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Optimize Dfg with relaxed live variable analysis (#7739)

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Relax the live variable analysis performed by Dfg to bail on fewer
cases. This analysis was already conservative (meaning it might think
variables are live when they are not), which is good enough for Dfg use.
This change in particular enables synthesizing more complex logic
involving arrays, e.g. those introduce by V3Table creating lookup
tables.
This commit is contained in:
Geza Lore
2026-06-10 15:59:44 +01:00
committed by GitHub
parent 75993ca9ea
commit d84af81a11
6 changed files with 30 additions and 26 deletions
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src/V3CfgLiveVariables.cpp
+11 -21
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@@ -51,28 +51,18 @@ class CfgLiveVariables final : VNVisitorConst {
// METHODS
// This is to match DFG, but can be extended independently - eventually should handle all
static bool isSupportedPackedDType(const AstNodeDType* dtypep) {
dtypep = dtypep->skipRefp();
static bool isPacked(const AstVarScope* vscp) {
AstNodeDType* const dtypep = vscp->dtypep()->skipRefp();
if (const AstBasicDType* const typep = VN_CAST(dtypep, BasicDType)) {
return typep->keyword().isIntNumeric();
}
if (const AstPackArrayDType* const typep = VN_CAST(dtypep, PackArrayDType)) {
return isSupportedPackedDType(typep->subDTypep());
}
if (VN_IS(dtypep, PackArrayDType)) return true;
if (const AstNodeUOrStructDType* const typep = VN_CAST(dtypep, NodeUOrStructDType)) {
return typep->packed();
}
return false;
}
// Check and return if variable is incompatible
bool incompatible(AstVarScope* vscp) {
if (!isSupportedPackedDType(vscp->dtypep())) return true;
if (vscp->varp()->ignoreSchedWrite()) return true;
return false;
}
void updateGen(AstNode* nodep) {
UASSERT_OBJ(!m_abort, nodep, "Doing useless work");
m_abort = nodep->exists([&](const AstVarRef* refp) {
@@ -80,8 +70,6 @@ class CfgLiveVariables final : VNVisitorConst {
if (refp->access().isWriteOnly()) return false;
// Grab referenced variable
AstVarScope* const vscp = refp->varScopep();
// Bail if not a compatible type
if (incompatible(vscp)) return true;
// Add to gen set, if not killed - assume whole variable read
if (m_currp->m_kill.count(vscp)) return false;
m_currp->m_gen.emplace(vscp);
@@ -96,11 +84,13 @@ class CfgLiveVariables final : VNVisitorConst {
if (refp->access().isReadOnly()) return false;
// Grab referenced variable
AstVarScope* const vscp = refp->varScopep();
// Bail if not a compatible type
if (incompatible(vscp)) return true;
// If whole written, add to kill set
if (refp->nextp()) return false;
if (VN_IS(refp->abovep(), Sel)) return false;
// Safety case: bail on variables with unusual semantics
if (vscp->varp()->ignoreSchedWrite()) return true;
// If not a packed variable, assume it's not a whole update, don't add to kill set
if (!isPacked(vscp)) return false;
// If packed, and updated partially, don't add to kill set
if (refp->backp()->nextp() != refp && VN_IS(refp->abovep(), Sel)) return false;
// Whole variable is writen, add to kill set
m_currp->m_kill.emplace(vscp);
return false;
});
@@ -198,7 +188,7 @@ public:
CfgLiveVariables analysis{cfg};
// If failed, return nullptr
if (analysis.m_abort) return nullptr;
// Gather variables live in to the entry blockstd::unique_ptr<std::vector<AstVarScope*>>
// Gather variables live in to the entry block
const std::unordered_set<AstVarScope*>& lin = analysis.m_blockState[cfg.enter()].m_liveIn;
std::vector<AstVarScope*>* const resultp
= new std::vector<AstVarScope*>{lin.begin(), lin.end()};