mirror of
https://github.com/verilator/verilator.git
synced 2026-10-06 01:54:01 +02:00
Improve Dfg variable removal and temporary insertion (#6401)
Combined Dfg variable elimination into the regularization pass that runs before converting back to Ast. This avoids introducing some unnecessary temporaries. Added replacing of variables with constants in the Ast if after the Dfg passes they are known to be constants. This is only done in final scoped Dfg application. Avoid introducing temporaries for common sub-expressions that are cheaper to re-compute than store in a temporary variable. Enable removal of redundant unpacked array variables. Also fixes #6394 as this patch involved changes to that code.
This commit is contained in:
Geza Lore
+30
-130
@@ -27,11 +27,18 @@ VL_DEFINE_DEBUG_FUNCTIONS;
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void V3DfgPasses::inlineVars(DfgGraph& dfg) {
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for (DfgVertexVar& vtx : dfg.varVertices()) {
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if (DfgVarPacked* const varp = vtx.cast<DfgVarPacked>()) {
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if (varp->hasSinks() && varp->isDrivenFullyByDfg()) {
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varp->replaceWith(varp->srcp());
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}
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}
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// Nothing to inline it into
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if (!vtx.hasSinks()) continue;
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// Nohting to inline
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DfgVertex* const srcp = vtx.srcp();
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if (!srcp) continue;
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// Value can differ from driver
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if (vtx.isVolatile()) continue;
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// Partial driver cannot be inlined
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if (srcp->is<DfgVertexSplice>()) continue;
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if (srcp->is<DfgUnitArray>()) continue;
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// Okie dokie, here we go ...
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vtx.replaceWith(srcp);
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}
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}
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@@ -92,7 +99,7 @@ void V3DfgPasses::removeUnused(DfgGraph& dfg) {
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}
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void V3DfgPasses::binToOneHot(DfgGraph& dfg, V3DfgBinToOneHotContext& ctx) {
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UASSERT(dfg.modulep(), "binToOneHot only works with unscoped DfgGraphs for now");
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if (!dfg.modulep()) return; // binToOneHot only works with unscoped DfgGraphs for now
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// Structure to keep track of comparison details
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struct Term final {
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@@ -330,138 +337,31 @@ void V3DfgPasses::binToOneHot(DfgGraph& dfg, V3DfgBinToOneHotContext& ctx) {
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}
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}
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void V3DfgPasses::eliminateVars(DfgGraph& dfg, V3DfgEliminateVarsContext& ctx) {
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// Worklist based algoritm
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DfgWorklist workList{dfg};
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// Add all variables to the work list
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for (DfgVertexVar& vtx : dfg.varVertices()) workList.push_front(vtx);
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// List of variables (AstVar or AstVarScope) we are replacing
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std::vector<AstNode*> replacedVariables;
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// AstVar::user2p() : AstVar* -> The replacement variables
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// AstVarScope::user2p() : AstVarScope* -> The replacement variables
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const VNUser2InUse user2InUse;
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// Whether we need to apply variable replacements
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bool doReplace = false;
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// Process the work list
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workList.foreach([&](DfgVertex& vtx) {
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// Remove unused non-variable vertices
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if (!vtx.is<DfgVertexVar>() && !vtx.hasSinks()) {
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// Add sources of removed vertex to work list
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vtx.foreachSource([&](DfgVertex& src) {
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workList.push_front(src);
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return false;
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});
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// Remove the unused vertex
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vtx.unlinkDelete(dfg);
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return;
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}
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// We can only eliminate DfgVarPacked vertices at the moment
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DfgVarPacked* const varp = vtx.cast<DfgVarPacked>();
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if (!varp) return;
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if (!varp->tmpForp()) {
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// Can't remove regular variable if it has external drivers
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if (!varp->isDrivenFullyByDfg()) return;
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} else {
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// Can't remove partially driven used temporaries
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if (!varp->isDrivenFullyByDfg() && varp->hasSinks()) return;
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}
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// Can't remove if referenced external to the module/netlist
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if (varp->hasExtRefs()) return;
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// Can't remove if written in the module
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if (varp->hasModWrRefs()) return;
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// Can't remove if referenced in other DFGs of the same module
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if (varp->hasDfgRefs()) return;
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// If it has multiple sinks, it can't be eliminated
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if (varp->hasMultipleSinks()) return;
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if (!varp->hasModRefs()) {
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// If it is only referenced in this DFG, it can be removed
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++ctx.m_varsRemoved;
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varp->replaceWith(varp->srcp());
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ctx.m_deleteps.push_back(varp->nodep()); // Delete variable at the end
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} else if (const DfgVarPacked* const driverp = varp->srcp()->cast<DfgVarPacked>()) {
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// If it's driven from another variable, it can be replaced by that.
