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Add a utility function that detects whether a loop is guaranteed to be finite.

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Use it to safely handle less-than-or-equals-to exit conditions in loops. These
also occur when the loop exit branch is exit on true because SCEV inverses the
icmp predicate.

Use it again to handle non-zero strides, but only with an unsigned comparison
in the exit condition.

llvm-svn: 59528
This commit is contained in:
Nick Lewycky 2008-11-18 15:10:54 +00:00
parent 3460a79979
commit c573f70ae4
4 changed files with 217 additions and 30 deletions
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156
lib/Analysis/ScalarEvolution.cpp
View File

@ -1477,7 +1477,7 @@ namespace {
/// specified less-than comparison will execute. If not computable, return
/// UnknownValue. isSigned specifies whether the less-than is signed.
SCEVHandle HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L,
bool isSigned);
bool isSigned, bool trueWhenEqual);
/// getPredecessorWithUniqueSuccessorForBB - Return a predecessor of BB
/// (which may not be an immediate predecessor) which has exactly one
@ -1487,7 +1487,13 @@ namespace {
/// executesAtLeastOnce - Test whether entry to the loop is protected by
/// a conditional between LHS and RHS.
bool executesAtLeastOnce(const Loop *L, bool isSigned, SCEV *LHS, SCEV *RHS);
bool executesAtLeastOnce(const Loop *L, bool isSigned, bool trueWhenEqual,
SCEV *LHS, SCEV *RHS);
/// potentialInfiniteLoop - Test whether the loop might jump over the exit value
/// due to wrapping.
bool potentialInfiniteLoop(SCEV *Stride, SCEV *RHS, bool isSigned,
bool trueWhenEqual);
/// getConstantEvolutionLoopExitValue - If we know that the specified Phi is
/// in the header of its containing loop, we know the loop executes a
@ -2025,24 +2031,46 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
break;
}
case ICmpInst::ICMP_SLT: {
SCEVHandle TC = HowManyLessThans(LHS, RHS, L, true);
SCEVHandle TC = HowManyLessThans(LHS, RHS, L, true, false);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
case ICmpInst::ICMP_SGT: {
SCEVHandle TC = HowManyLessThans(SE.getNotSCEV(LHS),
SE.getNotSCEV(RHS), L, true);
SE.getNotSCEV(RHS), L, true, false);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
case ICmpInst::ICMP_ULT: {
SCEVHandle TC = HowManyLessThans(LHS, RHS, L, false);
SCEVHandle TC = HowManyLessThans(LHS, RHS, L, false, false);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
case ICmpInst::ICMP_UGT: {
SCEVHandle TC = HowManyLessThans(SE.getNotSCEV(LHS),
SE.getNotSCEV(RHS), L, false);
SE.getNotSCEV(RHS), L, false, false);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
case ICmpInst::ICMP_SLE: {
SCEVHandle TC = HowManyLessThans(LHS, RHS, L, true, true);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
case ICmpInst::ICMP_SGE: {
SCEVHandle TC = HowManyLessThans(SE.getNotSCEV(LHS),
SE.getNotSCEV(RHS), L, true, true);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
case ICmpInst::ICMP_ULE: {
SCEVHandle TC = HowManyLessThans(LHS, RHS, L, false, true);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
case ICmpInst::ICMP_UGE: {
SCEVHandle TC = HowManyLessThans(SE.getNotSCEV(LHS),
SE.getNotSCEV(RHS), L, false, true);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
@ -2738,6 +2766,7 @@ ScalarEvolutionsImpl::getPredecessorWithUniqueSuccessorForBB(BasicBlock *BB) {
/// executesAtLeastOnce - Test whether entry to the loop is protected by
/// a conditional between LHS and RHS.
bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
bool trueWhenEqual,
SCEV *LHS, SCEV *RHS) {
BasicBlock *Preheader = L->getLoopPreheader();
BasicBlock *PreheaderDest = L->getHeader();
@ -2770,20 +2799,36 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
switch (Cond) {
case ICmpInst::ICMP_UGT:
if (isSigned) continue;
if (isSigned || trueWhenEqual) continue;
std::swap(PreCondLHS, PreCondRHS);
Cond = ICmpInst::ICMP_ULT;
break;
case ICmpInst::ICMP_SGT:
if (!isSigned) continue;
if (!isSigned || trueWhenEqual) continue;
std::swap(PreCondLHS, PreCondRHS);
Cond = ICmpInst::ICMP_SLT;
break;
case ICmpInst::ICMP_ULT:
if (isSigned) continue;
if (isSigned || trueWhenEqual) continue;
break;
case ICmpInst::ICMP_SLT:
if (!isSigned) continue;
if (!isSigned || trueWhenEqual) continue;
break;
case ICmpInst::ICMP_UGE:
if (isSigned || !trueWhenEqual) continue;
std::swap(PreCondLHS, PreCondRHS);
Cond = ICmpInst::ICMP_ULE;
break;
case ICmpInst::ICMP_SGE:
if (!isSigned || !trueWhenEqual) continue;
std::swap(PreCondLHS, PreCondRHS);
