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DFG: Support AstSel and AstConcat on LHS of assignments

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Added DfgVertexVariadic to represent DFG vetices with a varying number
of source operands. Converted DfgVar to be a variadic vertex, with each
driver corresponding to a fixed range of bits in the packed variable.
This allows us to handle AstSel on the LHS of assignments. Also added
support for AstConcat on the LHS by selecting into the RHS as
appropriate.

This improves OpenTitan ST speed by ~13%
This commit is contained in:
Geza Lore
2022-09-26 19:54:52 +01:00
parent 9c1cc5465d
commit 1b17acdb01
6 changed files with 406 additions and 141 deletions
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src/V3DfgPasses.cpp
+13 -5
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@@ -53,6 +53,7 @@ V3DfgOptimizationContext::V3DfgOptimizationContext(const std::string& label)
V3DfgOptimizationContext::~V3DfgOptimizationContext() {
const string prefix = "Optimizations, DFG " + m_label + " ";
V3Stats::addStat(prefix + "General, modules", m_modules);
V3Stats::addStat(prefix + "Ast2Dfg, coalesced assignments", m_coalescedAssignments);
V3Stats::addStat(prefix + "Ast2Dfg, input equations", m_inputEquations);
V3Stats::addStat(prefix + "Ast2Dfg, representable", m_representable);
V3Stats::addStat(prefix + "Ast2Dfg, non-representable (dtype)", m_nonRepDType);
@@ -79,8 +80,9 @@ void V3DfgPasses::inlineVars(DfgGraph& dfg) {
dfg.forEachVertex([](DfgVertex& vtx) {
// For each DfgVar that has a known driver
if (DfgVar* const varVtxp = vtx.cast<DfgVar>()) {
if (DfgVertex* const driverp = varVtxp->driverp()) {
if (varVtxp->isDrivenFullyByDfg()) {
// Make consumers of the DfgVar consume the driver directly
DfgVertex* const driverp = varVtxp->source(0);
varVtxp->forEachSinkEdge([=](DfgEdge& edge) { edge.relinkSource(driverp); });
}
}
@@ -123,7 +125,10 @@ void V3DfgPasses::removeVars(DfgGraph& dfg, DfgRemoveVarsContext& ctx) {
if (varp->hasSinks()) return;
// Can't remove if read in the module and driven here (i.e.: it's an output of the DFG)
if (varp->hasModRefs() && varp->driverp()) return;
if (varp->hasModRefs() && varp->isDrivenByDfg()) return;
// Can't remove if only partially driven by the DFG
if (varp->isDrivenByDfg() && !varp->isDrivenFullyByDfg()) return;
// Can't remove if referenced externally, or other special reasons
if (varp->keep()) return;
@@ -131,7 +136,8 @@ void V3DfgPasses::removeVars(DfgGraph& dfg, DfgRemoveVarsContext& ctx) {
// If the driver of this variable has multiple non-variable sinks, then we would need
// a temporary when rendering the graph. Instead of introducing a temporary, keep the
// first variable that is driven by that driver
if (DfgVertex* const driverp = varp->driverp()) {
if (varp->isDrivenByDfg()) {
DfgVertex* const driverp = varp->source(0);
unsigned nonVarSinks = 0;
const DfgVar* firstSinkVarp = nullptr;
const bool keepFirst = driverp->findSink<DfgVertex>([&](const DfgVertex& sink) {
@@ -147,7 +153,7 @@ void V3DfgPasses::removeVars(DfgGraph& dfg, DfgRemoveVarsContext& ctx) {
if (keepFirst && firstSinkVarp == varp) return;
}
// OK, we can delete this DfgVar!
// OK, we can delete this DfgVar
++ctx.m_removed;
// If not referenced outside the DFG, then also delete the referenced AstVar,
@@ -177,11 +183,13 @@ void V3DfgPasses::optimize(DfgGraph& dfg, V3DfgOptimizationContext& ctx) {
// There is absolutely nothing useful we can do with a graph of size 2 or less
if (dfg.size() <= 2) return;
// We consider a DFG trivial if it contains no more than 1 non-variable, non-constant vertex
// We consider a DFG trivial if it contains no more than 1 non-variable, non-constant vertex,
// or if if it contains a DfgConcat, which can be introduced through assinment coalescing.
unsigned excitingVertices = 0;
const bool isTrivial = !dfg.findVertex<DfgVertex>([&](const DfgVertex& vtx) { //
if (vtx.is<DfgVar>()) return false;
if (vtx.is<DfgConst>()) return false;
if (vtx.is<DfgConcat>()) return true;
return ++excitingVertices >= 2;
});