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Fix O(n*2) analysis in const-bit-op-tree (#6791)

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Note this might miss some cases where a sub-tree within an And/Or/Xor
tree is optimizeable, but not the whole tree, but in practice this seems
to work better than the alternative of keeping a set of failed nodes and
bail early.
This commit is contained in:
Geza Lore
2025-12-11 14:32:25 +00:00
committed by GitHub
parent afc4bed0f8
commit af1be26b96
6 changed files with 26 additions and 17 deletions
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src/V3Const.cpp
+21 -12
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@@ -1297,23 +1297,32 @@ class ConstVisitor final : public VNVisitor {
if (nodep->widthMin() != 1) return false;
if (!v3Global.opt.fConstBitOpTree()) return false;
string debugPrefix;
const int width = nodep->width();
AstNodeExpr* rootp = nodep;
unsigned externalOps = 0;
// Reach past a plain making AND
if (const AstAnd* const andp = VN_CAST(nodep, And)) {
if (isConst(andp->lhsp(), 1)) {
rootp = andp->rhsp();
externalOps = 1;
}
}
// Only optimize if rootp is in fact the root of a tree of identical
// operations, otherwise analysis on an eventually unoptimizable unablanced
// tree can go O(N^2) as we would re-analyze every time as we move up the
// tree, repeatedly re-discover the sub-tree is not optimizable.
if (rootp->type() == nodep->backp()->type()) return false;
std::string debugPrefix;
if (debug() >= 9) { // LCOV_EXCL_START
static int s_c = 0;
debugPrefix = "- matchBitOpTree[";
debugPrefix += cvtToStr(++s_c);
debugPrefix += "] ";
debugPrefix = "- matchBitOpTree[" + std::to_string(++s_c) + "] ";
nodep->dumpTree(debugPrefix + "INPUT: ");
} // LCOV_EXCL_STOP
AstNode* newp = nullptr;
const AstAnd* const andp = VN_CAST(nodep, And);
const int width = nodep->width();
if (andp && isConst(andp->lhsp(), 1)) { // 1 & BitOpTree
newp = ConstBitOpTreeVisitor::simplify(andp->rhsp(), width, 1, m_statBitOpReduction);
} else { // BitOpTree
newp = ConstBitOpTreeVisitor::simplify(nodep, width, 0, m_statBitOpReduction);
}
AstNodeExpr* const newp
= ConstBitOpTreeVisitor::simplify(rootp, width, externalOps, m_statBitOpReduction);
if (newp) {
nodep->replaceWithKeepDType(newp);