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Handle negative strides much more optimally. This compiles X86/lsr-negative-stride.ll
into: _t: movl 8(%esp), %ecx movl 4(%esp), %eax cmpl %ecx, %eax je LBB1_3 #bb17 LBB1_1: #bb cmpl %ecx, %eax jg LBB1_4 #cond_true LBB1_2: #cond_false subl %eax, %ecx cmpl %ecx, %eax jne LBB1_1 #bb LBB1_3: #bb17 ret LBB1_4: #cond_true subl %ecx, %eax cmpl %ecx, %eax jne LBB1_1 #bb jmp LBB1_3 #bb17 instead of: _t: subl $4, %esp movl %esi, (%esp) movl 12(%esp), %ecx movl 8(%esp), %eax cmpl %ecx, %eax je LBB1_4 #bb17 LBB1_1: #bb.outer movl %ecx, %edx negl %edx LBB1_2: #bb cmpl %ecx, %eax jle LBB1_5 #cond_false LBB1_3: #cond_true addl %edx, %eax cmpl %ecx, %eax jne LBB1_2 #bb LBB1_4: #bb17 movl (%esp), %esi addl $4, %esp ret LBB1_5: #cond_false movl %ecx, %edx subl %eax, %edx movl %eax, %esi addl %esi, %esi cmpl %ecx, %esi je LBB1_4 #bb17 LBB1_6: #cond_false.bb.outer_crit_edge movl %edx, %ecx jmp LBB1_1 #bb.outer git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37252 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -987,6 +987,20 @@ static bool PartitionByIsUseOfPostIncrementedValue(const BasedUser &Val) {
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return Val.isUseOfPostIncrementedValue;
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}
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/// isNonConstantNegative - REturn true if the specified scev is negated, but
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/// not a constant.
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static bool isNonConstantNegative(const SCEVHandle &Expr) {
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SCEVMulExpr *Mul = dyn_cast<SCEVMulExpr>(Expr);
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if (!Mul) return false;
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// If there is a constant factor, it will be first.
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SCEVConstant *SC = dyn_cast<SCEVConstant>(Mul->getOperand(0));
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if (!SC) return false;
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// Return true if the value is negative, this matches things like (-42 * V).
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return SC->getValue()->getValue().isNegative();
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}
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/// StrengthReduceStridedIVUsers - Strength reduce all of the users of a single
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/// stride of IV. All of the users may have different starting values, and this
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/// may not be the only stride (we know it is if isOnlyStride is true).
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@ -1104,15 +1118,24 @@ void LoopStrengthReduce::StrengthReduceStridedIVUsers(const SCEVHandle &Stride,
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// Add common base to the new Phi node.
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NewPHI->addIncoming(CommonBaseV, Preheader);
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// If the stride is negative, insert a sub instead of an add for the
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// increment.
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bool isNegative = isNonConstantNegative(Stride);
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SCEVHandle IncAmount = Stride;
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if (isNegative)
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IncAmount = SCEV::getNegativeSCEV(Stride);
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// Insert the stride into the preheader.
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Value *StrideV = PreheaderRewriter.expandCodeFor(Stride, PreInsertPt,
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Value *StrideV = PreheaderRewriter.expandCodeFor(IncAmount, PreInsertPt,
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ReplacedTy);
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if (!isa<ConstantInt>(StrideV)) ++NumVariable;
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// Emit the increment of the base value before the terminator of the loop
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// latch block, and add it to the Phi node.
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SCEVHandle IncExp = SCEVAddExpr::get(SCEVUnknown::get(NewPHI),
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SCEVUnknown::get(StrideV));
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SCEVHandle IncExp = SCEVUnknown::get(StrideV);
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if (isNegative)
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IncExp = SCEV::getNegativeSCEV(IncExp);
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IncExp = SCEVAddExpr::get(SCEVUnknown::get(NewPHI), IncExp);
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IncV = Rewriter.expandCodeFor(IncExp, LatchBlock->getTerminator(),
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ReplacedTy);
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