTED, "Unsupported: property local variable used across "
"composite sequence operator in consequent"
" (IEEE 1800-2023 16.10)");
errorEmitted = true;
return -1;
}
std::vector<AstVarRef*> refs;
@@ -3409,12 +3389,11 @@ class AssertNfaVisitor final : public VNVisitor {
matchItems[lhsRefp->varp()] = assignp->rhsp();
}
const unsigned startK = parts.isOverlapped ? 0 : 1;
bool errorEmitted = false;
walkSubstituteMatchItems(seqBodyp, startK, matchItems, errorEmitted);
const int length = walkSubstituteMatchItems(seqBodyp, startK, matchItems);
// Match-item substitution / strip mutates ancestor purity. Release
// builds don't auto-clear caches on edits, so refresh here.
VIsCached::clearCacheTree();
if (errorEmitted) return true;
if (length < 0) return true;
AstNodeExpr* const antBoolp = exprStmtp->resultp()->unlinkFrBack();
exprStmtp->replaceWith(antBoolp);
VL_DO_DANGLING(pushDeletep(exprStmtp), exprStmtp);