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[SCEV] Use context sensitive reasoning in howFarToZero #94525

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merged 3 commits into from
Jun 19, 2024
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[SCEV] Use context sensitive reasoning in howFarToZero #94525

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215810d
[SCEV] Use context sensative reasoning in howFarToZero
preames Jun 5, 2024
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preames Jun 19, 2024
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18 changes: 14 additions & 4 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -10485,16 +10485,19 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
const SCEV *Step = getSCEVAtScope(AddRec->getOperand(1), L->getParentLoop());
const SCEVConstant *StepC = dyn_cast<SCEVConstant>(Step);

if (!isLoopInvariant(Step, L) || !isKnownNonZero(Step))
if (!isLoopInvariant(Step, L))
return getCouldNotCompute();

// Specialize step for this loop so we get context sensitive facts below.
const SCEV *StepWLG = applyLoopGuards(Step, L);

// For positive steps (counting up until unsigned overflow):
// N = -Start/Step (as unsigned)
// For negative steps (counting down to zero):
// N = Start/-Step
// First compute the unsigned distance from zero in the direction of Step.
bool CountDown = isKnownNegative(Step);
if (!CountDown && !isKnownNonNegative(Step))
bool CountDown = isKnownNegative(StepWLG);
if (!CountDown && !isKnownNonNegative(StepWLG))
return getCouldNotCompute();

const SCEV *Distance = CountDown ? Start : getNegativeSCEV(Start);
Expand Down Expand Up @@ -10533,6 +10536,13 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
// will have undefined behavior due to wrapping.
if (ControlsOnlyExit && AddRec->hasNoSelfWrap() &&
loopHasNoAbnormalExits(AddRec->getLoop())) {

// If the stride is zero, the loop must be infinite. In C++, most loops
// are finite by assumption, in which case the step being zero implies
// UB must execute if the loop is entered.
if (!loopIsFiniteByAssumption(L) && !isKnownNonZero(StepWLG))
return getCouldNotCompute();

const SCEV *Exact =
getUDivExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
const SCEV *ConstantMax = getCouldNotCompute();
Expand All @@ -10547,7 +10557,7 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
}

// Solve the general equation.
if (!StepC)
if (!StepC || StepC->getValue()->isZero())
return getCouldNotCompute();
const SCEV *E = SolveLinEquationWithOverflow(StepC->getAPInt(),
getNegativeSCEV(Start), *this);
Expand Down
42 changes: 24 additions & 18 deletions llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
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Expand Up @@ -271,9 +271,10 @@ define void @ne_nsw_pos_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nsw_pos_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nsw_pos_step
; 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: backedge-taken count is (((-1 * %s) + %n) /u %s)
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%pos_step = icmp sgt i32 %s, 0
Expand All @@ -299,9 +300,10 @@ define void @ne_nsw_neg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nsw_neg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nsw_neg_step
; 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: backedge-taken count is (((-1 * %n) + %s) /u (-1 * %s))
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -2
; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %n) + %s) /u (-1 * %s))
; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%neg_step = icmp slt i32 %s, 0
Expand All @@ -327,9 +329,10 @@ define void @ne_nsw_nonneg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nsw_nonneg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nsw_nonneg_step
; 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: backedge-taken count is (((-1 * %s) + %n) /u %s)
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%nonneg_step = icmp sge i32 %s, 0
Expand Down Expand Up @@ -381,9 +384,10 @@ define void @ne_nuw_pos_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nuw_pos_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nuw_pos_step
; 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: backedge-taken count is (((-1 * %s) + %n) /u %s)
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%pos_step = icmp sgt i32 %s, 0
Expand All @@ -409,9 +413,10 @@ define void @ne_nuw_neg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nuw_neg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nuw_neg_step
; 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: backedge-taken count is (((-1 * %n) + %s) /u (-1 * %s))
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -2
; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %n) + %s) /u (-1 * %s))
; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%neg_step = icmp slt i32 %s, 0
Expand All @@ -437,9 +442,10 @@ define void @ne_nuw_nonneg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nuw_nonneg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nuw_nonneg_step
; 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: backedge-taken count is (((-1 * %s) + %n) /u %s)
; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%nonneg_step = icmp sge i32 %s, 0
Expand Down
11 changes: 1 addition & 10 deletions llvm/test/Transforms/LoopRotate/pr56260.ll
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Expand Up @@ -17,16 +17,7 @@ define void @main() {
; CHECK-NEXT: [[TOBOOL3_NOT1:%.*]] = icmp eq i32 [[INC]], 0
; CHECK-NEXT: br i1 [[TOBOOL3_NOT1]], label [[L0_PREHEADER_LOOPEXIT]], label [[L1_PREHEADER_LR_PH:%.*]]
; CHECK: L1.preheader.lr.ph:
; CHECK-NEXT: br label [[L1_PREHEADER:%.*]]
; CHECK: L1.preheader:
; CHECK-NEXT: [[SPEC_SELECT3:%.*]] = phi i32 [ [[INC]], [[L1_PREHEADER_LR_PH]] ], [ [[SPEC_SELECT:%.*]], [[L0_LATCH:%.*]] ]
; CHECK-NEXT: [[K_02:%.*]] = phi i32 [ 0, [[L1_PREHEADER_LR_PH]] ], [ [[SPEC_SELECT3]], [[L0_LATCH]] ]
; CHECK-NEXT: [[TOBOOL8_NOT:%.*]] = icmp eq i32 [[K_02]], 0
; CHECK-NEXT: br label [[L0_LATCH]]
; CHECK: L0.latch:
; CHECK-NEXT: [[SPEC_SELECT]] = add nsw i32 [[SPEC_SELECT3]], [[INC]]
; CHECK-NEXT: [[TOBOOL3_NOT:%.*]] = icmp eq i32 [[SPEC_SELECT]], 0
; CHECK-NEXT: br i1 [[TOBOOL3_NOT]], label [[L0_L0_PREHEADER_LOOPEXIT_CRIT_EDGE:%.*]], label [[L1_PREHEADER]]
; CHECK-NEXT: br label [[L0_L0_PREHEADER_LOOPEXIT_CRIT_EDGE:%.*]]
;
entry:
br label %L0.preheader
Expand Down
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