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[ValueTracking] Add dominating condition support in computeKnownBits() (
#73662) This adds support for using dominating conditions in computeKnownBits() when called from InstCombine. The implementation uses a DomConditionCache, which stores which branches may provide information that is relevant for a given value. DomConditionCache is similar to AssumptionCache, but does not try to do any kind of automatic tracking. Relevant branches have to be explicitly registered and invalidated values explicitly removed. The necessary tracking is done inside InstCombine. The reason why this doesn't just do exactly the same thing as AssumptionCache is that a lot more transforms touch branches and branch conditions than assumptions. AssumptionCache is an immutable analysis and mostly gets away with this because only a handful of places have to register additional assumptions (mostly as a result of cloning). This is very much not the case for branches. This change regresses compile-time by about ~0.2%. It also improves stage2-O0-g builds by about ~0.2%, which indicates that this change results in additional optimizations inside clang itself. Fixes #74242.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,56 @@ | ||
| //===- llvm/Analysis/DomConditionCache.h ------------------------*- C++ -*-===// | ||
| // | ||
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | ||
| // See https://llvm.org/LICENSE.txt for license information. | ||
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
| // | ||
| // Cache for branch conditions that affect a certain value for use by | ||
| // ValueTracking. Unlike AssumptionCache, this class does not perform any | ||
| // automatic analysis or invalidation. The caller is responsible for registering | ||
| // all relevant branches (and re-registering them if they change), and for | ||
| // removing invalidated values from the cache. | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #ifndef LLVM_ANALYSIS_DOMCONDITIONCACHE_H | ||
| #define LLVM_ANALYSIS_DOMCONDITIONCACHE_H | ||
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| #include "llvm/ADT/ArrayRef.h" | ||
| #include "llvm/ADT/DenseMap.h" | ||
| #include "llvm/ADT/DenseMapInfo.h" | ||
| #include "llvm/ADT/SmallVector.h" | ||
| #include "llvm/IR/ValueHandle.h" | ||
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| namespace llvm { | ||
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| class Value; | ||
| class BranchInst; | ||
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| class DomConditionCache { | ||
| private: | ||
| /// A map of values about which a branch might be providing information. | ||
| using AffectedValuesMap = DenseMap<Value *, SmallVector<BranchInst *, 1>>; | ||
| AffectedValuesMap AffectedValues; | ||
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| public: | ||
| /// Add a branch condition to the cache. | ||
| void registerBranch(BranchInst *BI); | ||
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| /// Remove a value from the cache, e.g. because it will be erased. | ||
| void removeValue(Value *V) { AffectedValues.erase(V); } | ||
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| /// Access the list of branches which affect this value. | ||
| ArrayRef<BranchInst *> conditionsFor(const Value *V) const { | ||
| auto AVI = AffectedValues.find_as(const_cast<Value *>(V)); | ||
| if (AVI == AffectedValues.end()) | ||
| return ArrayRef<BranchInst *>(); | ||
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| return AVI->second; | ||
| } | ||
| }; | ||
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| } // end namespace llvm | ||
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| #endif // LLVM_ANALYSIS_DOMCONDITIONCACHE_H |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,68 @@ | ||
| //===- DomConditionCache.cpp ----------------------------------------------===// | ||
| // | ||
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | ||
| // See https://llvm.org/LICENSE.txt for license information. | ||
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #include "llvm/Analysis/DomConditionCache.h" | ||
| #include "llvm/IR/PatternMatch.h" | ||
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| using namespace llvm; | ||
| using namespace llvm::PatternMatch; | ||
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| // TODO: This code is very similar to findAffectedValues() in | ||
| // AssumptionCache, but currently specialized to just the patterns that | ||
| // computeKnownBits() supports, and without the notion of result elem indices | ||
| // that are AC specific. Deduplicate this code once we have a clearer picture | ||
| // of how much they can be shared. | ||
| static void findAffectedValues(Value *Cond, | ||
| SmallVectorImpl<Value *> &Affected) { | ||
| auto AddAffected = [&Affected](Value *V) { | ||
| if (isa<Argument>(V) || isa<GlobalValue>(V)) { | ||
| Affected.push_back(V); | ||
| } else if (auto *I = dyn_cast<Instruction>(V)) { | ||
| Affected.push_back(I); | ||
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| // Peek through unary operators to find the source of the condition. | ||
| Value *Op; | ||
| if (match(I, m_PtrToInt(m_Value(Op)))) { | ||
| if (isa<Instruction>(Op) || isa<Argument>(Op)) | ||
| Affected.push_back(Op); | ||
| } | ||
| } | ||
| }; | ||
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| ICmpInst::Predicate Pred; | ||
| Value *A; | ||
| Constant *C; | ||
| if (match(Cond, m_ICmp(Pred, m_Value(A), m_Constant(C)))) { | ||
| AddAffected(A); | ||
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| if (ICmpInst::isEquality(Pred)) { | ||
| Value *X; | ||
| // (X & C) or (X | C) or (X ^ C). | ||
| // (X << C) or (X >>_s C) or (X >>_u C). | ||
| if (match(A, m_BitwiseLogic(m_Value(X), m_ConstantInt())) || | ||
| match(A, m_Shift(m_Value(X), m_ConstantInt()))) | ||
| AddAffected(X); | ||
| } else { | ||
| Value *X; | ||
| // Handle (A + C1) u< C2, which is the canonical form of A > C3 && A < C4. | ||
| if (match(A, m_Add(m_Value(X), m_ConstantInt()))) | ||
| AddAffected(X); | ||
| } | ||
| } | ||
| } | ||
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| void DomConditionCache::registerBranch(BranchInst *BI) { | ||
| assert(BI->isConditional() && "Must be conditional branch"); | ||
| SmallVector<Value *, 16> Affected; | ||
| findAffectedValues(BI->getCondition(), Affected); | ||
| for (Value *V : Affected) { | ||
| auto &AV = AffectedValues[V]; | ||
| if (!is_contained(AV, BI)) | ||
| AV.push_back(BI); | ||
| } | ||
| } |
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