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Optimize Sel patterns in DfgPeephole

This commit is contained in:
Geza Lore 2026-04-15 13:47:04 +01:00
parent ffd2c5c69e
commit f3c63d017a
1 changed files with 94 additions and 66 deletions
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160
src/V3DfgPeephole.cpp
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@ -1024,6 +1024,94 @@ class V3DfgPeephole final : public DfgVisitor {
return {nullptr, 0, 0};
}
// The following patterns all unwind a Sel throgh it's source and replace it with
// another single Sel. Doing this one at a time can take a long time with nested
// concatenations/selects/etc, so instead unwind as much as possible in one go.
std::pair<DfgVertex*, uint32_t> unwindSel(DfgVertex* fromp, uint32_t lsb,
const uint32_t width) {
while (true) {
const uint32_t msb = lsb + width - 1;
// Sel from Concat
if (DfgConcat* const concatp = fromp->cast<DfgConcat>()) {
DfgVertex* const lhsp = concatp->lhsp();
DfgVertex* const rhsp = concatp->rhsp();
if (msb < rhsp->width()) {
// If the select is entirely from rhs, then replace with sel from rhs
APPLYING(REMOVE_SEL_FROM_RHS_OF_CONCAT) {
fromp = rhsp;
continue;
}
} else if (lsb >= rhsp->width()) {
// If the select is entirely from the lhs, then replace with sel from lhs
APPLYING(REMOVE_SEL_FROM_LHS_OF_CONCAT) {
fromp = lhsp;
lsb -= rhsp->width();
continue;
}
}
}
if (DfgReplicate* const repp = fromp->cast<DfgReplicate>()) {
// If the Sel is wholly into the source of the Replicate, push the Sel through
// the Replicate and apply it directly to the source of the Replicate.
const uint32_t srcWidth = repp->srcp()->width();
if (width <= srcWidth) {
const uint32_t newLsb = lsb % srcWidth;
const uint32_t newMsb = newLsb + width - 1;
if (newMsb < srcWidth) {
APPLYING(PUSH_SEL_THROUGH_REPLICATE) {
fromp = repp->srcp();
lsb = newLsb;
continue;
}
}
}
}
// Sel from Sel
if (DfgSel* const selp = fromp->cast<DfgSel>()) {
APPLYING(REPLACE_SEL_FROM_SEL) {
// Select from the source of the source Sel with adjusted LSB
fromp = selp->fromp();
lsb += selp->lsb();
continue;
}
}
// Sel from a partial variable (including narrowed vertex)
if (DfgVarPacked* const varp = fromp->cast<DfgVarPacked>()) {
if (varp->srcp() && !varp->isVolatile()) {
// Must be a splice, otherwise it would have been inlined
DfgSplicePacked* splicep = varp->srcp()->as<DfgSplicePacked>();
DfgVertex* driverp = nullptr;
uint32_t driverLsb = 0;
splicep->foreachDriver([&](DfgVertex& src, const uint32_t dLsb) {
const uint32_t dMsb = dLsb + src.width() - 1;
// If it does not cover the whole searched bit range, move on
if (lsb < dLsb || dMsb < msb) return false;
// Save the driver
driverp = &src;
driverLsb = dLsb;
return true;
});
if (driverp) {
APPLYING(PUSH_SEL_THROUGH_SPLICE) {
fromp = driverp;
lsb -= driverLsb;
continue;
}
}
}
}
// No patterns matched, stop
break;
}
return {fromp, lsb};
}
// VISIT methods
void visit(DfgVertex*) override {}
@ -1151,38 +1239,12 @@ class V3DfgPeephole final : public DfgVisitor {
}
}
// Sel from Concat
if (DfgConcat* const concatp = fromp->cast<DfgConcat>()) {
DfgVertex* const lhsp = concatp->lhsp();
DfgVertex* const rhsp = concatp->rhsp();
if (msb < rhsp->width()) {
// If the select is entirely from rhs, then replace with sel from rhs
APPLYING(REMOVE_SEL_FROM_RHS_OF_CONCAT) { //
replace(make<DfgSel>(vtxp, rhsp, vtxp->lsb()));
return;
}
} else if (lsb >= rhsp->width()) {
// If the select is entirely from the lhs, then replace with sel from lhs
APPLYING(REMOVE_SEL_FROM_LHS_OF_CONCAT) {
replace(make<DfgSel>(vtxp, lhsp, lsb - rhsp->width()));
return;
}
}
}
if (DfgReplicate* const repp = fromp->cast<DfgReplicate>()) {
// If the Sel is wholly into the source of the Replicate, push the Sel through the
// Replicate and apply it directly to the source of the Replicate.
const uint32_t srcWidth = repp->srcp()->width();
if (width <= srcWidth) {
const uint32_t newLsb = lsb % srcWidth;
if (newLsb + width <= srcWidth) {
APPLYING(PUSH_SEL_THROUGH_REPLICATE) {
replace(make<DfgSel>(vtxp, repp->srcp(), newLsb));
return;
}
}
// Unwind through bit packing in one go
{
const auto res = unwindSel(fromp, lsb, width);
if (res.first != fromp || res.second != lsb) {
replace(make<DfgSel>(vtxp, res.first, res.second));
return;
}
}
@ -1200,15 +1262,6 @@ class V3DfgPeephole final : public DfgVisitor {
}
}
// Sel from Sel
if (DfgSel* const selp = fromp->cast<DfgSel>()) {
APPLYING(REPLACE_SEL_FROM_SEL) {
// Select from the source of the source Sel with adjusted LSB
replace(make<DfgSel>(vtxp, selp->fromp(), lsb + selp->lsb()));
return;
}
}
// Sel from Cond
if (DfgCond* const condp = fromp->cast<DfgCond>()) {
if (!condp->hasMultipleSinks()) {
@ -1242,31 +1295,6 @@ class V3DfgPeephole final : public DfgVisitor {
}
}
}
// Sel from a partial variable (including narrowed vertex)
if (DfgVarPacked* const varp = fromp->cast<DfgVarPacked>()) {
if (varp->srcp() && !varp->isVolatile()) {
// Must be a splice, otherwise it would have been inlined
DfgSplicePacked* splicep = varp->srcp()->as<DfgSplicePacked>();
DfgVertex* driverp = nullptr;
uint32_t driverLsb = 0;
splicep->foreachDriver([&](DfgVertex& src, const uint32_t dLsb) {
const uint32_t dMsb = dLsb + src.width() - 1;
// If it does not cover the whole searched bit range, move on
if (lsb < dLsb || dMsb < msb) return false;
// Save the driver
driverp = &src;
driverLsb = dLsb;
return true;
});
if (driverp) {
APPLYING(PUSH_SEL_THROUGH_SPLICE) {
replace(make<DfgSel>(vtxp, driverp, lsb - driverLsb));
return;
}
}
}
}
}
void visit(DfgMux* const vtxp) override {