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[SCEV] Consolidate code for proving wrap flags of controlling finite …
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…IVs (llvm#101404)

The canAssumeNoSelfWrap routine in howManyLessThans was doing two subtly
inter-related things. First, it was proving no-self-wrap. This exactly
duplicates the existing logic in the caller. Second, it was establishing
the precondition for the nw->nsw/nuw inference. Specifically, we need to
know that *this* exit must be taken for the inference to be sound.
Otherwise, another (possible abnormal) exit could be taken in the
iteration where this IV would become poison.

This change moves all of that logic into the caller, and caches the
resulting nuw/nsw flags in the AddRec. This centralizes the logic in one
place, and makes it clear that it all depends on controlling the sole
exit.

We do loose a couple cases with SCEV predication. Specifically, if SCEV
predication was able to convert e.g. zext(addrec) into an addrec(zext)
using predication, but didn't record the nuw fact on the new addrec,
then the consuming code can no longer fix this up. I don't think this
case particularly matters.

---------

Co-authored-by: Nikita Popov <github@npopov.com>
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@preames @nikic
preames and nikic authored Aug 12, 2024
1 parent d4f6fcf commit b812e57
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Showing 3 changed files with 29 additions and 55 deletions.
74 changes: 27 additions & 47 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9136,11 +9136,12 @@ ScalarEvolution::ExitLimit ScalarEvolution::computeExitLimitFromICmp(
}

// If this loop must exit based on this condition (or execute undefined
// behaviour), and we can prove the test sequence produced must repeat
// the same values on self-wrap of the IV, then we can infer that IV
// doesn't self wrap because if it did, we'd have an infinite (undefined)
// loop. Note that a stride of 0 is trivially no-self-wrap by definition.
// behaviour), see if we can improve wrap flags. This is essentially
// a must execute style proof.
if (ControllingFiniteLoop && isLoopInvariant(RHS, L)) {
// If we can prove the test sequence produced must repeat the same values
// on self-wrap of the IV, then we can infer that IV doesn't self wrap
// because if it did, we'd have an infinite (undefined) loop.
// TODO: We can peel off any functions which are invertible *in L*. Loop
// invariant terms are effectively constants for our purposes here.
auto *InnerLHS = LHS;
Expand All @@ -9156,6 +9157,25 @@ ScalarEvolution::ExitLimit ScalarEvolution::computeExitLimitFromICmp(
Flags = StrengthenNoWrapFlags(this, scAddRecExpr, Operands, Flags);
setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR), Flags);
}

// For a slt/ult condition with a positive step, can we prove nsw/nuw?
// From no-self-wrap, this follows trivially from the fact that every
// (un)signed-wrapped, but not self-wrapped value must be LT than the
// last value before (un)signed wrap. Since we know that last value
// didn't exit, nor will any smaller one.
if (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_ULT) {
auto WrapType = Pred == ICmpInst::ICMP_SLT ? SCEV::FlagNSW : SCEV::FlagNUW;
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(LHS);
AR && AR->getLoop() == L && AR->isAffine() &&
!AR->getNoWrapFlags(WrapType) && AR->hasNoSelfWrap() &&
isKnownPositive(AR->getStepRecurrence(*this))) {
auto Flags = AR->getNoWrapFlags();
Flags = setFlags(Flags, WrapType);
SmallVector<const SCEV*> Operands{AR->operands()};
Flags = StrengthenNoWrapFlags(this, scAddRecExpr, Operands, Flags);
setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR), Flags);
}
}
}

switch (Pred) {
Expand Down Expand Up @@ -12769,35 +12789,6 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,

const SCEVAddRecExpr *IV = dyn_cast<SCEVAddRecExpr>(LHS);
bool PredicatedIV = false;

auto canAssumeNoSelfWrap = [&](const SCEVAddRecExpr *AR) {
// Can we prove this loop *must* be UB if overflow of IV occurs?
// Reasoning goes as follows:
// * Suppose the IV did self wrap.
// * If Stride evenly divides the iteration space, then once wrap
// occurs, the loop must revisit the same values.
// * We know that RHS is invariant, and that none of those values
// caused this exit to be taken previously. Thus, this exit is
// dynamically dead.
// * If this is the sole exit, then a dead exit implies the loop
// must be infinite if there are no abnormal exits.
// * If the loop were infinite, then it must either not be mustprogress
// or have side effects. Otherwise, it must be UB.
// * It can't (by assumption), be UB so we have contradicted our
// premise and can conclude the IV did not in fact self-wrap.
if (!isLoopInvariant(RHS, L))
return false;

if (!isKnownToBeAPowerOfTwo(AR->getStepRecurrence(*this), /*OrZero=*/true,
/*OrNegative*/ true))
return false;

if (!ControlsOnlyExit || !loopHasNoAbnormalExits(L))
return false;

return loopIsFiniteByAssumption(L);
};

if (!IV) {
if (auto *ZExt = dyn_cast<SCEVZeroExtendExpr>(LHS)) {
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(ZExt->getOperand());
Expand Down Expand Up @@ -12948,21 +12939,10 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
Stride = getUMaxExpr(Stride, getOne(Stride->getType()));
}
}
} else if (!Stride->isOne() && !NoWrap) {
auto isUBOnWrap = [&]() {
// From no-self-wrap, we need to then prove no-(un)signed-wrap. This
// follows trivially from the fact that every (un)signed-wrapped, but
// not self-wrapped value must be LT than the last value before
// (un)signed wrap. Since we know that last value didn't exit, nor
// will any smaller one.
return canAssumeNoSelfWrap(IV);
};

