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Revert "Recommit "[SCEV] Use nw flag and symbolic iteration count to sharpen ranges of AddRecs"" 

This reverts commit 32b72c3165.

While better than before, this change still introduces a large
compile-time regression (>3% on mafft):
https://llvm-compile-time-tracker.com/compare.php?from=fbd62fe60fb2281ca33da35dc25ca3c87ec0bb51&to=32b72c3165bf65cca2e8e6197b59eb4c4b60392a&stat=instructions

Additionally, the logic here doesn't look quite right to me,
I will comment in more detail on the differential revision.
This commit is contained in:
Nikita Popov 2020-10-16 21:13:41 +02:00
parent f085b7cbc1
commit 74c8c2d903
5 changed files with 7 additions and 86 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
llvm/include/llvm/Analysis/ScalarEvolution.h
View File

@ -1489,13 +1489,6 @@ private:
ConstantRange getRangeForAffineAR(const SCEV *Start, const SCEV *Stop,
const SCEV *MaxBECount, unsigned BitWidth);
/// Determines the range for the affine non-self-wrapping SCEVAddRecExpr {\p
/// Start,+,\p Stop}<nw>.
ConstantRange getRangeForAffineNoSelfWrappingAR(const SCEVAddRecExpr *AddRec,
const SCEV *MaxBECount,
unsigned BitWidth,
RangeSignHint SignHint);
/// Try to compute a range for the affine SCEVAddRecExpr {\p Start,+,\p
/// Stop} by "factoring out" a ternary expression from the add recurrence.
/// Helper called by \c getRange.

72
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -5509,17 +5509,6 @@ ScalarEvolution::getRangeRef(const SCEV *S,
ConservativeResult =
ConservativeResult.intersectWith(RangeFromFactoring, RangeType);
}
// Now try symbolic BE count and more powerful methods.
MaxBECount = computeMaxBackedgeTakenCount(AddRec->getLoop());
if (!isa<SCEVCouldNotCompute>(MaxBECount) &&
getTypeSizeInBits(MaxBECount->getType()) <= BitWidth &&
AddRec->hasNoSelfWrap()) {
auto RangeFromAffineNew = getRangeForAffineNoSelfWrappingAR(
AddRec, MaxBECount, BitWidth, SignHint);
ConservativeResult =
ConservativeResult.intersectWith(RangeFromAffineNew, RangeType);
}
}
return setRange(AddRec, SignHint, std::move(ConservativeResult));
@ -5689,67 +5678,6 @@ ConstantRange ScalarEvolution::getRangeForAffineAR(const SCEV *Start,
return SR.intersectWith(UR, ConstantRange::Smallest);
}
ConstantRange ScalarEvolution::getRangeForAffineNoSelfWrappingAR(
const SCEVAddRecExpr *AddRec, const SCEV *MaxBECount, unsigned BitWidth,
ScalarEvolution::RangeSignHint SignHint) {
assert(AddRec->isAffine() && "Non-affine AddRecs are not suppored!\n");
assert(AddRec->hasNoSelfWrap() &&
"This only works for non-self-wrapping AddRecs!");
const bool IsSigned = SignHint == HINT_RANGE_SIGNED;
const SCEV *Step = AddRec->getStepRecurrence(*this);
// Let's make sure that we can prove that we do not self-wrap during
// MaxBECount iterations. We need this because MaxBECount is a maximum
// iteration count estimate, and we might infer nw from some exit for which we
// do not know max exit count (or any other side reasoning).
// TODO: Turn into assert at some point.
MaxBECount = getNoopOrZeroExtend(MaxBECount, AddRec->getType());
const SCEV *RangeWidth = getNegativeSCEV(getOne(AddRec->getType()));
const SCEV *StepAbs = getUMinExpr(Step, getNegativeSCEV(Step));
const SCEV *MaxItersWithoutWrap = getUDivExpr(RangeWidth, StepAbs);
if (!isKnownPredicate(ICmpInst::ICMP_ULE, MaxBECount, MaxItersWithoutWrap))
return ConstantRange::getFull(BitWidth);
ICmpInst::Predicate LEPred =
IsSigned ? ICmpInst::ICMP_SLE : ICmpInst::ICMP_ULE;
ICmpInst::Predicate GEPred =
IsSigned ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE;
const SCEV *Start = AddRec->getStart();
const SCEV *End = AddRec->evaluateAtIteration(MaxBECount, *this);
// We could handle non-constant End, but it harms compile time a lot.
if (!isa<SCEVConstant>(End))
return ConstantRange::getFull(BitWidth);
// We know that there is no self-wrap. Let's take Start and End values and
// look at all intermediate values V1, V2, ..., Vn that IndVar takes during
// the iteration. They either lie inside the range [Min(Start, End),
// Max(Start, End)] or outside it:
//
// Case 1: RangeMin ... Start V1 ... VN End ... RangeMax;
// Case 2: RangeMin Vk ... V1 Start ... End Vn ... Vk + 1 RangeMax;
//
// No self wrap flag guarantees that the intermediate values cannot be BOTH
// outside and inside the range [Min(Start, End), Max(Start, End)]. Using that
// knowledge, let's try to prove that we are dealing with Case 1. It is so if
// Start <= End and step is positive, or Start >= End and step is negative.
ConstantRange StartRange =
IsSigned ? getSignedRange(Start) : getUnsignedRange(Start);
ConstantRange EndRange =
IsSigned ? getSignedRange(End) : getUnsignedRange(End);
ConstantRange RangeBetween = StartRange.unionWith(EndRange);
// If they already cover full iteration space, we will know nothing useful
// even if we prove what we want to prove.
if (RangeBetween.isFullSet())
return RangeBetween;
if (isKnownPositive(Step) &&
isKnownViaNonRecursiveReasoning(LEPred, Start, End))
return RangeBetween;
else if (isKnownNegative(Step) &&
isKnownViaNonRecursiveReasoning(GEPred, Start, End))
return RangeBetween;
return ConstantRange::getFull(BitWidth);
}
ConstantRange ScalarEvolution::getRangeViaFactoring(const SCEV *Start,
const SCEV *Step,
const SCEV *MaxBECount,

