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Improve conditional merging to preserve branch prediction

This commit is contained in:
Geza Lore 2025-10-20 18:24:43 +01:00
parent f73dde6dd9
commit 8f13bc0477
1 changed files with 9 additions and 0 deletions
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9
src/V3MergeCond.cpp
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@ -655,6 +655,9 @@ class MergeCondVisitor final : public VNVisitor {
// Create equivalent 'if' statement and insert it before the first node // Create equivalent 'if' statement and insert it before the first node
AstIf* const resultp = new AstIf{m_mgCondp->fileline(), m_mgCondp}; AstIf* const resultp = new AstIf{m_mgCondp->fileline(), m_mgCondp};
m_mgFirstp->addHereThisAsNext(resultp); m_mgFirstp->addHereThisAsNext(resultp);
// Try to preserve branch prediction if we can
size_t nLikely = 0;
size_t nUnlikely = 0;
// Unzip the list and insert under branches // Unzip the list and insert under branches
AstNode* nextp = m_mgFirstp; AstNode* nextp = m_mgFirstp;
do { do {
@ -691,10 +694,16 @@ class MergeCondVisitor final : public VNVisitor {
if (AstNode* const listp = ifp->elsesp()) { if (AstNode* const listp = ifp->elsesp()) {
resultp->addElsesp(listp->unlinkFrBackWithNext()); resultp->addElsesp(listp->unlinkFrBackWithNext());
} }
// Record branch prediction
if (ifp->branchPred().likely()) ++nLikely;
if (ifp->branchPred().unlikely()) ++nUnlikely;
// Cleanup // Cleanup
VL_DO_DANGLING(ifp->deleteTree(), ifp); VL_DO_DANGLING(ifp->deleteTree(), ifp);
} }
} while (nextp); } while (nextp);
// If prediction is unanimous, assign it to the merged AstIf
if (nLikely && !nUnlikely) resultp->branchPred(VBranchPred::BP_LIKELY);
if (!nLikely && nUnlikely) resultp->branchPred(VBranchPred::BP_UNLIKELY);
// Merge the branches of the resulting AstIf after re-analysis // Merge the branches of the resulting AstIf after re-analysis
if (resultp->thensp()) m_workQueuep->push(resultp->thensp()); if (resultp->thensp()) m_workQueuep->push(resultp->thensp());
if (resultp->elsesp()) m_workQueuep->push(resultp->elsesp()); if (resultp->elsesp()) m_workQueuep->push(resultp->elsesp());