luke
/
verilator
mirror of https://github.com/verilator/verilator.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Optimize temporary insertion in Dfg (#7459) 

Insert temporaries for highest fanout vertices first. Omit them for
cheap comparisons.
This commit is contained in:
Geza Lore 2026-04-21 18:34:26 +01:00 committed by GitHub
parent a1a8b9624c
commit 686594b4ab
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
2 changed files with 50 additions and 25 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

38
src/V3Dfg.h
View File

@ -815,9 +815,11 @@ void DfgEdge::relinkSrcp(DfgVertex* srcp) {
bool DfgVertex::isCheaperThanLoad() const {
// Constants
if (is<DfgConst>()) return true;
// Variables
if (is<DfgVertexVar>()) return true;
// Array sels are just address computation
if (is<DfgArraySel>()) return true;
// Small constant select from variable
// Small select from variable
if (const DfgSel* const selp = cast<DfgSel>()) {
if (!selp->fromp()->is<DfgVarPacked>()) return false;
if (selp->fromp()->width() <= VL_QUADSIZE) return true;
@ -828,20 +830,32 @@ bool DfgVertex::isCheaperThanLoad() const {
// Zero extend of a cheap vertex - Extend(_) was converted to Concat(0, _)
if (const DfgConcat* const catp = cast<DfgConcat>()) {
if (catp->width() > VL_QUADSIZE) return false;
const DfgConst* const lCatp = catp->lhsp()->cast<DfgConst>();
if (!lCatp) return false;
if (!lCatp->isZero()) return false;
const DfgConst* const lConstp = catp->lhsp()->cast<DfgConst>();
if (!lConstp || !lConstp->isZero()) return false;
return catp->rhsp()->isCheaperThanLoad();
}
// Reduction of a cheap vertex
if (const DfgRedOr* const redOrp = cast<DfgRedOr>()) {
return redOrp->srcp()->isCheaperThanLoad();
// Reduction of a narrow cheap vertex
if (is<DfgRedOr>() //
|| is<DfgRedAnd>() //
|| is<DfgRedXor>()) {
const DfgVertex* const srcp = as<DfgVertexUnary>()->srcp();
return srcp->width() <= VL_QUADSIZE && srcp->isCheaperThanLoad();
}
if (const DfgRedAnd* const redAndp = cast<DfgRedAnd>()) {
return redAndp->srcp()->isCheaperThanLoad();
}
if (const DfgRedXor* const redXorp = cast<DfgRedXor>()) {
return redXorp->srcp()->isCheaperThanLoad();
// Comparisons of a narrow cheap vertex with constant
if (is<DfgEq>() //
|| is<DfgNeq>() //
|| is<DfgLt>() //
|| is<DfgLte>() //
|| is<DfgGt>() //
|| is<DfgGte>() //
|| is<DfgLtS>() //
|| is<DfgLteS>() //
|| is<DfgGtS>() //
|| is<DfgGteS>()) {
const DfgVertexBinary* const binp = as<DfgVertexBinary>();
const DfgVertex* const lhsp = binp->inputp(0);
const DfgVertex* const rhsp = binp->inputp(1);
return lhsp->width() <= VL_QUADSIZE && lhsp->is<DfgConst>() && rhsp->isCheaperThanLoad();
}
// Otherwise probably not
return false;

37
src/V3DfgRegularize.cpp
View File

@ -195,24 +195,35 @@ class DfgRegularize final {
// Scope cache for below
DfgVertex::ScopeCache scopeCache;
// Ensure intermediate values used multiple times are written to variables
// Build map from fanout to list of vertices with that fanout
std::vector<std::vector<DfgVertex*>> fanout2Vtxps;
for (DfgVertex& vtx : m_dfg.opVertices()) {
// LValue vertices feed into variables eventually and need no temporaries
if (vtx.is<DfgVertexSplice>()) continue;
if (vtx.is<DfgUnitArray>()) continue;
// Add to map
const uint32_t fanout = vtx.fanout();
if (!fanout) continue;
if (fanout >= fanout2Vtxps.size()) fanout2Vtxps.resize(2 * fanout);
fanout2Vtxps[fanout].push_back(&vtx);
}
if (fanout2Vtxps.empty()) return;
if (!needsTemporary(vtx, vtx)) continue;
// Need to create an intermediate variable
++m_ctx.m_temporariesIntroduced;
const std::string name = m_dfg.makeUniqueName("Regularize", m_nTmps);
FileLine* const flp = vtx.fileline();
AstScope* const scopep = vtx.scopep(scopeCache);
DfgVertexVar* const newp = m_dfg.makeNewVar(flp, name, vtx.dtype(), scopep);
++m_nTmps;
// Replace vertex with the variable, make it drive the variable
vtx.replaceWith(newp);
newp->srcp(&vtx);
// Ensure intermediate values used multiple times are written to variables
for (size_t fanout = fanout2Vtxps.size() - 1; fanout > 0; --fanout) {
for (DfgVertex* const vtxp : fanout2Vtxps[fanout]) {
if (!needsTemporary(*vtxp, *vtxp)) continue;
// Need to create an intermediate variable
++m_ctx.m_temporariesIntroduced;
const std::string name = m_dfg.makeUniqueName("Regularize", m_nTmps);
FileLine* const flp = vtxp->fileline();
AstScope* const scopep = vtxp->scopep(scopeCache);
DfgVertexVar* const newp = m_dfg.makeNewVar(flp, name, vtxp->dtype(), scopep);
++m_nTmps;
// Replace vertex with the variable, make it drive the variable
vtxp->replaceWith(newp);
newp->srcp(vtxp);
}
}
}