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Optimize combinational cycles through arrays in DFG (#6210) 

Extending V3DfgBreakCycles to handle common cases involving unpacked
arrays.
This commit is contained in:
Geza Lore 2025-07-22 08:23:45 +01:00 committed by GitHub
parent b5126a6abe
commit 9f04ee68c8
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3 changed files with 197 additions and 57 deletions
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223
src/V3DfgBreakCycles.cpp
View File

@ -385,6 +385,27 @@ class TraceDriver final : public DfgVisitor {
}
}
void visit(DfgArraySel* vtxp) override {
// Only constant select
DfgConst* const idxp = vtxp->bitp()->cast<DfgConst>();
if (!idxp) return;
// From a variable
DfgVarArray* varp = vtxp->fromp()->cast<DfgVarArray>();
if (!varp) return;
// Skip through intermediate variables
while (varp->srcp() && varp->srcp()->is<DfgVarArray>()) {
varp = varp->srcp()->as<DfgVarArray>();
}
// Find driver
if (!varp->srcp()) return;
DfgSpliceArray* const splicep = varp->srcp()->cast<DfgSpliceArray>();
if (!splicep) return;
DfgVertex* const driverp = splicep->driverAt(idxp->toSizeT());
if (!driverp) return;
// Trace the driver
SET_RESULT(trace(driverp, m_msb, m_lsb));
}
void visit(DfgConcat* vtxp) override {
DfgVertex* const rhsp = vtxp->rhsp();
DfgVertex* const lhsp = vtxp->lhsp();
@ -629,6 +650,27 @@ class IndependentBits final : public DfgVisitor {
if (DfgVertex* const srcp = vtxp->srcp()) MASK(vtxp) = MASK(srcp);
}
void visit(DfgArraySel* vtxp) override {
// Only constant select
DfgConst* const idxp = vtxp->bitp()->cast<DfgConst>();
if (!idxp) return;
// From a variable
DfgVarArray* varp = vtxp->fromp()->cast<DfgVarArray>();
if (!varp) return;
// Skip through intermediate variables
while (varp->srcp() && varp->srcp()->is<DfgVarArray>()) {
varp = varp->srcp()->as<DfgVarArray>();
}
// Find driver
if (!varp->srcp()) return;
DfgSpliceArray* const splicep = varp->srcp()->cast<DfgSpliceArray>();
if (!splicep) return;
DfgVertex* const driverp = splicep->driverAt(idxp->toSizeT());
if (!driverp) return;
// Update mask
MASK(vtxp) = MASK(driverp);
}
void visit(DfgConcat* vtxp) override {
const DfgVertex* const rhsp = vtxp->rhsp();
const DfgVertex* const lhsp = vtxp->lhsp();
@ -801,40 +843,78 @@ public:
};
class FixUpSelDrivers final {
public:
// Attempt to push Sel form Var through to the driving
// expression of the selected bits. This can fix things like
// 'a[1:0] = foo', 'a[2] = a[1]', which are somewhat common.
// Returns the number of drivers fixed up.
static size_t apply(DfgGraph& dfg, DfgVarPacked* varp) {
UINFO(9, "FixUpSelDrivers of " << varp->nodep()->name());
const uint32_t component = varp->getUser<uint32_t>();
static size_t fixUpSelSinks(DfgGraph& dfg, DfgVertex* vtxp) {
size_t nImprovements = 0;
varp->forEachSink([&](DfgVertex& sink) {
// Ignore if sink is not part of cycle
const uint32_t component = vtxp->getUser<uint32_t>();
vtxp->forEachSink([&](DfgVertex& sink) {
// Ignore if sink is not part of same cycle
if (sink.getUser<uint32_t>() != component) return;
// Only Handle Sels now
// Only handle Sel
DfgSel* const selp = sink.cast<DfgSel>();
if (!selp) return;
// Try to find of the driver of the selected bits outside the cycle
DfgVertex* const fixp
= TraceDriver::apply(dfg, varp, selp->lsb(), selp->width(), false);
= TraceDriver::apply(dfg, vtxp, selp->lsb(), selp->width(), false);
if (!fixp) return;
// Found an out-of-cycle driver. We can replace this sel with that.
