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author | Andrew Trick <atrick@apple.com> | 2011-11-02 17:19:57 +0000 |
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committer | Andrew Trick <atrick@apple.com> | 2011-11-02 17:19:57 +0000 |
commit | d3714b60b5adf15376a8803e6622c74694884b28 (patch) | |
tree | 8ad325293d2ec884f0e238fdf9936d1b941522ae /lib/Transforms/Scalar/IndVarSimplify.cpp | |
parent | a71b7b09116059e2a4359026f72eb429b6d2da8d (diff) | |
download | llvm-d3714b60b5adf15376a8803e6622c74694884b28.tar.gz llvm-d3714b60b5adf15376a8803e6622c74694884b28.tar.bz2 llvm-d3714b60b5adf15376a8803e6622c74694884b28.tar.xz |
Rewrite LinearFunctionTestReplace to handle pointer-type IVs.
We've been hitting asserts in this code due to the many supported
combintions of modes (iv-rewrite/no-iv-rewrite) and IV types. This
second rewrite of the code attempts to deal with these cases systematically.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143546 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Transforms/Scalar/IndVarSimplify.cpp')
-rw-r--r-- | lib/Transforms/Scalar/IndVarSimplify.cpp | 189 |
1 files changed, 116 insertions, 73 deletions
diff --git a/lib/Transforms/Scalar/IndVarSimplify.cpp b/lib/Transforms/Scalar/IndVarSimplify.cpp index 0ba327a1eb..1f21108152 100644 --- a/lib/Transforms/Scalar/IndVarSimplify.cpp +++ b/lib/Transforms/Scalar/IndVarSimplify.cpp @@ -1278,6 +1278,16 @@ static bool isHighCostExpansion(const SCEV *S, BranchInst *BI, /// canExpandBackedgeTakenCount - Return true if this loop's backedge taken /// count expression can be safely and cheaply expanded into an instruction /// sequence that can be used by LinearFunctionTestReplace. +/// +/// TODO: This fails for pointer-type loop counters with greater than one byte +/// strides, consequently preventing LFTR from running. For the purpose of LFTR +/// we could skip this check in the case that the LFTR loop counter (chosen by +/// FindLoopCounter) is also pointer type. Instead, we could directly convert +/// the loop test to an inequality test by checking the target data's alignment +/// of element types (given that the initial pointer value originates from or is +/// used by ABI constrained operation, as opposed to inttoptr/ptrtoint). +/// However, we don't yet have a strong motivation for converting loop tests +/// into inequality tests. static bool canExpandBackedgeTakenCount(Loop *L, ScalarEvolution *SE) { const SCEV *BackedgeTakenCount = SE->getBackedgeTakenCount(L); if (isa<SCEVCouldNotCompute>(BackedgeTakenCount) || @@ -1429,6 +1439,10 @@ static bool AlmostDeadIV(PHINode *Phi, BasicBlock *LatchBlock, Value *Cond) { /// FindLoopCounter - Find an affine IV in canonical form. /// +/// BECount may be an i8* pointer type. The pointer difference is already +/// valid count without scaling the address stride, so it remains a pointer +/// expression as far as SCEV is concerned. +/// /// FIXME: Accept -1 stride and set IVLimit = IVInit - BECount /// /// FIXME: Accept non-unit stride as long as SCEV can reduce BECount * Stride. @@ -1437,11 +1451,6 @@ static bool AlmostDeadIV(PHINode *Phi, BasicBlock *LatchBlock, Value *Cond) { static PHINode * FindLoopCounter(Loop *L, const SCEV *BECount, ScalarEvolution *SE, DominatorTree *DT, const TargetData *TD) { - // I'm not sure how BECount could be a pointer type, but we definitely don't - // want to LFTR that. - if (BECount->getType()->isPointerTy()) - return 