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DFG: make variable inlining part of the peephole optimizer 

This saves some traversals and prepares us to better handle cyclic DFGs.
This commit is contained in:
Geza Lore 2022-09-27 13:53:07 +01:00
parent 09e352ef66
commit 4a1a2def95
4 changed files with 37 additions and 51 deletions
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48
src/V3DfgPasses.cpp
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@ -75,20 +75,6 @@ V3DfgOptimizationContext::~V3DfgOptimizationContext() {
"Inconsistent statistics");
}
// 'Inline' DfgVar nodes with known drivers
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 (varVtxp->isDrivenFullyByDfg()) {
// Make consumers of the DfgVar consume the driver directly
DfgVertex* const driverp = varVtxp->source(0);
varVtxp->forEachSinkEdge([=](DfgEdge& edge) { edge.relinkSource(driverp); });
}
}
});
}
// Common subexpression elimination
void V3DfgPasses::cse(DfgGraph& dfg, V3DfgCseContext& ctx) {
DfgVertex::HashCache hashCache;
@ -183,16 +169,6 @@ 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,
// 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;
});
int passNumber = 0;
const auto apply = [&](int dumpLevel, const string name, std::function<void()> pass) {
@ -206,23 +182,15 @@ void V3DfgPasses::optimize(DfgGraph& dfg, V3DfgOptimizationContext& ctx) {
++passNumber;
};
if (!isTrivial) {
// Optimize non-trivial graph
if (dumpDfg() >= 8) { dfg.dumpDotAllVarConesPrefixed(ctx.prefix() + "input"); }
apply(3, "input ", [&]() {});
apply(4, "inlineVars ", [&]() { inlineVars(dfg); });
apply(4, "cse ", [&]() { cse(dfg, ctx.m_cseContext0); });
if (v3Global.opt.fDfgPeephole()) {
apply(4, "peephole ", [&]() { peephole(dfg, ctx.m_peepholeContext); });
}
if (dumpDfg() >= 8) dfg.dumpDotAllVarConesPrefixed(ctx.prefix() + "input");
apply(3, "input ", [&]() {});
apply(4, "cse ", [&]() { cse(dfg, ctx.m_cseContext0); });
if (v3Global.opt.fDfgPeephole()) {
apply(4, "peephole ", [&]() { peephole(dfg, ctx.m_peepholeContext); });
// Without peephole no variables will be redundant, and we just did CSE, so skip these
apply(4, "removeVars ", [&]() { removeVars(dfg, ctx.m_removeVarsContext); });
apply(4, "cse ", [&]() { cse(dfg, ctx.m_cseContext1); });
apply(3, "optimized ", [&]() { removeUnused(dfg); });
if (dumpDfg() >= 8) { dfg.dumpDotAllVarConesPrefixed(ctx.prefix() + "optimized"); }
} else {
// We can still eliminate redundancies from trivial graphs
apply(5, "trivial-input ", [&]() {});
apply(6, "trivial-inlineVars ", [&]() { inlineVars(dfg); });
apply(5, "trivial-optimized ", [&]() { removeVars(dfg, ctx.m_removeVarsContext); });
}
apply(3, "optimized ", [&]() { removeUnused(dfg); });
if (dumpDfg() >= 8) dfg.dumpDotAllVarConesPrefixed(ctx.prefix() + "optimized");
}

2
src/V3DfgPasses.h
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@ -100,8 +100,6 @@ AstModule* dfgToAst(DfgGraph&, V3DfgOptimizationContext&);
// Intermediate/internal operations
//===========================================================================
// Inline variables
void inlineVars(DfgGraph&);
// Common subexpression elimination
void cse(DfgGraph&, V3DfgCseContext&);
// Peephole optimizations

35
src/V3DfgPeephole.cpp
View File

@ -1102,19 +1102,30 @@ class V3DfgPeephole final : public DfgVisitor {
}
}
void visit(DfgVar* vtxp) override {
// Inline variables fully driven by the logic represented by the DFG
if (vtxp->hasSinks() && vtxp->isDrivenFullyByDfg()) {
APPLYING(INLINE_VAR) {
// Make consumers of the DfgVar consume the driver directly
DfgVertex* const driverp = vtxp->source(0);
vtxp->forEachSinkEdge([=](DfgEdge& edge) { edge.relinkSource(driverp); });
}
}
}
#undef APPLYING
// Process one vertex. Return true if graph changed
bool processVertex(DfgVertex& vtx) {
// Keep DfgVars in this pass, we will remove them later if they become redundant
// Note: We want to keep the original variables for non-var vertices that drive multiple
// sinks (otherwise we would need to introduce a temporary, it is better for debugging to
// keep the original variable name, if one is available), so we can't remove redundant
// variables here.
if (vtx.is<DfgVar>()) return false;
// Keep DfgVertexLValue vertices in this pass. We will remove them later if they become
// redundant. We want to keep the original variables for non-var vertices that drive
// multiple sinks (otherwise we would need to introduce a temporary, but it is better for
// debugging to keep the original variable name, if one is available), so we can't remove
// redundant variables here.
const bool keep = vtx.is<DfgVar>();
// If it has no sinks (unused), we can remove it
if (!vtx.hasSinks()) {
if (!keep && !vtx.hasSinks()) {
vtx.unlinkDelete(m_dfg);
return true;
}
@ -1122,7 +1133,15 @@ class V3DfgPeephole final : public DfgVisitor {
// Transform node
m_changed = false;
iterate(&vtx);
if (!vtx.hasSinks()) vtx.unlinkDelete(m_dfg); // If it became unused, we can remove it
// If it became unused, we can remove it
if (!keep && !vtx.hasSinks()) {
UASSERT_OBJ(m_changed, &vtx, "'m_changed' must be set if node became unused");
vtx.unlinkDelete(m_dfg);
return true;
}
// Return the changed status
return m_changed;
}

3
src/V3DfgPeephole.h
View File

@ -82,7 +82,8 @@
_FOR_EACH_DFG_PEEPHOLE_OPTIMIZATION_APPLY(macro, REPLACE_COND_WITH_THEN_BRANCH_ZERO) \
_FOR_EACH_DFG_PEEPHOLE_OPTIMIZATION_APPLY(macro, REPLACE_COND_WITH_THEN_BRANCH_ONES) \
_FOR_EACH_DFG_PEEPHOLE_OPTIMIZATION_APPLY(macro, REPLACE_COND_WITH_ELSE_BRANCH_ZERO) \
_FOR_EACH_DFG_PEEPHOLE_OPTIMIZATION_APPLY(macro, REPLACE_COND_WITH_ELSE_BRANCH_ONES)
_FOR_EACH_DFG_PEEPHOLE_OPTIMIZATION_APPLY(macro, REPLACE_COND_WITH_ELSE_BRANCH_ONES) \
_FOR_EACH_DFG_PEEPHOLE_OPTIMIZATION_APPLY(macro, INLINE_VAR)
// clang-format on