selp->replaceWith(fixp);
selp->unlinkDelete(dfg);
++nImprovements;
});
return nImprovements;
}
static size_t fixUpArraySelSinks(DfgGraph& dfg, DfgVertex* vtxp) {
size_t nImprovements = 0;
const uint32_t component = vtxp->getUser<uint32_t>();
vtxp->forEachSink([&](DfgVertex& sink) {
// Ignore if sink is not part of same cycle
if (sink.getUser<uint32_t>() != component) return;
// Only handle ArraySels
DfgArraySel* const aselp = sink.cast<DfgArraySel>();
if (!aselp) return;
// First, try to fix up the whole word
DfgVertex* const fixp = TraceDriver::apply(dfg, aselp, 0, aselp->width(), false);
if (fixp) {
// Found an out-of-cycle driver for the whole ArraySel
aselp->replaceWith(fixp);
aselp->unlinkDelete(dfg);
++nImprovements;
} else {
// Attempt to fix up piece-wise at Sels applied to the ArraySel
nImprovements += fixUpSelSinks(dfg, aselp);
// Remove if became unused
if (!aselp->hasSinks()) aselp->unlinkDelete(dfg);
}
});
return nImprovements;
}
public:
// Attempt to push Sel form Var through to the driving
// expression of the selected bits. This can fix things like
// 'a[1:0] = foo', 'a[2] = a[1]', which are somewhat common.
// Returns the number of drivers fixed up.
static size_t apply(DfgGraph& dfg, DfgVertexVar* varp) {
UINFO(9, "FixUpSelDrivers of " << varp->nodep()->name());
size_t nImprovements = 0;
if (varp->is<DfgVarPacked>()) {
// For Packed variables, fix up all Sels applied to it
nImprovements += fixUpSelSinks(dfg, varp);
} else if (varp->is<DfgVarArray>()) {
// For Array variables, fix up each ArraySel applied to it
nImprovements += fixUpArraySelSinks(dfg, varp);
}
UINFO(9, "FixUpSelDrivers made " << nImprovements << " improvements");
return nImprovements;
}
};
class FixUpIndependentRanges final {
// Returns a bitmask set if that bit of 'varp' is used (has a sink)
static V3Number computeUsedBits(DfgVarPacked* varp) {
V3Number result{varp->fileline(), static_cast<int>(varp->width()), 0};
varp->forEachSink([&](DfgVertex& sink) {
// Returns a bitmask set if that bit of 'vtxp' is used (has a sink)
static V3Number computeUsedBits(DfgVertex* vtxp) {
V3Number result{vtxp->fileline(), static_cast<int>(vtxp->width()), 0};
vtxp->forEachSink([&result](DfgVertex& sink) {
// If used via a Sel, mark the selected bits used
if (DfgSel* const selp = sink.cast<DfgSel>()) {
uint32_t lsb = selp->lsb();
@ -842,19 +922,29 @@ class FixUpIndependentRanges final {
for (uint32_t i = lsb; i <= msb; ++i) result.setBit(i, '1');
return;
}
// Used without a Sel, so all bits are used
result.setAllBits1();
});
return result;
}
// Trace drivers of independent bits of 'varp' in the range '[hi:lo]'
static std::string debugStr(DfgVertex* vtxp) {
if (DfgArraySel* const aselp = vtxp->cast<DfgArraySel>()) {
const size_t i = aselp->bitp()->as<DfgConst>()->toSizeT();
return debugStr(aselp->fromp()) + "[" + std::to_string(i) + "]";
}
if (DfgVertexVar* const varp = vtxp->cast<DfgVertexVar>()) {
return varp->nodep()->name();
}
vtxp->v3fatalSrc("Unhandled node type"); // LCOV_EXCL_LINE
}
// Trace drivers of independent bits of 'vtxp' in the range '[hi:lo]'
// append replacement terms to 'termps'. Returns number of successful
// replacements.
static size_t gatherTerms(std::vector<DfgVertex*>& termps, DfgGraph& dfg,
DfgVarPacked* const varp, const V3Number& indpBits,
const uint32_t hi, const uint32_t lo) {
DfgVertex* const vtxp, const V3Number& indpBits, const uint32_t hi,
const uint32_t lo) {
size_t nImprovements = 0;
// Iterate through consecutive dependent/non-dependet ranges within [hi:lo]
bool isIndependent = indpBits.bitIs1(lo);
@ -866,19 +956,19 @@ class FixUpIndependentRanges final {
DfgVertex* termp = nullptr;
// If the range is not self-dependent, attempt to trace its driver
if (isIndependent) {
termp = TraceDriver::apply(dfg, varp, lsb, width, true);
termp = TraceDriver::apply(dfg, vtxp, lsb, width, true);
if (termp) {
++nImprovements;
} else {
UINFO(5, "TraceDriver of independent range failed for "
<< varp->nodep()->name() << "[" << msb << ":" << lsb << "]");
<< debugStr(vtxp) << "[" << msb << ":" << lsb << "]");
}
}
// Fall back on using the part of the variable (if dependent, or trace failed)
if (!termp) {
AstNodeDType* const dtypep = DfgVertex::dtypeForWidth(width);
DfgSel* const selp = new DfgSel{dfg, varp->fileline(), dtypep};
// Do not connect selp->fromp yet, need to do afer replacing 'varp'
DfgSel* const selp = new DfgSel{dfg, vtxp->fileline(), dtypep};
// Do not connect selp->fromp yet, need to do afer replacing 'vtxp'
selp->lsb(lsb);
termp = selp;
}
@ -891,41 +981,37 @@ class FixUpIndependentRanges final {
return nImprovements;
}
public:
// Similar to FixUpSelDrivers, but first comptute which bits of the
// variable are self dependent, and fix up those that are independent
// but used.