Cond = ICmpInst::ICMP_SLE;
break;
case ICmpInst::ICMP_ULE:
if (isSigned || !trueWhenEqual) continue;
break;
case ICmpInst::ICMP_SLE:
if (!isSigned || !trueWhenEqual) continue;
break;
default:
continue;
@ -2802,11 +2847,46 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
return false;
}
/// potentialInfiniteLoop - Test whether the loop might jump over the exit value
/// due to wrapping around 2^n.
bool ScalarEvolutionsImpl::potentialInfiniteLoop(SCEV *Stride, SCEV *RHS,
bool isSigned, bool trueWhenEqual) {
// Return true when the distance from RHS to maxint > Stride.
if (!isa<SCEVConstant>(Stride))
return true;
SCEVConstant *SC = cast<SCEVConstant>(Stride);
if (SC->getValue()->isZero())
return true;
if (!trueWhenEqual && SC->getValue()->isOne())
return false;
if (!isa<SCEVConstant>(RHS))
return true;
SCEVConstant *R = cast<SCEVConstant>(RHS);
if (isSigned)
return true; // XXX: because we don't have an sdiv scev.
// If negative, it wraps around every iteration, but we don't care about that.
APInt S = SC->getValue()->getValue().abs();
APInt Dist = APInt::getMaxValue(R->getValue()->getBitWidth()) -
R->getValue()->getValue();
if (trueWhenEqual)
return !S.ult(Dist);
else
return !S.ule(Dist);
}
/// HowManyLessThans - Return the number of times a backedge containing the
/// specified less-than comparison will execute. If not computable, return
/// UnknownValue.
SCEVHandle ScalarEvolutionsImpl::
HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L, bool isSigned) {
HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L,
bool isSigned, bool trueWhenEqual) {
// Only handle: "ADDREC < LoopInvariant".
if (!RHS->isLoopInvariant(L)) return UnknownValue;
@ -2815,34 +2895,50 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L, bool isSigned) {
return UnknownValue;
if (AddRec->isAffine()) {
// FORNOW: We only support unit strides.
SCEVHandle One = SE.getIntegerSCEV(1, RHS->getType());
if (AddRec->getOperand(1) != One)
SCEVHandle Stride = AddRec->getOperand(1);
if (potentialInfiniteLoop(Stride, RHS, isSigned, trueWhenEqual))
return UnknownValue;
// We know the LHS is of the form {n,+,1} and the RHS is some loop-invariant
// m. So, we count the number of iterations in which {n,+,1} < m is true.
// Note that we cannot simply return max(m-n,0) because it's not safe to
// We know the LHS is of the form {n,+,s} and the RHS is some loop-invariant
// m. So, we count the number of iterations in which {n,+,s} < m is true.
// Note that we cannot simply return max(m-n,0)/s because it's not safe to
// treat m-n as signed nor unsigned due to overflow possibility.
// First, we get the value of the LHS in the first iteration: n
SCEVHandle Start = AddRec->getOperand(0);
if (executesAtLeastOnce(L, isSigned,
SE.getMinusSCEV(AddRec->getOperand(0), One), RHS)) {
// Since we know that the condition is true in order to enter the loop,
// we know that it will run exactly m-n times.
return SE.getMinusSCEV(RHS, Start);
} else {
// Then, we get the value of the LHS in the first iteration in which the
// above condition doesn't hold. This equals to max(m,n).
SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS, Start)
: SE.getUMaxExpr(RHS, Start);
SCEVHandle One = SE.getIntegerSCEV(1, RHS->getType());
// Finally, we subtract these two values to get the number of times the
// backedge is executed: max(m,n)-n.
return SE.getMinusSCEV(End, Start);
// Assuming that the loop will run at least once, we know that it will
// run (m-n)/s times.
SCEVHandle End = RHS;
if (!executesAtLeastOnce(L, isSigned, trueWhenEqual,
SE.getMinusSCEV(Start, One), RHS)) {
// If not, we get the value of the LHS in the first iteration in which
// the above condition doesn't hold. This equals to max(m,n).
End = isSigned ? SE.getSMaxExpr(RHS, Start)
: SE.getUMaxExpr(RHS, Start);
}
// If the expression is less-than-or-equal to, we need to extend the
// loop by one iteration.
//
// The loop won't actually run (m-n)/s times because the loop iterations
// won't divide evenly. For example, if you have {2,+,5} u< 10 the
// division would equal one, but the loop runs twice putting the
// induction variable at 12.
if (!trueWhenEqual)
// (Stride - 1) is correct only because we know it's unsigned.
// What we really want is to decrease the magnitude of Stride by one.
Start = SE.getMinusSCEV(Start, SE.getMinusSCEV(Stride, One));
else
Start = SE.getMinusSCEV(Start, Stride);
// Finally, we subtract these two values to get the number of times the
// backedge is executed: max(m,n)-n.
return SE.getUDivExpr(SE.getMinusSCEV(End, Start), Stride);
}
return UnknownValue;