0; - uint64_t BCWidth = SE->getTypeSizeInBits(BECount->getType()); Value *Cond = @@ -1458,6 +1467,10 @@ FindLoopCounter(Loop *L, const SCEV *BECount, if (!SE->isSCEVable(Phi->getType())) continue; + // Avoid comparing an integer IV against a pointer Limit. + if (BECount->getType()->isPointerTy() && !Phi->getType()->isPointerTy()) + continue; + const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Phi)); if (!AR || AR->getLoop() != L || !AR->isAffine()) continue; @@ -1503,6 +1516,82 @@ FindLoopCounter(Loop *L, const SCEV *BECount, return BestPhi; } +/// genLoopLimit - Help LinearFunctionTestReplace by generating a value that +/// holds the RHS of the new loop test. +static Value *genLoopLimit(PHINode *IndVar, const SCEV *IVCount, Loop *L, + SCEVExpander &Rewriter, ScalarEvolution *SE) { + const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(IndVar)); + assert(AR && AR->getLoop() == L && AR->isAffine() && "bad loop counter"); + const SCEV *IVInit = AR->getStart(); + + // IVInit may be a pointer while IVCount is an integer when FindLoopCounter + // finds a valid pointer IV. Sign extend BECount in order to materialize a + // GEP. Avoid running SCEVExpander on a new pointer value, instead reusing + // the existing GEPs whenever possible. + if (IndVar->getType()->isPointerTy() + && !IVCount->getType()->isPointerTy()) { + + Type *OfsTy = SE->getEffectiveSCEVType(IVInit->getType()); + const SCEV *IVOffset = SE->getTruncateOrSignExtend(IVCount, OfsTy); + + // Expand the code for the iteration count. + assert(SE->isLoopInvariant(IVOffset, L) && + "Computed iteration count is not loop invariant!"); + BranchInst *BI = cast<BranchInst>(L->getExitingBlock()->getTerminator()); + Value *GEPOffset = Rewriter.expandCodeFor(IVOffset, OfsTy, BI); + + Value *GEPBase = IndVar->getIncomingValueForBlock(L->getLoopPreheader()); + assert(AR->getStart() == SE->getSCEV(GEPBase) && "bad loop counter"); + // We could handle pointer IVs other than i8*, but we need to compensate for + // gep index scaling. See canExpandBackedgeTakenCount comments. + assert(SE->getSizeOfExpr( + cast<PointerType>(GEPBase->getType())->getElementType())->isOne() + && "unit stride pointer IV must be i8*"); + + IRBuilder<> Builder(L->getLoopPreheader()->getTerminator()); + return Builder.CreateGEP(GEPBase, GEPOffset, "lftr.limit"); + } + else { + // In any other case, convert both IVInit and IVCount to integers before + // comparing. This may result in SCEV expension of pointers, but in practice + // SCEV will fold the pointer arithmetic away as such: + // BECount = (IVEnd - IVInit - 1) => IVLimit = IVInit (postinc). + // + // Valid Cases: (1) both integers is most common; (2) both may be pointers + // for simple memset-style loops; (3) IVInit is an integer and IVCount is a + // pointer may occur when enable-iv-rewrite generates a canonical IV on top + // of case #2. + + const SCEV *IVLimit = 0; + // For unit stride, IVCount = Start + BECount with 2's complement overflow. + // For non-zero Start, compute IVCount here. + if (AR->getStart()->isZero()) + IVLimit = IVCount; + else { + assert(AR->getStepRecurrence(*SE)->isOne() && "only handles unit stride"); + const SCEV *IVInit = AR->getStart(); + + // For integer IVs, truncate the IV before computing IVInit + BECount. + if (SE->getTypeSizeInBits(IVInit->getType()) + > SE->getTypeSizeInBits(IVCount->getType())) + IVInit = SE->getTruncateExpr(IVInit, IVCount->getType()); + + IVLimit = SE->getAddExpr(IVInit, IVCount); + } + // Expand the code for the iteration count. + BranchInst *BI = cast<BranchInst>(L->getExitingBlock()->getTerminator()); + IRBuilder<> Builder(BI); + assert(SE->isLoopInvariant(IVLimit, L) && + "Computed iteration count is not loop invariant!"); + // Ensure that we generate the same type as IndVar, or a smaller integer + // type. In the presence of null pointer values, we have an integer type + // SCEV expression (IVInit) for a pointer type IV value (IndVar). + Type *LimitTy = IVCount->getType()->isPointerTy() ? + IndVar->getType() : IVCount->getType(); + return Rewriter.expandCodeFor(IVLimit, LimitTy, BI); + } +} + /// LinearFunctionTestReplace - This method rewrites the exit condition of the /// loop to be a canonical != comparison against the incremented loop induction /// variable. This pass is able to rewrite the exit tests of any loop where the @@ -1514,37 +1603,36 @@ LinearFunctionTestReplace(Loop *L, PHINode *IndVar, SCEVExpander &Rewriter) { assert(canExpandBackedgeTakenCount(L, SE) && "precondition"); - BranchInst *BI = cast<BranchInst>(L->getExitingBlock()->getTerminator()); // LFTR can ignore IV overflow and truncate to the width of // BECount. This avoids materializing the add(zext(add)) expression. Type *CntTy = !EnableIVRewrite ? BackedgeTakenCount->getType() : IndVar->getType(); - const SCEV *IVLimit = BackedgeTakenCount; + const SCEV *IVCount = BackedgeTakenCount; - // If the exiting block is not the same as the backedge block, we must compare - // against the preincremented value, otherwise we prefer to compare against - // the post-incremented value. + // If the exiting block is the same as the backedge block, we prefer to + // compare against the post-incremented value, otherwise we must compare + // against the preincremented value. Value *CmpIndVar; if (L->getExitingBlock() == L->getLoopLatch()) { // Add one to the "backedge-taken" count to get the trip count. // If this addition may overflow, we have to be more pessimistic and // cast the induction variable before doing the add. const SCEV *N = - SE->getAddExpr(IVLimit, SE->getConstant(IVLimit->getType(), 1)); - if (CntTy == IVLimit->getType()) - IVLimit = N; + SE->getAddExpr(IVCount, SE->getConstant(IVCount->getType(), 1)); + if (CntTy == IVCount->getType()) + IVCount = N; else { - const SCEV *Zero = SE->getConstant(IVLimit->getType(), 0); + const SCEV *Zero = SE->getConstant(IVCount->getType(), 0); if ((isa<SCEVConstant>(N) && !N->isZero()) || SE->isLoopEntryGuardedByCond(L, ICmpInst::ICMP_NE, N, Zero)) { // No overflow. Cast the sum. - IVLimit = SE->getTruncateOrZeroExtend(N, CntTy); + IVCount = SE->getTruncateOrZeroExtend(N, CntTy); } else { // Potential overflow. Cast before doing the add. - IVLimit = SE->getTruncateOrZeroExtend(IVLimit, CntTy); - IVLimit = SE->getAddExpr(IVLimit, SE->getConstant(CntTy, 1)); + IVCount = SE->getTruncateOrZeroExtend(IVCount, CntTy); + IVCount = SE->getAddExpr(IVCount, SE->getConstant(CntTy, 1)); } } // The BackedgeTaken expression contains the number of times that the @@ -1552,64 +1640,17 @@ LinearFunctionTestReplace(Loop *L, // number of times the loop executes, so use the incremented indvar. CmpIndVar = IndVar->getIncomingValueForBlock(L->getExitingBlock()); } else { - // We have to use the preincremented value... - IVLimit = SE->getTruncateOrZeroExtend(IVLimit, CntTy); + // We