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Optimize DFG partial assignments (#6176)

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This is mostly a refactoring, but also enables handling some more
UNOPTFLAT, when the variable is only partially assigned in the cycle.

Previously the way partial assignments to variables were handled were
through the DfgVerexVar types themselves, which kept track of all
drivers. This has been replaced by DfgVertexSplice (which always drives
a DfgVeretexVar), and all DfgVertexVar now only have a single source,
either a DfgVertexSplice, if partially assigned, or an arbitrary
DfgVertex when updated as a whole.
This commit is contained in:
Geza Lore
2025-07-14 17:09:34 -04:00
committed by GitHub
parent e2e5d9eaf1
commit 03e0d49d99
11 changed files with 399 additions and 307 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/V3DfgAstToDfg.cpp
+75 -43
View File
@@ -183,49 +183,37 @@ class AstToDfgVisitor final : public VNVisitor {
return m_foundUnhandled;
}
// Build DfgEdge representing the LValue assignment. Returns false if unsuccessful.
bool convertAssignment(FileLine* flp, AstNode* nodep, DfgVertex* vtxp) {
DfgVertexSplice* convertLValue(AstNode* nodep) {
FileLine* const flp = nodep->fileline();
if (AstVarRef* const vrefp = VN_CAST(nodep, VarRef)) {
m_foundUnhandled = false;
visit(vrefp);
// cppcheck-has-bug-suppress knownConditionTrueFalse
if (m_foundUnhandled) return false;
getVertex(vrefp)->template as<DfgVarPacked>()->addDriver(flp, 0, vtxp);
return true;
}
if (AstSel* const selp = VN_CAST(nodep, Sel)) {
AstVarRef* const vrefp = VN_CAST(selp->fromp(), VarRef);
const AstConst* const lsbp = VN_CAST(selp->lsbp(), Const);
if (!vrefp || !lsbp) {
++m_ctx.m_nonRepLhs;
return false;
if (m_foundUnhandled) return nullptr;
DfgVertexVar* const vtxp = getVertex(vrefp)->template as<DfgVertexVar>();
// Ensure driving splice vertex exists
if (!vtxp->srcp()) {
if (VN_IS(vtxp->dtypep(), UnpackArrayDType)) {
vtxp->srcp(new DfgSpliceArray{*m_dfgp, flp, vtxp->dtypep()});
} else {
vtxp->srcp(new DfgSplicePacked{*m_dfgp, flp, vtxp->dtypep()});
}
}
m_foundUnhandled = false;
visit(vrefp);
// cppcheck-has-bug-suppress knownConditionTrueFalse
if (m_foundUnhandled) return false;
getVertex(vrefp)->template as<DfgVarPacked>()->addDriver(flp, lsbp->toUInt(), vtxp);
return true;
}
if (AstArraySel* const selp = VN_CAST(nodep, ArraySel)) {
AstVarRef* const vrefp = VN_CAST(selp->fromp(), VarRef);
const AstConst* const idxp = VN_CAST(selp->bitp(), Const);
if (!vrefp || !idxp) {
++m_ctx.m_nonRepLhs;
return false;
}
m_foundUnhandled = false;
visit(vrefp);
// cppcheck-has-bug-suppress knownConditionTrueFalse
if (m_foundUnhandled) return false;
getVertex(vrefp)->template as<DfgVarArray>()->addDriver(flp, idxp->toUInt(), vtxp);
return true;
return vtxp->srcp()->as<DfgVertexSplice>();
}
++m_ctx.m_nonRepLhs;
return nullptr;
}
// Build DfgEdge representing the LValue assignment. Returns false if unsuccessful.