static size_t apply(DfgGraph& dfg, DfgVarPacked* varp) {
UINFO(9, "FixUpIndependentRanges of " << varp->nodep()->name());
static size_t fixUpPacked(DfgGraph& dfg, DfgVertex* vtxp) {
UASSERT_OBJ(VN_IS(vtxp->dtypep(), BasicDType), vtxp, "Should be a packed BasicDType");
size_t nImprovements = 0;
// Figure out which bits of 'varp' are used
const V3Number usedBits = computeUsedBits(varp);
UINFO(9, "Used bits of '" << varp->nodep()->name() << "' are "
<< usedBits.displayed(varp->nodep(), "%b"));
// Figure out which bits of 'vtxp' are used
const V3Number usedBits = computeUsedBits(vtxp);
UINFO(9, "Used bits of '" << debugStr(vtxp) << "' are "
<< usedBits.displayed(vtxp->fileline(), "%b"));
// Nothing to do if no bits are used
if (usedBits.isEqZero()) return 0;
// Figure out which bits of 'varp' are dependent on themselves
const V3Number indpBits = IndependentBits::apply(dfg, varp);
UINFO(9, "Independent bits of '" << varp->nodep()->name() << "' are "
<< indpBits.displayed(varp->nodep(), "%b"));
// Figure out which bits of 'vtxp' are dependent of themselves
const V3Number indpBits = IndependentBits::apply(dfg, vtxp);
UINFO(9, "Independent bits of '" << debugStr(vtxp) << "' are "
<< indpBits.displayed(vtxp->fileline(), "%b"));
// Can't do anything if all bits are dependent
if (indpBits.isEqZero()) return 0;
{
// Nothing to do if no used bits are independen (all used bits are dependent)
V3Number usedAndIndependent{varp->fileline(), static_cast<int>(varp->width()), 0};
V3Number usedAndIndependent{vtxp->fileline(), static_cast<int>(vtxp->width()), 0};
usedAndIndependent.opAnd(usedBits, indpBits);
if (usedAndIndependent.isEqZero()) return 0;
}
// We are computing the terms to concatenate and replace 'varp' with
// We are computing the terms to concatenate and replace 'vtxp' with
std::vector<DfgVertex*> termps;
// Iterate through consecutive used/unused ranges
FileLine* const flp = varp->fileline();
const uint32_t width = varp->width();
FileLine* const flp = vtxp->fileline();
const uint32_t width = vtxp->width();
bool isUsed = usedBits.bitIs1(0); // Is current range used
uint32_t lsb = 0; // LSB of current range
for (uint32_t msb = 0; msb < width; ++msb) {
@ -933,7 +1019,7 @@ public:
if (!endRange) continue;
if (isUsed) {
// The range is used, compute the replacement terms
nImprovements += gatherTerms(termps, dfg, varp, indpBits, msb, lsb);
nImprovements += gatherTerms(termps, dfg, vtxp, indpBits, msb, lsb);
} else {
// The range is not used, just use constant 0 as a placeholder
termps.emplace_back(new DfgConst{dfg, flp, msb - lsb + 1});
@ -957,16 +1043,47 @@ public:
replacementp = catp;
}
// Replace the variable
varp->replaceWith(replacementp);
// Replace the vertex
vtxp->replaceWith(replacementp);
// Connect the Sel nodes in the replacement
for (DfgVertex* const termp : termps) {
if (DfgSel* const selp = termp->cast<DfgSel>()) {
if (!selp->fromp()) selp->fromp(varp);
if (!selp->fromp()) selp->fromp(vtxp);
}
}
}
return nImprovements;
}
public:
// Similar to FixUpSelDrivers, but first comptute which bits of the
// variable are self dependent, and fix up those that are independent
// but used.
