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Teach ScalarEvolution how to reason about no-wrap flags on loops

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where the induction variable has a non-unit stride, such as {0,+,2}, and
there are expressions such as {1,+,2} inside the loop formed with
or or add nsw operators.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82151 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2009-09-17 18:05:20 +00:00
parent f9ca50e3dc
commit 1f96e67f78
3 changed files with 115 additions and 16 deletions
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3
include/llvm/Analysis/ScalarEvolution.h
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@ -253,7 +253,8 @@ namespace llvm {
/// CouldNotCompute if an intermediate computation overflows.
const SCEV *getBECount(const SCEV *Start,
const SCEV *End,
const SCEV *Step);
const SCEV *Step,
bool NoWrap);
/// getBackedgeTakenInfo - Return the BackedgeTakenInfo for the given
/// loop, lazily computing new values if the loop hasn't been analyzed

52
lib/Analysis/ScalarEvolution.cpp
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@ -2972,8 +2972,20 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
const SCEV *LHS = getSCEV(U->getOperand(0));
const APInt &CIVal = CI->getValue();
if (GetMinTrailingZeros(LHS) >=
(CIVal.getBitWidth() - CIVal.countLeadingZeros()))
return getAddExpr(LHS, getSCEV(U->getOperand(1)));
(CIVal.getBitWidth() - CIVal.countLeadingZeros())) {
// Build a plain add SCEV.
const SCEV *S = getAddExpr(LHS, getSCEV(CI));
// If the LHS of the add was an addrec and it has no-wrap flags,
// transfer the no-wrap flags, since an or won't introduce a wrap.
if (const SCEVAddRecExpr *NewAR = dyn_cast<SCEVAddRecExpr>(S)) {
const SCEVAddRecExpr *OldAR = cast<SCEVAddRecExpr>(LHS);
if (OldAR->hasNoUnsignedWrap())
const_cast<SCEVAddRecExpr *>(NewAR)->setHasNoUnsignedWrap(true);
if (OldAR->hasNoSignedWrap())
const_cast<SCEVAddRecExpr *>(NewAR)->setHasNoSignedWrap(true);
}
return S;
}
}
break;
case Instruction::Xor:
@ -4795,7 +4807,8 @@ ScalarEvolution::isImpliedCondOperandsHelper(ICmpInst::Predicate Pred,
/// CouldNotCompute if an intermediate computation overflows.
const SCEV *ScalarEvolution::getBECount(const SCEV *Start,
const SCEV *End,
const SCEV *Step) {
const SCEV *Step,
bool NoWrap) {
const Type *Ty = Start->getType();
const SCEV *NegOne = getIntegerSCEV(-1, Ty);
const SCEV *Diff = getMinusSCEV(End, Start);
@ -4805,15 +4818,17 @@ const SCEV *ScalarEvolution::getBECount(const SCEV *Start,
// the division will effectively round up.
const SCEV *Add = getAddExpr(Diff, RoundUp);
// Check Add for unsigned overflow.
// TODO: More sophisticated things could be done here.
const Type *WideTy = IntegerType::get(getContext(),
getTypeSizeInBits(Ty) + 1);
const SCEV *EDiff = getZeroExtendExpr(Diff, WideTy);
const SCEV *ERoundUp = getZeroExtendExpr(RoundUp, WideTy);
const SCEV *OperandExtendedAdd = getAddExpr(EDiff, ERoundUp);
if (getZeroExtendExpr(Add, WideTy) != OperandExtendedAdd)
return getCouldNotCompute();
if (!NoWrap) {
// Check Add for unsigned overflow.
// TODO: More sophisticated things could be done here.
const Type *WideTy = IntegerType::get(getContext(),
getTypeSizeInBits(Ty) + 1);
const SCEV *EDiff = getZeroExtendExpr(Diff, WideTy);
const SCEV *ERoundUp = getZeroExtendExpr(RoundUp, WideTy);
const SCEV *OperandExtendedAdd = getAddExpr(EDiff, ERoundUp);
if (getZeroExtendExpr(Add, WideTy) != OperandExtendedAdd)
return getCouldNotCompute();
}
return getUDivExpr(Add, Step);
}
@ -4831,6 +4846,10 @@ ScalarEvolution::HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
if (!AddRec || AddRec->getLoop() != L)
return getCouldNotCompute();
// Check to see if we have a flag which makes analysis easy.
bool NoWrap = isSigned ? AddRec->hasNoSignedWrap() :
AddRec->hasNoUnsignedWrap();
if (AddRec->isAffine()) {
// FORNOW: We only support unit strides.
unsigned BitWidth = getTypeSizeInBits(AddRec->getType());
@ -4843,7 +4862,10 @@ ScalarEvolution::HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
if (CStep->isOne()) {
// With unit stride, the iteration never steps past the limit value.
} else if (CStep->getValue()->getValue().isStrictlyPositive()) {
if (const SCEVConstant *CLimit = dyn_cast<SCEVConstant>(RHS)) {
if (NoWrap) {
// We know the iteration won't step past the maximum value for its type.
;
} else if (const SCEVConstant *CLimit = dyn_cast<SCEVConstant>(RHS)) {
// Test whether a positive iteration iteration can step past the limit
// value and past the maximum value for its type in a single step.
if (isSigned) {
@ -4896,11 +4918,11 @@ ScalarEvolution::HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
// Finally, we subtract these two values and divide, rounding up, to get
// the number of times the backedge is executed.
const SCEV *BECount = getBECount(Start, End, Step);
const SCEV *BECount = getBECount(Start, End, Step, NoWrap);
// The maximum backedge count is similar, except using the minimum start
// value and the maximum end value.
const SCEV *MaxBECount = getBECount(MinStart, MaxEnd, Step);
const SCEV *MaxBECount = getBECount(MinStart, MaxEnd, Step, NoWrap);
return BackedgeTakenInfo(BECount, MaxBECount);
}