31
test/Analysis/ScalarEvolution/2008-11-18-LessThanOrEqual.ll Normal file
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@ -0,0 +1,31 @@
; RUN: llvm-as < %s | opt -analyze -scalar-evolution |& \
; RUN: grep {Loop bb: (7 + (-1 \\* %argc)) iterations!}
define i32 @main(i32 %argc, i8** %argv) nounwind {
entry:
%0 = icmp ugt i32 %argc, 7 ; <i1> [#uses=1]
br i1 %0, label %bb2, label %bb.nph
bb.nph: ; preds = %entry
br label %bb
bb: ; preds = %bb.nph, %bb1
%indvar = phi i32 [ 0, %bb.nph ], [ %indvar.next, %bb1 ] ; <i32> [#uses=2]
%argc_addr.04 = add i32 %indvar, %argc ; <i32> [#uses=1]
tail call void (...)* @Test() nounwind
%1 = add i32 %argc_addr.04, 1 ; <i32> [#uses=1]
br label %bb1
bb1: ; preds = %bb
%phitmp = icmp ugt i32 %1, 7 ; <i1> [#uses=1]
%indvar.next = add i32 %indvar, 1 ; <i32> [#uses=1]
br i1 %phitmp, label %bb1.bb2_crit_edge, label %bb
bb1.bb2_crit_edge: ; preds = %bb1
br label %bb2
bb2: ; preds = %bb1.bb2_crit_edge, %entry
ret i32 0
}
declare void @Test(...)

30
test/Analysis/ScalarEvolution/2008-11-18-Stride1.ll Normal file
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@ -0,0 +1,30 @@
; RUN: llvm-as < %s | opt -analyze -scalar-evolution |& grep {/u 3}
define i32 @f(i32 %x) nounwind readnone {
entry:
%0 = icmp ugt i32 %x, 4 ; <i1> [#uses=1]
br i1 %0, label %bb.nph, label %bb2
bb.nph: ; preds = %entry
br label %bb
bb: ; preds = %bb.nph, %bb1
%indvar = phi i32 [ 0, %bb.nph ], [ %indvar.next, %bb1 ] ; <i32> [#uses=2]
%tmp = mul i32 %indvar, -3 ; <i32> [#uses=1]
%x_addr.04 = add i32 %tmp, %x ; <i32> [#uses=1]
%1 = add i32 %x_addr.04, -3 ; <i32> [#uses=2]
br label %bb1
bb1: ; preds = %bb
%2 = icmp ugt i32 %1, 4 ; <i1> [#uses=1]
%indvar.next = add i32 %indvar, 1 ; <i32> [#uses=1]
br i1 %2, label %bb, label %bb1.bb2_crit_edge
bb1.bb2_crit_edge: ; preds = %bb1
%.lcssa = phi i32 [ %1, %bb1 ] ; <i32> [#uses=1]
br label %bb2
bb2: ; preds = %bb1.bb2_crit_edge, %entry
%x_addr.0.lcssa = phi i32 [ %.lcssa, %bb1.bb2_crit_edge ], [ %x, %entry ] ; <i32> [#uses=1]
ret i32 %x_addr.0.lcssa
}

30
test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll Normal file
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@ -0,0 +1,30 @@
; RUN: llvm-as < %s | opt -analyze -scalar-evolution |& grep {/u 3}
define i32 @f(i32 %x) nounwind readnone {
entry:
%0 = icmp ugt i32 %x, 999 ; <i1> [#uses=1]
br i1 %0, label %bb2, label %bb.nph
bb.nph: ; preds = %entry
br label %bb
bb: ; preds = %bb.nph, %bb1
%indvar = phi i32 [ 0, %bb.nph ], [ %indvar.next, %bb1 ] ; <i32> [#uses=2]
%tmp = mul i32 %indvar, 3 ; <i32> [#uses=1]
%x_addr.04 = add i32 %tmp, %x ; <i32> [#uses=1]
%1 = add i32 %x_addr.04, 3 ; <i32> [#uses=2]
br label %bb1
bb1: ; preds = %bb
%2 = icmp ugt i32 %1, 999 ; <i1> [#uses=1]
%indvar.next = add i32 %indvar, 1 ; <i32> [#uses=1]
br i1 %2, label %bb1.bb2_crit_edge, label %bb
bb1.bb2_crit_edge: ; preds = %bb1
%.lcssa = phi i32 [ %1, %bb1 ] ; <i32> [#uses=1]
br label %bb2
bb2: ; preds = %bb1.bb2_crit_edge, %entry
%x_addr.0.lcssa = phi i32 [ %.lcssa, %bb1.bb2_crit_edge ], [ %x, %entry ] ; <i32> [#uses=1]
ret i32 %x_addr.0.lcssa
}