static size_t apply(DfgGraph& dfg, DfgVertexVar* varp) {
UINFO(9, "FixUpIndependentRanges of " << varp->nodep()->name());
size_t nImprovements = 0;
if (varp->is<DfgVarPacked>()) {
// For Packed variables, fix up as whole
nImprovements += fixUpPacked(dfg, varp);
} else if (varp->is<DfgVarArray>()) {
// For array variables, fix up element-wise
const uint32_t component = varp->getUser<uint32_t>();
varp->forEachSink([&](DfgVertex& sink) {
// Ignore if sink is not part of cycle
if (sink.getUser<uint32_t>() != component) return;
// Only handle ArraySels with constant index
DfgArraySel* const aselp = sink.cast<DfgArraySel>();
if (!aselp) return;
if (!aselp->bitp()->is<DfgConst>()) return;
// Fix up the word
nImprovements += fixUpPacked(dfg, aselp);
// Remove if became unused
if (!aselp->hasSinks()) aselp->unlinkDelete(dfg);
});
}
UINFO(9, "FixUpIndependentRanges made " << nImprovements << " improvements");
return nImprovements;
}
@ -1022,14 +1139,11 @@ V3DfgPasses::breakCycles(const DfgGraph& dfg, V3DfgOptimizationContext& ctx) {
// Method 1: FixUpSelDrivers
for (DfgVertexVar& vtx : res.varVertices()) {
// Only handle DfgVarPacked at this point
DfgVarPacked* const varp = vtx.cast<DfgVarPacked>();
if (!varp) continue;
// If Variable is not part of a cycle, move on
const uint32_t component = varp->getUser<uint32_t>();
const uint32_t component = vtx.getUser<uint32_t>();
if (!component) continue;
const size_t nFixed = FixUpSelDrivers::apply(res, varp);
const size_t nFixed = FixUpSelDrivers::apply(res, &vtx);
if (nFixed) {
nImprovements += nFixed;
ctx.m_breakCyclesContext.m_nImprovements += nFixed;
@ -1042,14 +1156,11 @@ V3DfgPasses::breakCycles(const DfgGraph& dfg, V3DfgOptimizationContext& ctx) {
// Method 2. FixUpIndependentRanges
for (DfgVertexVar& vtx : res.varVertices()) {
// Only handle DfgVarPacked at this point
DfgVarPacked* const varp = vtx.cast<DfgVarPacked>();
if (!varp) continue;
// If Variable is not part of a cycle, move on
const uint32_t component = varp->getUser<uint32_t>();
const uint32_t component = vtx.getUser<uint32_t>();
if (!component) continue;
const size_t nFixed = FixUpIndependentRanges::apply(res, varp);
const size_t nFixed = FixUpIndependentRanges::apply(res, &vtx);
if (nFixed) {
nImprovements += nFixed;
ctx.m_breakCyclesContext.m_nImprovements += nFixed;

2
src/V3Number.h
View File

@ -564,7 +564,6 @@ private:
}
}
static string displayPad(size_t fmtsize, char pad, bool left, const string& in) VL_PURE;
string displayed(FileLine* fl, const string& vformat) const VL_MT_STABLE;
string displayed(const string& vformat) const VL_MT_STABLE {
return displayed(m_fileline, vformat);
}
@ -592,6 +591,7 @@ public:
// ACCESSORS
string ascii(bool prefixed = true, bool cleanVerilog = false) const VL_MT_STABLE;
string displayed(AstNode* nodep, const string& vformat) const VL_MT_STABLE;
string displayed(FileLine* fl, const string& vformat) const VL_MT_STABLE;
static bool displayedFmtLegal(char format, bool isScan); // Is this a valid format letter?
int width() const VL_MT_SAFE { return m_data.width(); }
int widthToFit() const; // Minimum width that can represent this number (~== log2(num)+1)

29
test_regress/t/t_dfg_break_cycles.v
View File

@ -111,4 +111,33 @@ module t (
assign PARTIAL[0] = rand_a[0];
// PARTIAL[1] intentionally unconnected
assign PARTIAL[3:2] = rand_a[3:2] ^ {PARTIAL[2], PARTIAL[0]};
wire [2:0] array_0 [2];
assign array_0[0] = rand_a[2:0];
assign array_0[1] = array_0[0];
`signal(ARRAY_0, 3);
assign ARRAY_0 = array_0[1];
wire [2:0] array_1 [1];
assign array_1[0][0] = rand_a[0];
assign array_1[0][1] = array_1[0][0];
assign array_1[0][2] = array_1[0][1];
`signal(ARRAY_1, 3);
assign ARRAY_1 = array_1[0];
wire [2:0] array_2a [2];
wire [2:0] array_2b [2];
assign array_2a[0][0] = rand_a[0];
assign array_2a[0][1] = array_2b[1][0];
assign array_2a[0][2] = array_2b[1][1];
assign array_2a[1] = array_2a[0];
assign array_2b = array_2a;
`signal(ARRAY_2, 3);
assign ARRAY_2 = array_2a[0];
wire [2:0] array_3 [2];
assign array_3[0] = rand_a[2:0] ^ array_3[1] >> 1;
assign array_3[1] = array_3[0];
`signal(ARRAY_3, 3);
assign ARRAY_3 = array_3[0];
endmodule