must use the preincremented value... + IVCount = SE->getTruncateOrZeroExtend(IVCount, CntTy); CmpIndVar = IndVar; } - // For unit stride, IVLimit = Start + BECount with 2's complement overflow. - // So for non-zero start compute the IVLimit here. - Type *CmpTy = CntTy; - const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(IndVar)); - assert(AR && AR->getLoop() == L && AR->isAffine() && "bad loop counter"); - if (!AR->getStart()->isZero()) { - assert(AR->getStepRecurrence(*SE)->isOne() && "only handles unit stride"); - const SCEV *IVInit = AR->getStart(); - - // For pointer types, sign extend BECount in order to materialize a GEP. - // Note that for without EnableIVRewrite, we never run SCEVExpander on a - // pointer type, because we must preserve the existing GEPs. Instead we - // directly generate a GEP later. - if (CmpIndVar->getType()->isPointerTy()) { - CmpTy = SE->getEffectiveSCEVType(IVInit->getType()); - IVLimit = SE->getTruncateOrSignExtend(IVLimit, CmpTy); - } - // For integer types, truncate the IV before computing IVInit + BECount. - else { - if (SE->getTypeSizeInBits(IVInit->getType()) - > SE->getTypeSizeInBits(CmpTy)) - IVInit = SE->getTruncateExpr(IVInit, CmpTy); - - IVLimit = SE->getAddExpr(IVInit, IVLimit); - } - } - // Expand the code for the iteration count. - IRBuilder<> Builder(BI); - - assert(SE->isLoopInvariant(IVLimit, L) && - "Computed iteration count is not loop invariant!"); - assert((EnableIVRewrite || !IVLimit->getType()->isPointerTy()) && - "Should not expand pointer types" ); - Value *ExitCnt = Rewriter.expandCodeFor(IVLimit, CmpTy, BI); - - // Create a gep for IVInit + IVLimit from on an existing pointer base. - // - // In the presence of null pointer values, the SCEV expression may be an - // integer type while the IV is a pointer type. Ensure that the compare - // operands are always the same type by checking the IV type here. - if (CmpIndVar->getType()->isPointerTy()) { - Value *IVStart = IndVar->getIncomingValueForBlock(L->getLoopPreheader()); - assert(AR->getStart() == SE->getSCEV(IVStart) && "bad loop counter"); - assert(SE->getSizeOfExpr( - cast<PointerType>(IVStart->getType())->getElementType())->isOne() - && "unit stride pointer IV must be i8*"); - - Builder.SetInsertPoint(L->getLoopPreheader()->getTerminator()); - ExitCnt = Builder.CreateGEP(IVStart, ExitCnt, "lftr.limit"); - Builder.SetInsertPoint(BI); - } + Value *ExitCnt = genLoopLimit(IndVar, IVCount, L, Rewriter, SE); + assert(ExitCnt->getType()->isPointerTy() == IndVar->getType()->isPointerTy() + && "genLoopLimit missed a cast"); // Insert a new icmp_ne or icmp_eq instruction before the branch. + BranchInst *BI = cast<BranchInst>(L->getExitingBlock()->getTerminator()); ICmpInst::Predicate P; if (L->contains(BI->getSuccessor(0))) P = ICmpInst::ICMP_NE; @@ -1621,11 +1662,13 @@ LinearFunctionTestReplace(Loop *L, << " op:\t" << (P == ICmpInst::ICMP_NE ? "!=" : "==") << "\n" << " RHS:\t" << *ExitCnt << "\n" - << " Expr:\t" << *IVLimit << "\n"); + << " IVCount:\t" << *IVCount << "\n"); + IRBuilder<> Builder(BI); if (SE->getTypeSizeInBits(CmpIndVar->getType()) - > SE->getTypeSizeInBits(CmpTy)) { - CmpIndVar = Builder.CreateTrunc(CmpIndVar, CmpTy, "lftr.wideiv"); + > SE->getTypeSizeInBits(ExitCnt->getType())) { + CmpIndVar = Builder.CreateTrunc(CmpIndVar, ExitCnt->getType(), + "lftr.wideiv"); } Value *Cond = Builder.CreateICmp(P, CmpIndVar, ExitCnt, "exitcond"); |