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// Mark it for replacement
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++ctx.m_varsReplaced;
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UASSERT_OBJ(!varp->hasSinks(), varp, "Variable inlining should make this impossible");
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// Grab the AstVar/AstVarScope
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AstNode* const nodep = varp->nodep();
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UASSERT_OBJ(!nodep->user2p(), nodep, "Replacement already exists");
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doReplace = true;
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ctx.m_deleteps.push_back(nodep); // Delete variable at the end
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nodep->user2p(driverp->nodep());
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} else {
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// Otherwise this *is* the canonical var, keep it
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return;
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}
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// Add sources of redundant variable to the work list
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vtx.foreachSource([&](DfgVertex& src) {
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workList.push_front(src);
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return false;
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});
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// Remove the redundant variable
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vtx.unlinkDelete(dfg);
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});
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// Job done if no replacements possible
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if (!doReplace) return;
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// Apply variable replacements
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if (AstModule* const modp = dfg.modulep()) {
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modp->foreach([&](AstVarRef* refp) {
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AstVar* varp = refp->varp();
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while (AstVar* const replacep = VN_AS(varp->user2p(), Var)) varp = replacep;
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refp->varp(varp);
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});
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} else {
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v3Global.rootp()->foreach([&](AstVarRef* refp) {
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AstVarScope* vscp = refp->varScopep();
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while (AstVarScope* const replacep = VN_AS(vscp->user2p(), VarScope)) vscp = replacep;
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refp->varScopep(vscp);
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refp->varp(vscp->varp());
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});
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}
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}
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void V3DfgPasses::optimize(DfgGraph& dfg, V3DfgContext& ctx) {
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// There is absolutely nothing useful we can do with a graph of size 2 or less
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if (dfg.size() <= 2) return;
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int passNumber = 0;
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const auto apply = [&](int dumpLevel, const string& name, std::function<void()> pass) {
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const auto run = [&](const std::string& name, bool dump, std::function<void()> pass) {
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// Apply the pass
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pass();
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if (dumpDfgLevel() >= dumpLevel) {
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const string strippedName = VString::removeWhitespace(name);
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const string label
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= ctx.prefix() + "pass-" + cvtToStr(passNumber) + "-" + strippedName;
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dfg.dumpDotFilePrefixed(label);
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}
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// Debug dump
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if (dump) dfg.dumpDotFilePrefixed(ctx.prefix() + "opt-" + VString::removeWhitespace(name));
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// Internal type check
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if (v3Global.opt.debugCheck()) V3DfgPasses::typeCheck(dfg);
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++passNumber;
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};
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apply(3, "input ", [&]() {});
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apply(4, "inlineVars ", [&]() { inlineVars(dfg); });
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apply(4, "cse0 ", [&]() { cse(dfg, ctx.m_cseContext0); });
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if (dfg.modulep()) {
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apply(4, "binToOneHot ", [&]() { binToOneHot(dfg, ctx.m_binToOneHotContext); });
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}
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if (v3Global.opt.fDfgPeephole()) {
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apply(4, "peephole ", [&]() { peephole(dfg, ctx.m_peepholeContext); });
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// We just did CSE above, so without peephole there is no need to run it again these
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apply(4, "cse1 ", [&]() { cse(dfg, ctx.m_cseContext1); });
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}
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// Currend debug dump level
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const uint32_t dumpLvl = dumpDfgLevel();
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// Run passes
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run("input ", dumpLvl >= 3, [&]() { /* debug dump only */ });
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run("inlineVars ", dumpLvl >= 4, [&]() { inlineVars(dfg); });
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run("cse0 ", dumpLvl >= 4, [&]() { cse(dfg, ctx.m_cseContext0); });
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run("binToOneHot ", dumpLvl >= 4, [&]() { binToOneHot(dfg, ctx.m_binToOneHotContext); });
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run("peephole ", dumpLvl >= 4, [&]() { peephole(dfg, ctx.m_peepholeContext); });
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run("cse1 ", dumpLvl >= 4, [&]() { cse(dfg, ctx.m_cseContext1); });
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run("output ", dumpLvl >= 3, [&]() { /* debug dump only */ });
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// Accumulate patterns for reporting
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if (v3Global.opt.stats()) ctx.m_patternStats.accumulate(dfg);
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apply(4, "regularize", [&]() { regularize(dfg, ctx.m_regularizeContext); });
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}
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