4
llvm/test/Analysis/ScalarEvolution/no-wrap-symbolic-becount.ll
View File

@ -7,7 +7,7 @@ define i32 @test_01(i32 %start, i32* %p, i32* %q) {
; CHECK-NEXT: %0 = zext i32 %start to i64
; CHECK-NEXT: --> (zext i32 %start to i64) U: [0,4294967296) S: [0,4294967296)
; CHECK-NEXT: %indvars.iv = phi i64 [ %indvars.iv.next, %backedge ], [ %0, %entry ]
; CHECK-NEXT: --> {(zext i32 %start to i64),+,-1}<nsw><%loop> U: [0,4294967296) S: [0,4294967296) Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
; CHECK-NEXT: --> {(zext i32 %start to i64),+,-1}<nsw><%loop> U: [-4294967295,4294967296) S: [-4294967295,4294967296) Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
; CHECK-NEXT: %iv = phi i32 [ %start, %entry ], [ %iv.next, %backedge ]
; CHECK-NEXT: --> {%start,+,-1}<%loop> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
; CHECK-NEXT: %iv.next = add i32 %iv, -1
@ -21,7 +21,7 @@ define i32 @test_01(i32 %start, i32* %p, i32* %q) {
; CHECK-NEXT: %stop = load i32, i32* %load.addr, align 4
; CHECK-NEXT: --> %stop U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop: Variant }
; CHECK-NEXT: %indvars.iv.next = add nsw i64 %indvars.iv, -1
; CHECK-NEXT: --> {(-1 + (zext i32 %start to i64))<nsw>,+,-1}<nsw><%loop> U: [-4294967296,4294967295) S: [-1,4294967295) Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
; CHECK-NEXT: --> {(-1 + (zext i32 %start to i64))<nsw>,+,-1}<nsw><%loop> U: [-4294967296,4294967295) S: [-4294967296,4294967295) Exits: <<Unknown>> LoopDispositions: { %loop: Computable }
; CHECK-NEXT: Determining loop execution counts for: @test_01
; CHECK-NEXT: Loop %loop: <multiple exits> Unpredictable backedge-taken count.
; CHECK-NEXT: exit count for loop: (zext i32 %start to i64)