76
test/Analysis/ScalarEvolution/nsw-offset.ll Normal file
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@ -0,0 +1,76 @@
; RUN: opt < %s -S -analyze -scalar-evolution -disable-output | FileCheck %s
; ScalarEvolution should be able to fold away the sign-extensions
; on this loop with a primary induction variable incremented with
; a nsw add of 2.
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
define void @foo(i32 %n, double* nocapture %d, double* nocapture %q) nounwind {
entry:
%0 = icmp sgt i32 %n, 0 ; <i1> [#uses=1]
br i1 %0, label %bb.nph, label %return
bb.nph: ; preds = %entry
br label %bb
bb: ; preds = %bb.nph, %bb1
%i.01 = phi i32 [ %16, %bb1 ], [ 0, %bb.nph ] ; <i32> [#uses=5]
; CHECK: %1 = sext i32 %i.01 to i64
; CHECK: --> {0,+,2}<bb>
%1 = sext i32 %i.01 to i64 ; <i64> [#uses=1]
; CHECK: %2 = getelementptr inbounds double* %d, i64 %1
; CHECK: --> {%d,+,16}<bb>
%2 = getelementptr inbounds double* %d, i64 %1 ; <double*> [#uses=1]
%3 = load double* %2, align 8 ; <double> [#uses=1]
%4 = sext i32 %i.01 to i64 ; <i64> [#uses=1]
%5 = getelementptr inbounds double* %q, i64 %4 ; <double*> [#uses=1]
%6 = load double* %5, align 8 ; <double> [#uses=1]
%7 = or i32 %i.01, 1 ; <i32> [#uses=1]
; CHECK: %8 = sext i32 %7 to i64
; CHECK: --> {1,+,2}<bb>
%8 = sext i32 %7 to i64 ; <i64> [#uses=1]
; CHECK: %9 = getelementptr inbounds double* %q, i64 %8
; CHECK: {(8 + %q),+,16}<bb>
%9 = getelementptr inbounds double* %q, i64 %8 ; <double*> [#uses=1]
; Artificially repeat the above three instructions, this time using
; add nsw instead of or.
%t7 = add nsw i32 %i.01, 1 ; <i32> [#uses=1]
; CHECK: %t8 = sext i32 %t7 to i64
; CHECK: --> {1,+,2}<bb>
%t8 = sext i32 %t7 to i64 ; <i64> [#uses=1]
; CHECK: %t9 = getelementptr inbounds double* %q, i64 %t8
; CHECK: {(8 + %q),+,16}<bb>
%t9 = getelementptr inbounds double* %q, i64 %t8 ; <double*> [#uses=1]
%10 = load double* %9, align 8 ; <double> [#uses=1]
%11 = fadd double %6, %10 ; <double> [#uses=1]
%12 = fadd double %11, 3.200000e+00 ; <double> [#uses=1]
%13 = fmul double %3, %12 ; <double> [#uses=1]
%14 = sext i32 %i.01 to i64 ; <i64> [#uses=1]
%15 = getelementptr inbounds double* %d, i64 %14 ; <double*> [#uses=1]
store double %13, double* %15, align 8
%16 = add nsw i32 %i.01, 2 ; <i32> [#uses=2]
br label %bb1
bb1: ; preds = %bb
%17 = icmp slt i32 %16, %n ; <i1> [#uses=1]
br i1 %17, label %bb, label %bb1.return_crit_edge
bb1.return_crit_edge: ; preds = %bb1
br label %return
return: ; preds = %bb1.return_crit_edge, %entry
ret void
}
; CHECK: Loop bb: backedge-taken count is ((-1 + %n) /u 2)
; CHECK: Loop bb: max backedge-taken count is 1073741823