bool convertAssignment(FileLine* flp, AstNode* nodep, DfgVertex* vtxp) {
// Concatenation on the LHS. Select parts of the driving 'vtxp' then convert each part
if (AstConcat* const concatp = VN_CAST(nodep, Concat)) {
AstNode* const lhsp = concatp->lhsp();
AstNode* const rhsp = concatp->rhsp();
{
{ // Convet LHS of concat
FileLine* const lFlp = lhsp->fileline();
DfgSel* const lVtxp = new DfgSel{*m_dfgp, lFlp, DfgVertex::dtypeFor(lhsp)};
lVtxp->fromp(vtxp);
@@ -233,7 +221,7 @@ class AstToDfgVisitor final : public VNVisitor {
if (!convertAssignment(flp, lhsp, lVtxp)) return false;
}
{
{ // Convert RHS of concat
FileLine* const rFlp = rhsp->fileline();
DfgSel* const rVtxp = new DfgSel{*m_dfgp, rFlp, DfgVertex::dtypeFor(rhsp)};
rVtxp->fromp(vtxp);
@@ -241,7 +229,37 @@ class AstToDfgVisitor final : public VNVisitor {
return convertAssignment(flp, rhsp, rVtxp);
}
}
++m_ctx.m_nonRepLhs;
if (AstSel* const selp = VN_CAST(nodep, Sel)) {
AstVarRef* const vrefp = VN_CAST(selp->fromp(), VarRef);
const AstConst* const lsbp = VN_CAST(selp->lsbp(), Const);
if (!vrefp || !lsbp) {
++m_ctx.m_nonRepLhs;
return false;
}
if (DfgVertexSplice* const splicep = convertLValue(vrefp)) {
splicep->template as<DfgSplicePacked>()->addDriver(flp, lsbp->toUInt(), vtxp);
return true;
}
} else if (AstArraySel* const selp = VN_CAST(nodep, ArraySel)) {
AstVarRef* const vrefp = VN_CAST(selp->fromp(), VarRef);
const AstConst* const idxp = VN_CAST(selp->bitp(), Const);
if (!vrefp || !idxp) {
++m_ctx.m_nonRepLhs;
return false;
}
if (DfgVertexSplice* const splicep = convertLValue(vrefp)) {
splicep->template as<DfgSpliceArray>()->addDriver(flp, idxp->toUInt(), vtxp);
return true;
}
} else if (VN_IS(nodep, VarRef)) {
if (DfgVertexSplice* const splicep = convertLValue(nodep)) {
splicep->template as<DfgSplicePacked>()->addDriver(flp, 0, vtxp);
return true;
}
} else {
++m_ctx.m_nonRepLhs;
}
return false;
}
@@ -315,18 +333,23 @@ class AstToDfgVisitor final : public VNVisitor {
for (DfgVarPacked* const varp : m_varPackedps) {
// Delete variables with no sinks nor sources (this can happen due to reverting
// uncommitted vertices, which does not remove variables)
if (!varp->hasSinks() && varp->arity() == 0) {
if (!varp->hasSinks() && !varp->srcp()) {
VL_DO_DANGLING(varp->unlinkDelete(*m_dfgp), varp);
continue;
}
// Nothing to do for un-driven (input) variables
if (!varp->srcp()) continue;
DfgSplicePacked* const splicep = varp->srcp()->as<DfgSplicePacked>();
// Gather (and unlink) all drivers
std::vector<Driver> drivers;
drivers.reserve(varp->arity());
varp->forEachSourceEdge([this, varp, &drivers](DfgEdge& edge, size_t idx) {
drivers.reserve(splicep->arity());
splicep->forEachSourceEdge([this, splicep, &drivers](DfgEdge& edge, size_t idx) {
DfgVertex* const driverp = edge.sourcep();
UASSERT(driverp, "Should not have created undriven sources");
addDriver(varp->driverFileLine(idx), varp->driverLsb(idx), driverp, drivers);
addDriver(splicep->driverFileLine(idx), splicep->driverLsb(idx), driverp, drivers);
edge.unlinkSource();
});
@@ -424,10 +447,10 @@ class AstToDfgVisitor final : public VNVisitor {
}
// Reinsert drivers in order
varp->resetSources();
splicep->resetSources();
for (const Driver& driver : drivers) {
if (!driver.m_vtxp) break; // Stop at end of compacted list
varp->addDriver(driver.m_fileline, driver.m_lsb, driver.m_vtxp);
splicep->addDriver(driver.m_fileline, driver.m_lsb, driver.m_vtxp);
}
// Prune vertices potentially unused due to resolving multiple drivers.
@@ -442,6 +465,15 @@ class AstToDfgVisitor final : public VNVisitor {
vtxp->forEachSource([&](DfgVertex& src) { prune.emplace(&src); });
vtxp->unlinkDelete(*m_dfgp);
}
// If the whole variable is driven, remove the splice node
if (splicep->arity() == 1 //
&& splicep->driverLsb(0) == 0 //
&& splicep->source(0)->width() == varp->width()) {
varp->srcp(splicep->source(0));
varp->driverFileLine(splicep->driverFileLine(0));
splicep->unlinkDelete(*m_dfgp);
}
}
}
@@ -450,7 +482,7 @@ class AstToDfgVisitor final : public VNVisitor {
for (DfgVarArray* const varp : m_varArrayps) {
// Delete variables with no sinks nor sources (this can happen due to reverting
// uncommitted vertices, which does not remove variables)
if (!varp->hasSinks() && varp->arity() == 0) {
if (!varp->hasSinks() && !varp->srcp()) {
VL_DO_DANGLING(varp->unlinkDelete(*m_dfgp), varp);
}
}