} else if (!NoWrap) {
// Avoid proven overflow cases: this will ensure that the backedge taken
// count will not generate any unsigned overflow. Relaxed no-overflow
// conditions exploit NoWrapFlags, allowing to optimize in presence of
// undefined behaviors like the case of C language.
if (canIVOverflowOnLT(RHS, Stride, IsSigned) && !isUBOnWrap())
// count will not generate any unsigned overflow.
if (canIVOverflowOnLT(RHS, Stride, IsSigned))
return getCouldNotCompute();
}

Expand Down
6 changes: 0 additions & 6 deletions llvm/test/Analysis/ScalarEvolution/trip-count-implied-addrec.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -239,12 +239,6 @@ define void @neg_rhs_wrong_range(i16 %n.raw) mustprogress {
; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
; CHECK-NEXT: Loop %for.body: Predicated backedge-taken count is ((-1 + (2 umax (-1 + (zext i8 (trunc i16 %n.raw to i8) to i16))<nsw>)) /u 2)
; CHECK-NEXT: Predicates:
; CHECK-NEXT: {2,+,2}<nw><%for.body> Added Flags: <nusw>
; CHECK-NEXT: Loop %for.body: Predicated symbolic max backedge-taken count is ((-1 + (2 umax (-1 + (zext i8 (trunc i16 %n.raw to i8) to i16))<nsw>)) /u 2)
; CHECK-NEXT: Predicates:
; CHECK-NEXT: {2,+,2}<nw><%for.body> Added Flags: <nusw>
;
entry:
%n.and = and i16 %n.raw, 255
Expand Down
4 changes: 2 additions & 2 deletions llvm/test/Analysis/ScalarEvolution/trip-count-scalable-stride.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -377,7 +377,7 @@ define void @vscale_slt_noflags(ptr nocapture %A, i32 %n) mustprogress vscale_ra
; CHECK-NEXT: %arrayidx = getelementptr inbounds i32, ptr %A, i32 %i.05
; CHECK-NEXT: --> {%A,+,(4 * vscale)<nuw><nsw>}<%for.body> U: full-set S: full-set Exits: ((4 * vscale * ((-1 + %n) /u vscale)) + %A) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: %add = add i32 %i.05, %vscale
; CHECK-NEXT: --> {vscale,+,vscale}<nw><%for.body> U: full-set S: full-set Exits: (vscale * (1 + ((-1 + %n) /u vscale))<nuw>) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: --> {vscale,+,vscale}<nuw><nsw><%for.body> U: [2,-2147483648) S: [2,-2147483648) Exits: (vscale * (1 + ((-1 + %n) /u vscale))<nuw>) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: Determining loop execution counts for: @vscale_slt_noflags
; CHECK-NEXT: Loop %for.body: backedge-taken count is ((-1 + %n) /u vscale)
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 1073741822
Expand Down Expand Up @@ -415,7 +415,7 @@ define void @vscalex4_ult_noflags(ptr nocapture %A, i32 %n) mustprogress vscale_
; CHECK-NEXT: %arrayidx = getelementptr inbounds i32, ptr %A, i32 %i.05
; CHECK-NEXT: --> {%A,+,(16 * vscale)<nuw><nsw>}<%for.body> U: full-set S: full-set Exits: ((16 * vscale * ((-1 + %n) /u (4 * vscale)<nuw><nsw>)) + %A) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: %add = add i32 %i.05, %VF
; CHECK-NEXT: --> {(4 * vscale)<nuw><nsw>,+,(4 * vscale)<nuw><nsw>}<nw><%for.body> U: [0,-3) S: [-2147483648,2147483645) Exits: (vscale * (4 + (4 * ((-1 + %n) /u (4 * vscale)<nuw><nsw>))<nuw><nsw>)<nuw>) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: --> {(4 * vscale)<nuw><nsw>,+,(4 * vscale)<nuw><nsw>}<nuw><%for.body> U: [8,-3) S: [-2147483648,2147483645) Exits: (vscale * (4 + (4 * ((-1 + %n) /u (4 * vscale)<nuw><nsw>))<nuw><nsw>)<nuw>) LoopDispositions: { %for.body: Computable }
; CHECK-NEXT: Determining loop execution counts for: @vscalex4_ult_noflags
; CHECK-NEXT: Loop %for.body: backedge-taken count is ((-1 + %n) /u (4 * vscale)<nuw><nsw>)
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 536870910
Expand Down

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