2
llvm/test/Transforms/IndVarSimplify/X86/eliminate-trunc.ll
View File

@ -474,7 +474,7 @@ define void @test_10(i32 %n) {
; CHECK-NEXT: [[TMP1:%.*]] = zext i32 [[TMP0]] to i64
; CHECK-NEXT: [[TMP2:%.*]] = icmp ult i64 [[TMP1]], 90
; CHECK-NEXT: [[UMIN:%.*]] = select i1 [[TMP2]], i64 [[TMP1]], i64 90
; CHECK-NEXT: [[TMP3:%.*]] = add nuw nsw i64 [[UMIN]], -99
; CHECK-NEXT: [[TMP3:%.*]] = add i64 [[UMIN]], -99
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ -100, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]

8
llvm/test/Transforms/IndVarSimplify/promote-iv-to-eliminate-casts.ll
View File

@ -196,7 +196,7 @@ define void @promote_latch_condition_decrementing_loop_01(i32* %p, i32* %a) {
; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], [[LOOP]] ], [ [[TMP0]], [[PREHEADER]] ]
; CHECK-NEXT: [[EL:%.*]] = getelementptr inbounds i32, i32* [[A:%.*]], i64 [[INDVARS_IV]]
; CHECK-NEXT: store atomic i32 0, i32* [[EL]] unordered, align 4
; CHECK-NEXT: [[LOOPCOND:%.*]] = icmp ult i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[LOOPCOND:%.*]] = icmp slt i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nsw i64 [[INDVARS_IV]], -1
; CHECK-NEXT: br i1 [[LOOPCOND]], label [[LOOPEXIT_LOOPEXIT:%.*]], label [[LOOP]]
;
@ -241,7 +241,7 @@ define void @promote_latch_condition_decrementing_loop_02(i32* %p, i32* %a) {
; CHECK-NEXT: [[EL:%.*]] = getelementptr inbounds i32, i32* [[A:%.*]], i64 [[INDVARS_IV]]
; CHECK-NEXT: store atomic i32 0, i32* [[EL]] unordered, align 4
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nsw i64 [[INDVARS_IV]], -1
; CHECK-NEXT: [[LOOPCOND:%.*]] = icmp ult i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[LOOPCOND:%.*]] = icmp slt i64 [[INDVARS_IV]], 1
; CHECK-NEXT: br i1 [[LOOPCOND]], label [[LOOPEXIT_LOOPEXIT:%.*]], label [[LOOP]]
;
@ -285,7 +285,7 @@ define void @promote_latch_condition_decrementing_loop_03(i32* %p, i32* %a) {
; CHECK-NEXT: [[EL:%.*]] = getelementptr inbounds i32, i32* [[A:%.*]], i64 [[INDVARS_IV]]
; CHECK-NEXT: store atomic i32 0, i32* [[EL]] unordered, align 4
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nsw i64 [[INDVARS_IV]], -1
; CHECK-NEXT: [[LOOPCOND:%.*]] = icmp ult i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[LOOPCOND:%.*]] = icmp slt i64 [[INDVARS_IV]], 1
; CHECK-NEXT: br i1 [[LOOPCOND]], label [[LOOPEXIT_LOOPEXIT:%.*]], label [[LOOP]]
;
@ -336,7 +336,7 @@ define void @promote_latch_condition_decrementing_loop_04(i32* %p, i32* %a, i1 %
; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], [[LOOP]] ], [ [[TMP0]], [[PREHEADER]] ]
; CHECK-NEXT: [[EL:%.*]] = getelementptr inbounds i32, i32* [[A:%.*]], i64 [[INDVARS_IV]]
; CHECK-NEXT: store atomic i32 0, i32* [[EL]] unordered, align 4
; CHECK-NEXT: [[LOOPCOND:%.*]] = icmp ult i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[LOOPCOND:%.*]] = icmp slt i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nsw i64 [[INDVARS_IV]], -1
; CHECK-NEXT: br i1 [[LOOPCOND]], label [[LOOPEXIT_LOOPEXIT:%.*]], label [[LOOP]]
;